Nikon DSLR Exposure
In photography, exposure is the total amount of light allowed to fall on the photographic medium (silver halide film or digital image sensor) for the time the shutter is open when we take a single photograph. Understanding exposure is the key to understanding photography.
In this note, we shall describe the types of exposure metering found in a modern Nikon DSLR. The note also discusses how the Nikon exposure metering system has evolved, and how to make best use of Nikon-specific modes and innovations such as 3D Matrix Metering and Active D-lighting.
The text below is not a general introduction to exposure. It deals specifically with the metering system and the exposure modes you'll find on a Nikon DSLR. If you are not familiar with the fundamentals of exposure, and how such elements as ISO, aperture and shutter speed work, I strongly suggest you first read this tutorial about the exposure triangle.
2. Metering Light
All exposure starts with metering the light. In order to expose a photograph correctly, we need to meter the total amount of light illuminating the scene we want to photograph.
The light on the scene may come from two sources:
- The Ambient light is continuous light such as natural light (sunlight), tungsten lamps or fluorescent lamps – as opposed to flash light.
- Flash light is a brief, intense burst of light made by a Nikon Speedlight or a similar flash unit.
For a scene lit by both ambient light and flash light, we need to meter both.
In all modern Nikon DSLRs (since the Nikon D200), both the ambient light and the flash light is metered by a RGB CCD sensor that is located on top of the prism, near the viewfinder (see drawing on the right).
The RGB CCD sensor makes separate measurements of the ambient light and the flash light. The ambient light is monitored continuously, while the flash light is measured by firing a low-power pre-flash just before the mirror is lifted and the flash light reflected by scene is measured in synchronisation with this pre-flash.
Note that this happens so quickly the human brain is not able to tell the pre-flash apart apart from the main flash. However, if you use FV lock, or set a 0.4 second exposure delay, the extra delay will allow you to notice the pre-flash as a separate event.
In the current generation of Nikon DSLRs, the RGB CCD is a matrix that consists of between 420 to 91000 segments, and can be switched between 3D-Colour Matrix, Centre-Weighted and Spot modes.
Evolution of TTL Flash Exposure Control
Before there was TTL (Through The Lens) flash exposure control, flash power output (which determine the exposure when flash is main light source) was either controlled manually or by a so-called “auto-thyristor” (a thyristor circuit inside the flash unit controlled by a light sensor on the flash that quenched the flash when the scene was adequately exposed). Today, the auto-thyristor flash control mode is sometimes referred to as “non-TTL auto”, because it provides automatic flash exposure control without making use of the camera's TTL capability.
In March 1980, with the Nikon F3 film SLR and Speedlight SB-12, Nikon introduced the TTL mode for automatic flash exposure control. As the name says the TTL-sensor that measures the light put out by the flash is metering through the lens. This is a big improvement compared to the auto-thyristor mode, because the sensor reading would be correct also when the flash was moved off-camera, placed inside soft-boxes, etc, where the reading from the sensor on the flash would be useless.
When metering TTL, there is a minor problem. During exposure, the mirror in a SLR is raised and the light path to the RGB CCD sensor on top of the prism is blocked. Because of this, Nikon fitted a secondary metering system at the bottom of the F3 mirror box for the sole purpose of metering and controlling TTL flash. There is schematic drawing of this secondary system to the right. In this system, flash light is metered by a dedicated five-segment TTL flash sensor and the metering is not influenced by the exposure mode or focus point selected on the camera, but always centre weighted.
This original system for TTL flash exposure control was just called “TTL” by Nikon. Because of the secondary sensor, it measured the flash light reflected by the scene in real-time during actual exposure OTF (Off The Film). And it issued, still in real-time, a quench signal to the flash when the logic of the system determined that the subject was correctly exposed. Here is a description of the sequence of operation for the original TTL system:
- The mirror raise.
- Shutter opens.
- Main flash, any wired Speedlights, and any remote flash units controlled by a SU-4 unit fires.
- Five segment TTL flash sensor monitors flash exposure off the film.
- Computer analysing data from sensor in real-time determines that the scene is correctly exposed and sends quench signal to the flash.
- Shutter closes.
- The mirror is lowered.
This system also supported wireless off-camera flash by means of a unit known as SU-4. The SU-4 had a manual mode where it was just a simple optical slave trigger that would trigger a remote flash whenever it saw a master flash fire. However, it also had a mode called “auto” where it would quench the remote flash when the main flash stopped firing. Since the TTL logic it operated in real-time, the in-camera computer could also take into account the light contributed to the scene by remote flashes during this time. The computer would issue the quench signal when the combined output of all Speedlights had resulted in correct exposure.
When digital replaced film, there was a problem. In the Nikon D1, introduced i 1999, there was no longer any film for the secondary sensor at the bottom of the mirror box to meter off. The shiny surface of a digital sensor turned out to be unsuitable for metering. Nikon's engineers solved this by giving up real-time metering and instead introduce a pre-flash that would fire before the shutter opened. The dull grey surface of the shutter curtain turned out to be perfect for metering off. The modified sequence of operation went like this:
- The mirror raise.
- On camera flash and any wired Speedlights fire a pre-flash.
- Five segment TTL flash sensor measures flash exposure off the shutter curtain.
- In-camera computer analyses this measurement and computes how long flash should last to provide correct exposure.
- Shutter opens.
- Main flash fires.
- After the time computed in step 4, in-camera computer sends quench signal to the flash(es).
- Shutter closes.
- The mirror is lowered.
Nikon called this modified system “D-TTL” (D for “Digital”). D-TTL is only found in the following Nikon DSLRs: D1, D1H, D1X, D2H, D2Hs, D2X, D2Xs and D100.
In many ways, D-TTL was a step back from the original system. The pre-flash made people blink, and introduced a delay that made the system less responsive. Also, the ability to control the power of remote flashes by means of the SU-4 auto mode was lost.D-TTL only lasted four years and Nikon only released five flash units supporting D-TTL: SB-28DX, SB-50DX, SB-80DX, SB-600 and SB-800 (the last two also support i-TTL). In 2003, with the Nikon D2H, Nikon made a major redesign of its flash system. Instead of using a dedicated five-segment secondary TTL flash sensor at the bottom of the mirror box to measure the pre-flash, Nikon moved this task if metering flash light TTL to the main multisegment RGB CCD exposure sensor located at top of the prism. The new system was called “i-TTL” (i for “intelligent”) and enabled Nikon to use its superior 3D-Colour Matrix metering for flash exposure as well.
With the introduction of i-TTL, Nikon re-introduced support for controlling remote Speedlights wirelessly, and the sequence of operation was modified again. Now it went like this:
- On camera flash and any active remote groups fire a pre-flash.
- Multisegment RGB CCD at top of prism measures flash exposure directly.
- In-camera computer analyses this measurement and computes flash power.
- The mirror raise.
- Shutter opens.
- Main flash sends a digitally coded signal to remotes, telling each group how long to fire.
- Main flash fires and remote groups fire for the pre-set times.
- Shutter closes.
- The mirror is lowered.
Nikon retained the five-segment sensor in the F6 film SLR and the entire D2-series of DSLRs. As a result, the F6 can be used with both TTL or i-TTL compatible Speedlights, and the D2-series can be used with both D-TTL and i-TTL compatible Speedlights. Newer Nikon DSLRs (starting with the D70 in 2004) are only compatible with i-TTL.
I used the original TTL-system that metered flash exposure in real time on my F90x for many years, and I found it to be excellent! Unfortunately, with digital cameras, real-time OTF metering is no longer technically feasible, so we have to rely on the use of pre-flash for flash exposure control through the lens. If you want to avoid the pre-flash and have real-time automatic measurement these days, Nikon's top-of-the-line Speedlights (SB-800, SB-900 and SB-910) still offers the good, old non-TTL Auto-thyristor mode. For simple lighting arrangements, with the flash on camera, I prefer this mode to i-TTL. For complex light, with multiple flashes off-camera, i-TTL is hard to beat.
I shall not say more about flash exposure control in this article. For an introduction to Nikon flash and Nikon's Creative Lighting System, see this article.
Evolution of Matrix Metering
Unlike Average, Centre Weighted, and Spot metering, you'll not find the Matrix Metering on non-Nikon cameras. Matrix Metering is a proprietary metering system that you'll only find on Nikon cameras. (However, Canon offers something the company calls Evaluative Metering that is somewhat similar.)
Today, Nikon uses Matrix Metering not only for automatic exposure control, but also to enhance AF (autofocus), AWB (auto white balance), and for CLS (Creative Light System) control.
Nikon's matrix metering with 5 monochrome sensors first appeared on the Nikon FA in 1983. The system has evolved considerably over time. The latest version uses an CCD array with around 91000 RGB sensors. It also takes light, colour, distance and active AF focus point into account.
The first version of Nikon's 3D-Colour Matrix Metering was incorporated in the Nikon F5 in 1996 with 1005 RGB sensors. When the Nikon D2-series was introduced in 2003, Nikon merged the metering systems for TTL flash exposure control and ambient exposure control and changed its name to 3D-Colour Matrix Metering II. In 2012, with the D4, Nikon upgraded the system again with a 91000 segment RGB CCD and face detection, and started referring to it as 3D-Colour Matrix Metering III. That's a mouthful, so I shall hereafter refer to it as 3DMM.http://imaging.nikon.com/lineup/microsite/d-technology/autofocus/01scene/index.htm
Here is the number of segments used for Matrix Metering in various cameras.
- FA, F4: 5.
- F70, F90x: 8.
- F100, D100: 10.
- F75: 25.
- D50, D40, D40x, D80, D90, D3000, D3100, D5000, D5100: 420.
- F5, F6, D70, D70s, D200, D300, D300s, D700, D1, D1h, D1x, D2h, D2x, D3, D3x D3s: 1005.
- D7000: 2016.
- D4: 91000 (approx.)
The schematic below shows the first generation 3DMM system introduced in the Nikon F5 in 1996. The 1005 segment RGB CCD that is the core of the system measures the brightness and colour at 1005 different points in the image. This information is fed into a scanning module that extracts average brightness, the contrast between the upper and lower parts of the image, the brightness of the selected focus area. This information is then processed by a computer that also takes into consideration colour information, the position of the selected focus area, and (provided you use a D or G-type lens) distance information. The end result of all this is what Nikon calls the «optimum exposure».
Note that only Nikon lenses designated D or G transmits distance information to the camera. For 3DMM to be most efficient, you need to use lenses of this type.
The quotes below, excerpted from Nikon's history is Nikon's own description of Matrix Metering and how it evolved.
The traditional center-weighted metering system was still popular for its consistent results, but there were some gripes. Backlight compensation, for example, required considerable guesswork as even experienced users made errors in judgement at times. With the Nikon FA's matrix metering system, however, the finder image was divided into five distinct metering areas. The ratio of light to dark is measured for each area individually and a microcomputer (CPU) analyzes data to calculate the optimal exposure. (Source: The development of the matrix metering system)
Thirteen (13) years after the introduction of Nikon FA, which incorporated the Matrix Metering system to provide proper exposure even in complicated lighting conditions, the exposure metering system in the Nikon F5 adopted the 1005 (-pixels) CCD metering elements with the world's first R (red), G (green) and B (blue) filters, which changed the concept of Matrix Metering systems. The metering system called “3D-Color Matrix Metering” was a revolutionary new technology, using the color tone of the subject as one of the factors for proper exposure in addition to the brightness and contrast, and the information about distance obtained from the lenses. This contributed to photographic images that more closely resembled what we see especially under fluorescent or tungsten lamp lighting, or in such conditions where yellow, green and blue account for the greater part of the projected area. (Source: Debut of Nikon F5)
The F6's 3D-Color Matrix Metering offers enhanced precision thanks largely to an improved scene-detecting algorithm. It analyzes numerous aspects of the scene conditions including brightness, contrast, selected focus area, subject-to-camera distance and color and compares them to the reference information of more than 30000 actual scenes in the database, ensuring super-precise exposure control and faithfully preserving the ambience of the scene. (Source: Nikon F6 2003 brochure)
After a process of steady evolution, 3D Color Matrix Metering II was first used on the D2x. (Source: Nikon's Scene Recognition microsite)
Technically, the Scene Recognition System uses color, brightness and other information obtained from the 1,005-pixel RGB sensor to analyze the subject prior to capture and applies the results to achieving greater accuracy in AF (autofocus), AE (auto exposure), and AWB (auto white balance) control. The 1,005-pixel RGB sensor that was first introduced in the F5 has been improved in the area of exposure accuracy, mainly as a light meter. By adding a diffraction grating between the prism and the lens for light metering, the 1,005-pixel RGB sensor is able to detect the color and brightness of the subject more accurately, significantly improving accuracy in AE and AWB. AF benefits as well, because the sensor even recognizes changes of the subject's position within the viewfinder. (Source: Nikon D3 2007 interview)
When a high-resolution 91 Kpx metering sensor was introduced in Nikon D4, Nikon added face recognition to the 3DMM, and upgraded the version number to roman numeral III. Face recognition is used to control on the brightness of that face so that the face is optimally exposed even with backlighting. The face recognition is also used to lock focus on human faces in Auto Area AF mode.
3. Metering Mode
All Nikon DSLRs have a built in exposure meter and a electronic analog exposure display at the bottom of the viewfinder.
You can see the result of built-in exposure meter's measurement of the ambient light in the electronic analog exposure display It may look something like the figure above (the electronic analog exposure display is highlighted in green).
In this particular example the analog exposure display extends to the right, and goes off the scale. This indicates that with the present settings, taking a photograph without providing more light will result in severe underexposure.
Nikon DSLRs have three exposure metring modes: 3D-Colour Matrix, Centre-Weighted and Spot.
3D-Color Matrix Metering
This is Nikon's most advanced exposure metering mode. How it actually works is still somewhat of a mystery.
From the official interviews cited above, we learn that 3DMM is multi-pattern metering system. The meter does not look at a single area or an average of areas, but instead samples many different points in a scene. In the F6 brochure quoted above, Nikon claims it works by comparing a rendering of the scene on a large array of RGB segments to a huge database of actual scenes. In the interview about the D3, also quoted above, Nikon's Hiroshi Takeuchi calls it a ”Scene Recognition System”. Here is Nikon's own description about how the Scene Recognition System is utilized to improve the accuracy of Auto White Balance (AWB):
For a conventional AWB system, for example, under a mercury vapor light white tinged with green and the green of vegetation would appear exactly the same. How could Nikon deal with this? When the subject is green vegetation, the light source is most often natural daylight. Nikon felt that if the light source could be identified, the camera could accurately make decisions whether or not to remove green (with mercury vapor lights), or let the green remain (with natural daylight). Using a simulator, we analyzed approximately 20,000 images of actual situations and constructed algorithms that would dramatically improve AWB accuracy by determining the light source. (Source: Nikon's Scene Recognition microsite)
I think the best way to approach Nikon's 3DMM is to think of it as an embedded zone system. It tries to compute the optimal exposure and dynamic range for a scene based on a complicated algorithm with input from its segment CCD, the selected focus point, and its scene recognition system and database. The system seems to factor out extremes of brightness and darkness, just as you would if you were personally evaluating the scene using the zone system.
Tests of 3DMM suggests that is tend to optimise exposure for in-focus objects near the active and central focus points, but that it also takes other factors into consideration, including colour and focus distance as reported by a D- or G-lens.
3DMM does not work well with filters. If you alter the scene brightness or contrast with filters, in particular neutral density filters, polarisers, graduated filters, infrared filters, and strongly coloured filters designed for B&W photography, then the 3DMM-logic may not make the best exposure decision. I suppose the main reason would be that the matrix metering decisions are based on a set of normal photographs, so if you do abnormal things with filters, then you are working outside it's range of knowledge of the prevailing conditions. Anyway, with matters like this you try a few comparison shots between 3DMM and centre-weighted or spot when you think you may have set up an abnormal condition and then compare the results.
Centre-weighted mode is best suited when your main subject fills the foreground and you do not care about the background. Using centre-weighted mode lets the meter put emphasis on the centre of the frame.
Unless you are using the zone system (when you'll use spot metering for everything), spot metering is for extreme lighting scenarios, such as stage or theatre work, where you encounter wild swings between the areas that are lit, and the areas that are not. Spot metering nails the exposure for a very small, key area of the photo, and has the advantage of being linked with the focus point. The highlighted focus area becomes the active spot for measuring exposure. Good for off-centre subjects.
4. Exposure Modes
To control exposure on the DSLR, you first need to select the mode you want to use. On Nikon's professional cameras, this is done by pressing the mode button and rotating the main command wheel. On Nikon's entry level and mid-range models cameras, you do this by means of the mode wheel. The mode wheel on the Nikon D80 is shown on the right. You will find a similar mode wheel on all Nikon's entry level and mid-range models. The mode wheel is divided in two parts.
The first part, from the nine o'clock position to twelve o'clock on the illustration to the right, is made up of the four standard exposure modes that you will find available on almost every modern camera. They are signified by the letters «PASM».
The second part of the mode wheel is made of point & shoot modes that you will not find on a professional camera. They are signified by small pictograms. On the D80 mode wheel shown above, they are (clockwise from the one o'clock position): Green Auto, Portrait, Landscape, Close up, Sports, Night landscape, and Night portrait. These modes are discussed in a separate note titled Nikon DSLR Point & Shoot Modes.
The standard exposure modes give you much more control of the
creative process than the point & shoot modes. I think most
photographers are best served by avoiding the Point & Shoot modes
altogether (just forget that they exist), and concentrate on
mastering the four standard exposure modes:
Programmed Auto (P)
On the surface, this mode is very similar to green Auto Point & Shoot mode. However, this mode has a lot more to offer than green Auto.
Like the green Auto, it leaves picking a suitable aperture and shutter speed to the camera. Unlike green Auto, it lets you use the camera's controls to override the aperture and shutter speed picked by the camera. And one of the most annoying features of green Auto, the automatic activation of the camera's built-in flash in low-light situations, is absent.
If you change one of the settings, the camera will automatically adjust the other to maintain constant exposure. E.g.: If the settings picket by the camera are f/2.0 and 1/60 second, doubling the shutter speed to 1/30 second will make the camera automatically change the aperture to f/2.8.
The situation where Programmed Auto really shines is when you are using flash in TTL BL mode. Provided the ambient is bright enough for this, Programmed Auto will pick a combination of aperture and shutter speed that will nicely balance the flash and the ambient.
Shutter Priority (S)
In this mode, you set the shutter speed and the camera picks the aperture that it thinks will give the best exposure.
You should use this mode when the shutter speed is important for the photograph, for example when shooting fast moving sports, or for blurring running water.
Aperture Priority (A)
In this mode, you set the aperture and the camera picks the shutter speed that it thinks will give the best exposure.
This is probably the exposure mode most photographers use most of the time. It allows the photographer to set the depth of field he or she wants, which for many is the most important characteristic of a photograph.
The only time Aperture Priority is really unsuitable is when working with flash as the main light in dark surroundings. Here, Aperture Priority may give you unacceptable long shutter times. In such a situation, you will probably prefer Manual exposure mode.
In this mode, you set both the shutter and aperture.
Note that the built-in electronic analog exposure display still operates and will tell you the difference between your manual settings and the exposure metered by the built-in light meter.
The manual mode is usually the best mode to use when you are using TTL flash. With TTL flash, the separate flash exposure meter will make sure that flash power is adjusted for correct exposure of the main subject, and you can control the exposure of the ambient by juggling shutter speed and aperture.
You may also want to turn to manual mode as an alternative to using exposure compensation to correct for errors in the camera's exposure meter's evaluation of a scene.
Nikon's exposure control system includes some special features that may assist you in controlling exposure and dynamic range. These are:
D-Lighting is a post-processing feature that is can be selected from the Retouching menu. It only works on JPEG images, and does not alter RAW data or affect the exposure.
What it does is to selectively lighten the dark levels of the photograph. This is sometimes useful in back-lit situations.
An alternative to using D-Lighting in-camera is to use similar features when post-processing images on a computer. You'll find a very similar D-Lighting function in Nikon's post-processing package Capture NX2, and the «Shadows/Highlights» sliders in Photoshop seems to be very similar.
Using Active D-Lighting
Active D-Lighting is a special Nikon image processing technology that was introduced with the Nikon D3. It is not available on the on older models.
Nikon has never stated what it actually does, but it seems to perform a 3DMM prior to exposure and in order to analyse the frame looking for blown highlights. If the logic determines that some of the highlights are burnt out, it reduces the exposure in order to salvage the highlights. This leads to an overall darker image as RAW data. When converting to JPEG, in-camera software and Nikon's Capture NX2 will automatically lighten shadow levels by applying a compensating tone-curve, probably by means of the ordinary D-Lighten feature. If you do do not use Nikon's own software, the Shadows-Highlight tool in Photoshop can be used to lighten the shadows in a very similar fashion.
ADL does not seem to increase exposure (i.e. it does not «expose to right» to reduce noise in the shadows when a scene is of sufficiently low contrast to allow this). In fact, because ADL tends to push the camera towards underexposure, the extended headroom in the highlights comes at the expense of increased noise in the shadows. If you are shooting at high ISO, your images will probably have more objectionable noise if ADL is turned on.
ADL is using all 16 (linear) bits, even to create 8-bit (log) JPGs. This requires more number-crunching (so it will take longer to empty the buffer) and halves the size of the buffer if you are shooting JPGs. For high speed shooting, you may want to deactivate ADL.
ADL is off by default. To turn it on or off, go to the Shooting Menu and pick one of settings are Off (default), Low, Normal and High. The difference between the various levels is mostly in the amount of lift applied to the shadows.
Auto ISO is available on all current Nikon DSLRs. It is one of the most useful features of Nikon's exposure system. Some people seem to dislike because of the word «Auto». An alternative name would be «Aperture and Shutter speed Priority», because it let you set the Aperture and Shutter speed limits you want, and then the camera will juggle the ISO to stay within those limits.
Here's an example of how you may use this feature: Imagine that you're walking down a path along a ridge on a sunny day photographing the landscape illuminated by the sun. The camera is set to ISO 200, an appropriate sensitivity setting for a bright day. All of sudden you look into the shady underbrush beside the path and see a small animal or something else something worth photographing. With the camera set to Auto-ISO, you simply frame and shoot. If the camera needs to run the ISO up to 6400 (whatever is needed to give a usable exposure), it will do so automatically, and you will be able to get the shot. Without Auto ISO enabled you need to take the time to judge what ISO setting might be required, to set it, and then to frame and shoot.
The idea behind Auto-ISO is to be able to get the shot when the light changes faster than you have time to change your ISO. In moderately low lighting conditions, I may set my base ISO on my D700 to 200 and my max ISO to 1600, use aperture priority set at f/4, and a minimum shutter of 1/80 second. As long as the light is good, I get ISO 200, f/4 and shutter speeds faster than 1/80 second. If the light fails, the camera will increase the ISO to preserve aperture f/4 and a minimum shutter speed of 1/80 second.
Using Exposure Compensation (EC)
Nikon's exposure metering usually do a good job in determining the correct exposure, but it is not 100% – sometimes it misses. The exposure meter is just a machine. If you leave the final decision to the meter, a bright background may silhouette people in the foreground, or a groom in a dark suit may fool the meter into burning out the bride's white dress. Sometimes, you need to override the meter to nail the exposure. You can, of course, do this by switching over to fully Manual mode, but most photographers prefer to use exposure compensation to correct the exposure in the cases where the camera's automatic exposure meter gets the exposure wrong.
Setting the exposure compensation to a positive or negative value biases the camera's metering with the specified amount.
The unit used for exposure compensation is EV (Exposure Value). One EV equals one stop. To indicate an exposure compensation adjustment of a magnitude of a third stop darker, one would dial in -0.3 EV.
On most cameras, the button to press to set exposure compensation is located next to the shutter button and marked with a «+/-»-symbol. With some practice, you should be able to slide your index finger over to it without taking your eye from the viewfinder, and deal in the required EV (exposure value) in compensation with the main command wheel your thumb. You can see the EV amount set directly in electronic analog exposure display at the bottom of the viewfinder.
The best guide to how much exposure compensation to apply is the blinking highlights display and the histogram. Both are available on the rear LCD. If the specular highlights in your image is blinking, this is probably OK. That means your specular highlights are pure white. But if a large portion of the frame, say the entire sky area, is blinking, you may have a problem. However. it is not always possible to capture the full dynamic range of scene in a single exposure. If you are shooting a city street in shadow against a southern sky, the sky will probably burn out unless you are willing to sacrifice details in the shadows.
Nikon says that exposure compensation works better in the modes other than with 3DMM: «It is most effective when used with centre-weighted or spot metering.» (p. 116 of the D300 manual). What they really mean to say is that 3DMM is too complex to predict, so you don't know the amount of exposure compensation to dial in beforehand. I often use spot metering to establish the amount of compensation to use. Once you get the compensation right, 3DMM works perfectly with it.
You only use exposure compensation in one of the “automatic” modes (P, S, A, auto, or vari-modes) and its purpose is override the automatic metering without going to full manual. In M, you don't use exposure compensation, you just change the shutter speed (or aperture or ISO) to correct the exposure.
Learning to set the correct amount of exposure compensation comes with experience. In centre-weighted mode you can train yourself into just looking at a scene and before making an exposure, you know exactly the amount of exposure compensation to apply. Skilled photographers also use exposure compensation with film - when there are no screens at all to look at.
Using Exposure Bracketing
Bracketing means that you set up the camera to take several pictures, with a setting (e.g. exposure) that is automatically changed from picture to picture.
This function is available on all Nikon's mid-level and professional cameras, but as far as I know, only the D50 and D5000 entry level models have this function built-in.
You use bracketing to get you several pictures of the same scene with different settings. This let you pick the shot with the best exposure. Or you can use it to combine several shots taken with different exposure into a composite with higher dynamic range.
The following bracketing options can be selected through the Custom Settings menu:
- AE & Flash: Camera varies flash level and exposure with each shot.
- AE only: Camera varies exposure with each shot.
- Flash only: Camera varies flash level with each shot.
- WB bracketing: Camera creates multiple white balance conversions from a single shot (not available when shooting RAW).
After selecting what you want to bracket in the Custom Settings menu, you activate the bracketing function. On some bodies (e.g. the D80 and D90) you do this by holding down the BKT button located on the left hand side of the lens, just under the flash release button, while using the main command wheel (rear) to select the number of frames per bracket, and the sub-command wheel (front) to select the steps (in EV if you are doing exposure bracketing) between the pictures in the set. If you select three frames and AE only bracketing, you will get pictures with -2 EV, 0, and +2 EV in steps of 2 EV, and -1/3 EV, 0, and +1/3 EV in steps of 1/3 EV.
Now press the Shutter Mode button (on the D80 and D90, it is on the right hand side of the top panel, next to the AF button) until you see the continuous shooting icon at the top right of your screen.
Finally, set camera to shutter (S) or aperture priority (A). Just keep your finger on the shutter and you will take 3 shots with different exposures. (If you don't set continuous shooting, the camera will still bracket. It that case, you press the shutter three times to get three shots with different exposures.)
6. Final Word
Be aware that light coming back in through the eyepiece also gets to the RGB CCD sensor at the top of the prism. Stray light coming through the eyepiece may confuse the sensor and cause underexposure. This usually happens when you are taking tripod shots and your eye is not blocking the light from getting in. It can be avoided by blocking the eyepiece with the little plastic clip provided or with the eyepiece shutter. What means to use depends on the camera model.
You can also avoid underexposure by using manual mode. Set the aperture/shutter while the eye is at the eyepiece, blocking stray light from entering the prism. When you move your head away it no longer matters as the settings are already fixed when you use manual mode..