Lens alphabet soup decoded
Lens specifications – and in particular the string of numbers and letters that make up the long name of a lens – can be difficult to grasp. But when you are considering a particular lens, it is very useful to understand what all the numbers and letters that appear as part of the long lens lens name means.
By being familiar with the alphabet soup, that specify the various properties of a lens, and such terms as “focal length”, “maximum aperture” and “CFD magnification” (if it is a macro lens), you should be able to pinpoint whether a particular lens is suitable for your intended use or not.
As an example, look at the image to the left. It shows a Nikon-lens, and the long name, the label that is marked on the lens, is DX AF-S Nikkor 18-135 mm 1:3.5-5.6 G ED.
I.e.: This particular lens label consists of seven elements, starting with “DX” and ending with “ED”. Below is a brief explanation of each of the seven elements.
- “DX”:. This two-letter code is specific to Nikon (and Tokina), and tells us that this lens is a lens designed with to be used on a camera with a reduced image circle (RIC) and should only be used on a digital camera where the sensor covers a smaller area than the so-called FX-size. The DX-size format is designed to cover a digital sensor measuring 23.6 x 15.8 mm, with a 28.4 mm diagonal. The FX-size format is roughly the same size of the negative of the very popular 135-format film, i.e. 24 x 36 mm with a 43.3 mm diagonal.
- “AF-S”: This is a Nikon-specific code, and tells us two things. First, that is an autofocus (AF) lens, and second, that the lens is fitted with an integral motor motor that drives the autofocus.
- “NIKKOR”: This is a brand name for Nikon lenses.
- “18-135mm”: This is the focal lengths of the lens. Since this is a zoom lens, two different values are given. The lens is 18 mm at its wide end, and 135 mm at its tele end.
- “1:3.5-5.6”: These are the maximum apertures of the lens.
- “G”: This is a Nikon-specific code and indicates that a lens is without a mechanical aperture ring. All Nikon G-lenses will relay distance information to the body. As far as I know “G” is not an abbreviation for anything.
- “ED”: This is a Nikon-specific code and indicates that at least one special glass element, called a Extra-low Dispersion element, is used in the lens.
Below, we shall describe these and other elements used in lens labels in much greater detail.
2. Alphabet soup
Below is a more detailed breakdown of some of the major elements that make up lens labels.
Reduced Image Circle
A fundamental characteristic of a lens is its image circle. The image circle is the circle that contains the image projected on to the film or digital sensor.
In the film era, the most popular film format was called the 135-format and produced negatives the measured 24 x 36 mm with a 43.3 mm diagonal. There exists digital cameras with a sensor that is roughly the same size. Nikon labels these cameras “FX”. Following up on Nikon's terminology, I call a sensor measuring 24 x 36 mm an “FX-sized” sensor.
Most DSLRs have a sensor than is smaller than FX-size. I call these: Cameras with a reduced image circle (RIC).
DSLRs with a smaller sensor (RIC) may use lenses that are specifically designed for the smaller area (compared to the FX-format). Such lenses can be built smaller and lighter than a comparable lens designed to cover the image circle of an FX-sized sensor. They may physically fit on a 135-format film camera, or a digital camera with an FX-format sensor, but its use will result in vignetting in at least part of its focal length range.
Buyers need to able to tell the lenses designed for a RIC apart from those designed for a FX-sized sensor. All lens manufacturers except those that produce lenses for the 4/3rds mount labels lenses designed for a RIC with a special string. I list these below:
|Canon||EF-S||will not physically fit on bodies with larger sensors|
|Leica||-||all lenses in 4/3rds mount are RIC|
|Olympus||-||all lenses in 4/3rds mount are RIC|
|Panasonic||-||all lenses in 4/3rds mount are RIC|
|Sigma||DG||Sigma uses DG for the FX DSLRs|
|Sigma||DC||Sigma uses DC for the RIC DSLRs|
|Sigma||DN||Sigma uses DN (Digital Neo) for RIC MILCs.|
|Zeiss||DT||co-branded with Sony|
Example: The string “Di-II” that is part of the official label for the Tamron 17-50mm f/2.8 SP AF XR Di-II LD IF , tells us that this is a lens designed for a camera with a RIC. This also means that it will not perform well on a digital camera with an FX-sized sensor, or a 135-format film camera.
Lenses lenses that work well on a camera with a FX-sized sensor will also work well on a camera with a RIC. Therefore, most DSLR lens manufacturers does not label such lenses. However, Canon lenses marked “EF”, and Sigma lenses marked “DG” always have an image circle suitable for an FX-sized sensor.
Note that the 4/3rds mount that is used by Leica, Olympus and Panasonic is RIC by design (17.3 x 13.0 mm). This is why most lenses designed for the 4/3rds mount are not explicitly marked. These lenses do not fit physically on any DSLR with a larger image sensor.
The purpose of stabilisation is to reduce motion blur caused by hand-holding the camera on long shutter speeds.
Olympus and Pentax exclusively use a mechanical in-body system called sensor-shift, where the camera's image sensor is moved for image stabilisation. A similar system is used in some Leica and Panasonic bodies for the Four-Thirds system.
Canon, Nikon Tamron and Sigma exclusively use an optical system for image stabilisation. A similar system is used in some Leica and Panasonic lenses for the Four-Thirds system. These optical systems have different names (see table below), but they are all built around the same design principle: A floating lens element is moved orthogonally to the optical axis of the lens using electromagnets, modifying the optical path to cancel vibrations caused by angular movement of the lens.
In 2009, Canon introduced an improved stabilisation system dubbed Hybrid Image Stabilisation (H-IS). The H-IS system also compensates for camera shift, which may be pronounced in macro and close-range photography.
|Canon||H-IS||Hybrid Image Stabilisation|
|Leica||OIS||Optical Image Stabilisation. Also sensor-shift (on some models).|
|Olympus||-||Sensor-shift IS (on some models).|
|Panasonic||OIS||Optical Image Stabilisation. Also sensor-shift (on some models).|
|Pentax||-||Sensor-shift SR (on some models).|
|Sony||-||Sensor-shift SteadyShot (on all models)|
The stabilisation system in a modern DSLR or lens does not engage until the shutter is pressed half-way. It may take some time (up to 1 second) before the stabilisation settles and gives you the most stable result.
Some Canon lenses have a switch that let you toggle VR between modes “1” and “2”. Mode “1” stabilises in both the horizontal and vertical directions. Mode “2” is used when panning the lens and turns off stabilisation in the direction in which the lens is being panned.
Likewise, some Nikon lenses have a switch that let you toggle VR between “Normal” and “Active” mode. Use “Normal” for hand-holding from a fixed position, and “Active” when shooting from a moving vehicle and similar situations.
Note that activating the stabilisation system only affects your ability to photograph a stationary object at longer shutter speeds without getting blur from camera shake. If the object is moving long shutter speeds will give you motion blur whether the stabilisation system is active or not.
Some compact cameras are said by their manufacturer to offer “stabilisation” because they can be set to use a higher ISO (which means that you use a higher shutter speed that reduces the effect of motion blur). As all cameras these days can set to a higher ISO, calling this “stabilisation” is just marketing hype.
Canon's EOS-system, introduced in 1987, was designed from the beginning to have the focus motor for driving autofocus in the lens, rather than in the camera body. Other camera systems initially had the focus motor in the camera body, and used a mechanical coupling to transfer torque to gears inside the lens to move the focus ring. Later, these systems also added an electronic connection to their lens mount that permitted putting the focus motor in the lens and controlling it from the body.
There are lens labels that sometimes will tell you whether a lens comes with a focus motor, and/or what type the focus motor is. However, in many cases you can not rely on the labels that are part of the lens name, but must look up the lens' detailed specifications to find this out.
The most advanced type of in-lens focus motor is the so-called ring ultrasonic motor introduced by Canon as part of the EOS-system. This type of motor uses supersonic sound waves to transfer energy without any moving parts. As the term “ring” implies, the motor is shaped like a doughnut that surrounds the optics of the camera. This type of motor is superior to other designs because it is fast, silent, precise, robust, and it also let you to do manual focus override when in autofocus mode.
Canon adopted the term “USM” (for “UltraSonic Motor”) for this type of motor. When Nikon started offering lenses with this type of ring motor, they called it “AF-S” (for “Autofocus” – Silent wave motor). Now most lens manufacturers offer lenses with a supersonic ring motor. They are all build around the same technology, but each manufacturer uses its own name for it (see table below).
To make things even more confusing, some Canon lenses labelled “USM” are slow to focus, and noisy. Likewise, there are Nikon lenses labelled “AF-S” and Sigma lenses labelled “HSM” with similar characteristics. Indeed, the more “consumer-oriented” an USM-, AF-S-, or HSM-labelled lens is, the more noise it makes and the slower the autofocus speed is.
This is because all three companies use the “USM-”, “AF-S-”, or “HSM”-label for two different types of focus motor: The first is the original “ring-motor” described above, the second is something called a “micro-motor”.
A micro-motor is not silent and fast like a ring-motor, nor will it let you override with manual focus while in autofocus mode. Micro-motor driven autofocus uses moving mechanical parts, such as cogwheels (gear wheels) and shafts.
To see whether a lens with built-in focus motor has a ring-motor or a micro-motor, you can look at the manufacturer's detailed specifications.
For Nikon lenses, a quick way to determine the type of focus motor is to examine the lens' autofocus control panel (show above). A lens equipped with a micro-motor will only let you choose between autofocus (A), and manual focus (M). A lens equipped with a ring-motor will let you choose between manual+autofocus (M/A), and manual focus (M).
Nikon autofocus lenses without the “AF-S” or “AF-I”-label do not have a built-in focus motor. These lenses instead use a screwdriver-like mechanical coupling where a focus motor in the body transfers torque to the autofocus mechanism in the lens. Consumer-end Nikon bodies, such as the D40, D40x, D60, D3000, D3100, D5000 and D5100, do not have a built-in focus motor. If you fit one of these older autofocus Nikkors to such a body, you will not have autofocus.
A list of F-mount lenses that can be used on consumer-end Nikon bodies is maintained on this site. Please see: Nikon lenses with built-in focus motor.
All autofocus lenses designed for the Four-Thirds system used by Leica, Panasonic and Olympus have a built-in focus motor. The same is the case for all lenses with the Sigma SD mount (used by Sigma DSLRs).
So far, all digital Pentax and Sony bodies both have a drive shaft that transfers torque from an in-body focus motor to the lens. All Sony and newer Pentax DSLRs also have an electrical contact that can be used to control an in-lens focus motor. Older lenses for the Pentax and Sony system make use of the camera's in-body focus motor, while newer lens designs have a built-in focus motor. The Pentax *ist-series and the Pentax K100D can not make use of an in-lens focus motor. These Pentax bodies only let you use manual focus with third-party lenses with built-in focus motors (e.g. Sigma HSM-lenses).
Third party autofocus lenses from Tamron, Tokina and Sigma for Canon's EOS system have a built-in focus motor. Most of these does not carry a special label, unless the motor is a ring-motor.
Third-party autofocus lenses for Nikon's F-mount did not come with a built-in focus motor until very recently. Instead, they made use of the focus motor in the body, just like Nikon's own lenses prior to the introduction of AF-S. After Nikon introduced several new DSLR models (starting with the D40 in 2006) without a focus motor in the body, third party manufacturers has been busy redesigning their current lenses for the Nikon mount so that they come with a built-in focus motor. Unless the motor is a ring-motor, there is usually nothing in the name of the less that indicates that it has a built-in focus motor. This means that if you are buying third-party autofocus lenses for Nikon's F-mount, and need a version with a focus motor, you need to be very careful. An old version of a particular lens may come with a mechanical coupling and no focus motor. A newer version of the lens will have the same name, but may come with an electronic connection and a built-in focus motor.
The DPanswers' lens finder will tell you whether a particular lens has a built-in focus motor.
The table below list the labels used by different manufacturers to say something about an in-lens focus motor. NB: You need to read the notes to get the full picture. (The notes also explains how you can find for yourself whether a particular lens has a built-in motor.)
|Canon||AFD||Arc Form Drive (older than USM and STM).|
|Canon||USM||Ultra-Sonic Motor (1).|
|Leica||-||No ring-motor AF lenses in program?|
|Nikon||AF-I||AF Integrated (older than AF-S)|
|Nikon||AF-S||AF with built-in motor in lens (2).|
|Olympus||SWD||Supersonic Wave Drive|
|Pentax||SDM||Sonic Drive Motor (3)|
|Tamron||PZD||PieZo Drive (4).|
|Tamron||USD||Ultrasonic Silent Drive (4).|
|Tokina||SD-M||Silent Drive-Module (5).|
|Sigma||HSM||Hyper-Sonic Motor (6).|
|Sony||SAM||Smooth Autofocus Motor (i.e. micro motor)|
|Sony||SSM||Super-Sonic Motor (i.e. ring type motor)|
- Canon's “USM”-label is used for both ring-motor USM and for micro-motor USM. All Canon EF and EF-S autofocus lenses have a built-in focus motor. Some Canon autofocus lenses does not carry the USM-label. They contain a regular focus motor that is a bit more noisy, but otherwise is very similar to micro-motor USM.
- Nikon's “AF-S”-label is used for both ring-motor silent wave AF-S and for micro-motor AF-S. Not all Nikon autofocus lenses have a built-in focus motor, but all Nikon lenses with the “AF-S” and “AF-I”-label have a built-in focus motor. For list of all current Nikon lenses with a built-in focus motor, see our lens finder.
- Currently all Pentax lenses carrying the SDM-label is fitted with an internal micro-motor type drive (i.e. not an ultrasonic ring-motor). The Pentax SDM-motor is fairly slow and noisy. Pentax' SDM-lenses is also fitted with a mechanical coupling that will make these lenses work with the in-body focus motor in old cameras (i.e. the *ist-series or the K100D) that lack the electrical contact used by newer bodies to control the focus motor in the lens. Third party lenses that come fitted with a built-in focus motor does not provide such a mechanical coupling, and will not autofocus with these old Pentax cameras.
- As of January 2012, almost all currently available F-mount Tamron lenses have a built-in focus motors. Those with a ring-type motor are marked PZD or USD, (depending on the type of motor), while those with a micro-motor does not carry any special label. Note that Tamron has since around 2007 silently replaced the screw-drive AF in their F-mount lenses without changing the name of the lens. If you're shooting with a DSLR that requires the focus motor to be in the lens, and are buying second-hand Tamron lenses, you may need to check the type of AF in he lens by looking at the mount (or a picture of the mount).
- As of January 2012, not all Tokina lenses has a built-in focus motor. Tokina is updating their range to have focus-motors in more Nikon-mount models. To check for updates, go to Tokina's webpage and click on the lens you're interested in. The description of the lens will mention it if the lens has a built-in AF motor drive. In Tokina literature, you may see reference to a “SD-M” or a “DC motor“. According to Tokina, SD-M stands for “Silent Drive-Module”, which is described as a very quiet integral gear reducer DC motor. Some thinks that “DC” means “Direct Coupled” (i.e. a micro-motor). Anyway, Tokina does not put the type of focus motor in the long name of the lens, only in its description.
- Sigma's “HSM”-label is used for both ring-motor HSM and for micro-motor HSM. Some Sigma lenses with a micro-motor is not marked with any special label. Currently, not all Sigma lenses have a built-in focus motor in the Nikon mount, but many have. To find out whether a current Sigma lens is available in the Nikon mount with a built-in focus motor, go to the lens-finder on Sigma's webpage and search for the lens you're interested in. Those with a built-in focus motor are marked with “HSM” or “Motorised”.
In 1976, Nikon first introduced some lenses where focus was achieved by moving elements inside the lens barrel. This was called “Internal Focusing” (IF) and resulted in less glass being moved and therefore faster autofocus than conventional designs. Another benefit was designs with a closer minimum focus distance, that the front element don't move during focusing (useful for macro), and that the front ring don't rotate (useful with certain types of filter).
Internal focusing widens the lens' field of view as it focuses closer. Such lenses will only have the FOV normally associated with its rated focal length when the focus is set to infinity. Focused at its close focus distance (CFD), most IF lenses have a FOV that corresponds to the FOV of a shorter focal length.
“Rear Focusing” is similar to IF, except just the rear element or group retracts as the lens is focused closer.
“Dual Focus” is used by Sigma on some lenses to indicate that it has a system where you push the focus ring forward to dis-engage it, so it don't move during autofocus.
Some lenses uses glass elements such as fluorite (an artificial crystal) or other special types of glass with extra low dispersion. Some manufacturers use the word “apochromatic” (abbreviated APO) to indicate that low dispersion glass is used in a lens. The reason that such special glass is used is to correct chromatic aberration. This is particular important in long tele lenses (200 mm and above). These days, one often see a reference to special glass elements in non-tele lenses. This is (IMHO) a marketing gimmick.
Tamron uses glass with extra high refractive index for the same purpose.
Some lens design also make use of so-called aspherical surfaces. This allows a more compact design without loss of quality.
Below are the labels various manufacturers use to indicate the use of special glass and/or surfaces:
|Canon||L||Lens using fluorite elements and aspherical surfaces.|
|Tamron||LAH||LD Aspherical Hybrid.|
|Tamron||XR||Extra Refractive index.|
Sometimes, the word “Aspherical” is not abbreviated, but is spelt out in full as part of the lens' long name.
Both fluoride and low dispersion glass is less stable with temperature than conventional glass. This means that the optical characteristics of these lenses change slightly with temperature. Canon colour most of their L-lenses white to make them less susceptible to temperature change. Nikon also do this with a few. Also, there is no hard infinity focus stop on Nikon ED-lenses because the point of infinity focus will change with temperature.
Fluoride and low dispersion glass has a lower refractive index than conventional glass, so such designs requires more deeply curved elements for the same focal length, than conventional designs. Tamron's XR-glass, however, has a higher refractive index than conventional glass. Tamron explains why it uses XR-glass in some designs as follows. “Glass having a refractive index of more than 1.69 is called extra refractive or high index glass. This numerical expression compares the speed of light in a transparent medium, like glass, with the speed of light in air. The higher the index number, the thinner a given element will be. The refractive index its ability to bend light plays a critical role in the creation of the power and thickness of an element. Extra Refractive Index (XR) glass bends light more effectively than normal or lower refractive index glass and can therefore compensate for specific aberrations within an optical design.”
Some manufacturers uses a special label on a lens to indicate that it is of exceptional high quality, or “professional grade”. This usually means that the lens is very good mechanically, often with environmental sealings, that it has a large aperture, and that it is well corrected for aberrations with the use of special optical materials.
|Canon||L||Luxury. Also red ring on lens.|
|Leica||-||All Leica lenses are of exceptional quality.|
|Nikon||-||No letter. Gold ring on lens indicates superior quality.|
|Pentax||*||E.g. “FA*” means a top-of-the-line autofocus lens.|
|Pentax||Limited||More recently, Pentax uses this label for quality optics.|
|Tokina||AT-X||Advanced Technology – Extra|
|Sony||G||Gold. Also gold ring on lens.|
In some literature you'll see several interpretations of the letter “L” that Canon uses in the label of its professional grade lenses. The official word from Canon is that it stands for “Luxury” (source: EF Lens Work III, the section named Canon's Challenges). In the same document, Canon states the “L”-label is reserved for lenses “using fluorite (an artificial crystal), a ground and polished aspherical surface, UD, super UD lenses, or other special optical materials.”. This is probably why an otherwise professional grade lens, the Canon 100mm f/2.0 EF USM, is not honoured with an “L”-label.
In some literature. you will see that the writer thinks that the term “Nikkor” is used by Nikon to signify that a lens is of high quality. That may have been the case a long time ago, but now Nikon calls almost every lens “Nikkor”, including some cheap plastic lenses of inferior quality. The same can be said for Nikon's use of the term “ED”. When ED-glass was introduced, Nikon only used it on lenses that was expensive and mechanical very good. Now ED-glass is also used in cheaper designs, so it no longer has significance as a quality designator. The best indicator (apart from price) that Nikon consider a lens to be of superior quality, is a gold ring. However, Nikon only puts a gold ring on lenses with ED glass elements. That means that some very good professional grade fixed focal length lenses (e.g. the Nikkor 85mm f/1.4 AF-D IF) does not have a gold ring. However, consumer grade zooms with ED glass elements does not have a gold ring (but some of them have a silver ring). Overall, Nikon's policy with the gold ring seems to be similar to Canon's policy with its “L” label and red ring.
Konica-Minolta and Sony uses the letter “G” to indicate a high-quality lens, indicating high mechanical quality, excellent optical performance, and large aperture. However, no Minolta G lens has “G” written anywhere on the lens. You can find the “G” label only on the box. Only Sony has started putting the “G” label also on the lens itself.
Some lenses relay distance information to body. This is used by advanced TTL flash systems for more accurate flash exposure.
|Canon||-||All recent Canon lenses relay distance information.|
|Nikon||D||Autofocus lens that relay distance information to body.|
|Nikon||G||All G-lenses relay distance information to the body.|
|Tamron||-||All recent Tamron lenses relay distance information.|
|Tokina||-||All recent Tokina lenses relay distance information.|
|Sigma||-||All recent Sigma lenses relay distance information.|
|Sony||D||Lens relay distance information to body.|
Newer Canon cameras use an improved version of the E-TTL system for flash exposure, called E-TTL II. The most significant difference from E-TTL is that E-TTL II is capable of taking the focus distance as reported by the lens into account when computing flash output. Not all Canon lenses report focus distance to the body, but all newer lenses do. For older lenses, this chart at Lensplay.com lists those that does not.
According to Nikon support, distance information (relayed by D- and G-lenses) is important for getting the most accurate exposure from Nikon's flash system CLS (Creative Lighting System). The use of Nikon D- or G-type lenses is recommended as the distance information is used to make the computation of what flash power to use more accurate.
Sony uses the information from a D-lens for ADI (Advanced Distance Integration) flash, and the Dynax 7 can also calculate DOF with this information. With non-D lenses you can not use ADI flash.
Newer aftermarket lenses from Tamron, Tokina and Sigma relay distance information to body, but doesn't indicate this on the lens. Thom Hogan maintains a set of list of lenses with Nikon mount that do this. If the Nikon-mount version of a lens relay distance information the body, other versions probably do it as well. Thom's lists are here:
The table below summarises miscellaneous other special designations and letters you may come across on current lenses. I've left out designations and letters you may come across on older mounts.
|Canon||EF-S||Electronic Focus – Short back focus|
|Canon||MP-E||Macro Photo – Electronic. Non-AF lens for macro photography.|
|Canon||SF||Soft Focus. Let the photographer add deliberate spherical aberration.|
|Canon||TS-E||Tilt-shift – Electronic. Lens offers perspective control.|
|Nikon||DC||Defocus Control. Gives some control over bokeh.|
|Nikon||G||Aperture is controlled electronically (no aperture ring).|
|Nikon||N||Nano Crystal Coat.|
|Tamron||Di||Digital integrated. FX-lens with special coatings.|
|Tamron||ZL||Zoom Lock. Lock to avoid zoom creep.|
|Tokina||GMR||Magnetic sensor to locate the focus element's current position.|
|Tokina||Pro||MF/AF switch by pulling focus ring|
|Sigma||DG||Digital Grade. FX-lens with special coatings.|
|Sony||STF||Smooth Trans Focus – Lens with improved bokeh.|
Nikon puts a symbol containing the letter “N” inside an oblong hexagon (see illustration on the left) on the barrel of lenses that features Nikon's Nano Crystal Coat. However, the “N” is not normally used as part of the lens name.
Some lenses also come with the term “Macro” as part of their long name (e.g.: “Tamron 18-270 mm f/3.5-6.3 Di II VC LD Aspherical IF Macro”). This may indicate that the lens is a macro lens, with a design optimised for close-up photography. But it may also just be a marketing gimmick and only indicate that the lens will focus fairly close. To actually find out if a lens is a true macro lens, you need to look up its CFD magnification in the manufacturer's specifications.
Nikon uses the term “Micro-Nikkor” to indicate that a lens is a macro lens (e.g.: “Micro-Nikkor 105mm f/2.8 G AF-S VR IF-ED”).
3. Optical parameters
The optical parameters that determines the characteristics of a fixed focal length lens are the focal length, the maximum aperture, and the magnification at the close focus distance (CFD).
A zoom lens has always two focal lengths associated with it, indicating the focal length at the start and at the end of the zoom range. The name of a zoom lens may also list two different maximum apertures, then the first is associated with the start of the zoom range, and the second is associated with the end of the zoom range. For a zoom lens, the CFD magnification is always when the zoom is set to its longest focal length.
The focal length of a lens expressed as one or two numbers followed by the label “mm” (millimetres). Examples: “50 mm” and “18-135 mm”.
The focal length is a physical property of an optical system. When a photographic lens is set to “infinity“, its rear nodal point is separated from the sensor or film, at the focal plane, by the lens' focal length.
When the focal length is a single number, the lens is called a fixed focal length lens. When the focal length is expressed as two numbers, the lens is called a zoom lens or variable focal length lens. A fixed focal length lens has a single focal length that cannot be changed. A zoom lens allow the photographer to select any focal length within the range given by the two numbers. For example, a zoom lens designated “18-135 mm” can be set to a focal length equal to 18 mm at its wide end, and 135 mm at its tele end, and also any value between the two extremes.
For a photographer, the focal length is useful to determine the field of view (FOV) he or she will get when using the lens on a camera with a sensor with a given size. For practical and historical reasons, the FOV itself have never been marked on the lens. Instead, the focal length is given, and the photographer is expected to know how focal lengths corresponds to FOV.
For example, if you use a camera that has a DX-format sensor, you may know that a focal length equal to 19 mm corresponds to a wide angle lens with a FOV equal to 75°, a focal length equal to 33 mm corresponds to a so-called normal lens with a FOV equal to 47°, and a focal length equal to 100 mm corresponds to a so-called tele (or tele) lens with a FOV equal to 16°. Notice that the smaller the number designating the focal length is, the wider the FOV.
You should also know that the relationship between a specific focal length and and a specific FOV is only true for a specific sensor size. The FOV angles listed above are all true for a camera with an DX-format sensor, but if you use the same focal lengths on a camera with a FX-format sensor, the FOV angles will be different, as shown in the tables below.
Some people are confused when they encounter a lens that is a reduced image circle lens. They believe that they also need to take this into account to determine the FOV of the lens. This is not so. Setting a specific focal length (e.g. 19 mm) will always produce a FOV equal to 75° when placed on a camera with a DX-format sensor – it does not matter if you choose that setting on a zoom lens designed for a DX-format sensor, or a zoom lens designed for a larger format sensor.
When selecting what lens to use for a task, you should be familiar with the common names used for different classes of lenses (i.e. super-wide, wide, normal, short tele, tele and super-tele), and how these classes of lenses maps onto specific focal lengths depending upon sensor size. This is done in the two tables below.
The table shows the focal length (in millimetres) for for a number of common DSLR formats. The rows labelled “FOV” shows the field of view in degrees for the focal lengths listed. The top FOV row shows diagonal field of view, and the bottom FOV shows horizontal field of view for sensors with a 3:2 aspect ratio (i.e. all except Four-Thirds, which has a 4:3 aspect ratio). You can use the table to figure out the focal lengths to look for if you want a specific class of lens, given the sensor size of the camera you want to use the lens on.
|Format||Crop f.||Super-wide||Wide||Normal||Short tele||Tele||Super-tele|
When using the table, you will need to match focal length to the nearest focal length that is actually available. E.g. if you want a “normal” fixed focal length lens, matching the popular 50 mm on a camera with an FX-format sensor on a camera with a DX-format sensor, the table tells you that an exact match is 33 mm. However, a 33 mm lens does not exist. In this case, you should pick a 35 mm or a 30 mm.
Let's say that you want to buy a super-wide class lens. You can see from the table that for a camera with an FX-format sensor, you need to find a lens where the focal length is 24 mm or shorter. If your camera uses a DX-format sensor, you need to use a lens that is 16 mm or shorter for the same super-wide effect.
This means that a lens such as the Nikkor 14-24 mm will be clearly classified as super-wide for use on a camera with a FX-format sensor, but only marginally so for a camera with a DX-format sensor. If we use this Nikkor with an adaptor on a camera with a Four-Thirds sensor, it will be classified as wide-to-normal (not super-wide).
We can also see from the table that for a camera with a DX-format sensor, you would need a lens with a zoom range 9.3 mm to 16 mm to match the FOV of the Nikkor 14-24 mm when used on a FX-format sensor. Unfortunately, such a lens does not exist for the DX-format, so one must use something like the Sigma 10-20 mm instead. This lens, however, does not go as wide as 114° on the wide end. On a camera with DX-format sensor, f=10 mm gives a diagonal FOV=110°.
While there exists lenses with zoom ranges to goes from wide to tele (e.g. the Canon EF 28-300mm f/3.5-5.6 L IS USM and the Nikon 18-200mm f/3.5-5.6G IF-ED AF-S DX VR), the compromises that is inherent in such designs make many photographers build a collection of quality zooms that covers the range from super-wide (FOV ≈114°) to tele (FOV ≈12°) spread over three lenses. The table below shows the typical zoom lenses in such a collection. The table also shows the diagonal FOV at both ends of the range.
|Format||Crop f.||FOV||Super-wide zoom||Standard zoom||Tele zoom||FOV|
|FX:||1.0x||114°||14-24 mm||24-70 mm||70-200 mm||12.4°|
|DX:||1.5x||110°||10-20 mm||17-55 mm||50-135 mm||12.0°|
|Canon (1.6x):||1.6x||107°||10-22 mm||17-55 mm||50-135 mm||11.4°|
|Four-Thirds:||2.0x||114°||7-14 mm||14-35 mm||35-100 mm||12.4°|
Currently, users of cameras with FX-format sensors have access to original zooms with a maximum aperture of f/2.8 in the FOV-range from super-wide) to tele. For users of smaller sensors, third party lenses must be used to cover at least part of this range if one want wide or fast zooms.
While FOV is the most important parameter decided by the focal length, the focal length also influences the depth of field (DOF) at a given aperture. But DOF is much more complicated than FOV, so I skip the details. Briefly, a short focal length (small mm-number) gives you a deeper DOF than a long focal length (large mm-number). For a more comprehensive discussion of the relationship between focal length and DOF, see this webpage.
There are many notations in use for indicating the maximum aperture as part of the lens' label. Here is a list of examples: “f/1.4”, “1:1.4”, “F1.4”, or just “1.4”.
All the aperture labels above refers to the same aperture, and the number repeated in all of them (1.4) is often referred to as the “f-number”.
What this boils down to, is that the f-number is a numerical label for a lens' aperture. For more about the f-number and what it means, see our article: The Aperture and the F-number.
When looking at the aperture label for a zoom lens, you may find two different f-numbers. This means that the zoom has a design where the maximum aperture changes when the focal length changes. The two maximum apertures listed are the maximum aperture available at the zoom's lowest and highest focal length. For example the Nikkor 18-135mm f/3.5-5.6 will have a maximum aperture equal to f/3.5 when used with the zoom set to at 18 mm, and it will have a maximum aperture equal to f/5.6 when used with the zoom set to 135 mm. Not all zoom lenses list two aperture values. Some (usually more expensive) zoom lenses provide constant aperture over its entire zoom range. Then only a single maximum aperture is given.
The minimum aperture of a lens is never listed as part of the lens label, but can be found if you look up the manufacturers data sheet for the lens.
Macro – CFD magnification
A macro lens is a lens that is constructed to let you focus on objects very close to the camera. A major characteristic of a macro lens, is its CFD magnification. This tells us the ratio between the real life size of an object, and the projection of that object on to the sensor or film, when the lens is focused as its close focus distance (CFD). Sometimes this datum is referred to as maximum magnification.
The number indicating the lens' magnification is sometimes written as a ordinary fraction (e.g. 1:2) and sometimes a decimal fraction followed by an “X” (e.g. 0.5X). These two are equivalent and both refer to a magnification that is half life size.
A CFD magnification of 1:1 or 1.0X means that the projection of the object on the sensor will measure the same as it does in real life (i.e. life size). If you took a picture of a ruler at magnification 1:1, a 10 mm segment of the ruler would occupy exactly 10 mm on the digital sensor or film negative. A magnification of 1:5 or 0.2X means a fifth of life size, and a magnification of 3:1 or 3X will magnify the object to three times life size, and so on.
Most photographers do not classify a lens as a real macro lens unless it has a construction that is corrected to provide a flat field at close range, and that its CFD magnification is at least half life size. Most real macro lenses has a CFD magnification of life size.
Sigma and Tamron like to put the word “macro” in the long name of cheap zoom lenses capable of focusing fairly close. These lenses, however, are not corrected to provide a flat field, and instead of offering a magnification equal to 1:1 / 1.0X, they have a “macro mode” that can only be used at the long end of the zoom range. As a general rule, do not assume that the word “macro” in the name of a lens means anything. You must read a full description of such a lens to determine whether it is suitable for macro photography, or not. The DPanswers lens finder let you search for true macro lenses. The DPansers lens finder also let you look up the data for CFD and CFD magnification for many pseudo-macro zooms.
In the macro region (i.e. at and around a 1:1 magnification) the depth of field will be very shallow, and it will be determined by magnification, not focal length. In other words, there will be no difference between the depth of field of a 50 mm lens and a 150 mm lens if both are used at life size (1:1 or 1.0X) and both are set to the same aperture.
4. Filter Diameter
Most lenses take threaded round filters, where the front of lens has female threads, and the filter comes with a male thread.
The most common filter diameter sizes for circular filters for DSLR cameras are 46 mm, 49 mm, 52 mm, 55 mm, 58 mm, 62 mm, 67 mm, 72 mm, 77 mm, 82 mm, all with a pitch of 0.75 mm.
To find the filter diameter for a particular lens, you can look up the technical specifications for a particular lens in the brochure describing the lens, or on the manufacturer's web pages and look for the data on “filter diameter” or “filter size”.
In addition, the thread diameter for the filters that will fit a particular lens is usually marked on the lens' surface by the symbol “Ø”. The photo on the right shows the marking on a Nikon 17-55 mm lens that uses filters with a thread diameter equal to 77 mm.
Filters are expensive, and lenses come in different sizes with different front thread diameters. It is a costly option to buy filters that fit every lens you own. A cheaper alternative is to standardise and buy all filters in just one standard size (e.g. 77 mm), and then use stepping rings to fit them to different lenses. A stepping ring have one end (with male thread) to be mounted on the camera lens, and the other end (with female thread) is used for accepting a filter of a different size.
There are two types of stepping rings, step-up rings and step-down rings. Both types have two size markings in millimetre such as “52mm→58mm” (step-up ring), and “77mm→72mm” (step-down-ring). The directions is always from the camera lens. A step-up ring is used to mount a filter with a diameter larger than the camera lens' threads. A step-down ring let you use a filter smaller than the camera lens' threads. Note that a step-up ring may interfere with the use of a lens hood, and a step-down ring may cause vignetting.
- Thom Hogan:
- Thom Hogan: Acronyms
- Michael Hohner:
- Minolta AF/Sony Alpha F.A.Q.
- Lens Specifications Explained
- Ken Rockwell:
- Nikon Lens Technology
- Photography Acronyms