Exposure Compensation
Technique - Technique

EXPOSURE CORRECTION

Approved translation of the original (German) text from Marco Silbernagel. Some enhancements plus style changes by Klaus Schroiff.

Topics:

Exposure Value (EV)

The "Exposure Value" (EV) is the common unit to explain exposure differences so it's useful to understand its concept. In simple words: 1 EV difference is identical with either +-1 f-stop or the half or double shutter speed.

Example:
The camera meters something like 125s at f/8.
If you correct the setting by +1 EV this can mean 1/60s at f/8, 1/90s at f/6.7 or 1/125s at f/5.6 (based on half stop steps). At either setting the lens will transmit the double amount of light to the film. Furthermore 1/125s at f/8 has the same EV like 1/90s at f/9.5 or 1/60s at f/11. The amount of light transmitted to the film remains the same in all these three samples here.

Modern SLRs cameras offer at least possible corrections in 1EV steps, half steps are standard and third steps are a common option in pro cameras. Back in the old days when camera had no internal metering sensors a photographer had to use an external light meter which show also the absolute EV, like EV 6, to provide information about the light situation - EV 0 means 1 sec at f/1.0 here.

The 3 major Auto Exposure Programs:

  • Program (P) - The camera suggests a shutter speed/aperture combination based on the chosen focal length. Some of the smarter camera models allow to "shift" the program ( changes the setting to the next nearest shutter speed/aperture setting. The exposure value (EV) remains unchanged ). A "P" program without Shift mode cannot be used for more than just P&S - including Shift it is a good general purpose program.
  • Shutter Priority - This program is targeted for Action photography where you want to insure that you have a fixed shutter speed e.g. in order to prevent blurred images. The camera cares about an appropriate aperture setting here. The exposure value (EV) remains unchanged again.
  • Aperture Priority - This is usually used to control DOF ( Depth-of-Field ) - portraits may be a good application for this program. The camera automatically cares about the appropriate shutter speed. The exposure value (EV) remains untouched again here.

There's no uniform naming convention in regard to these programs. The "Program" mode is usually designated with a "P" but the sign for shutter and aperture priority differs between the manufacturers (T, Tv, S, A, Av etc. pp.). Look into the manual to find out the representation in your world.

The story behind exposure correction

The exposure meter of today's SLRs measure the amount of light (and sometimes also the color) reflected from a scene. This metering is usually done Through-the-Lens (TTL). This works pretty good for most of the time but there're several situations where the limit of the method is exceeded.
The metering sensor is calibrated to provide appropriate exposure settings for a scene which reflects light "like" 18% gray. What does that mean ? Have a look outside your window: e.g you may see some red cars which reflect light like 12% gray, the blue sky (5% gray), grand mother's doberman (80% gray), the next house with some white walls (0-5% gray) etc. pp. If you average all these values you'll quite often end up with a scene that reflects light around 18% gray. If this averaged value is different we have a problem. This is the reason for the various metering modes of a camera. e.g. multi-segment metering systems try to emphasize certain areas in the scene to overcome the 18% gray restriction - with often limited success. Via spot metering you can be the one who selects a typical part of the scene that resembles pretty much this gray value. Have a look at the following chart. All these colors reflect light like 18% gray:
 

It's quite amazing that these colors look so dark, doesn't it ? However, you have to be aware that there're usually lots of shadows in a scene so the average reflection rate is not as high as you "feel" it - the human brain is pretty smart in correcting light differences.
Please note that many metering sensors have color preferences - e.g they may receive red light in a different intensity than blue light. In the real world it usually doesn't make a big difference but you have to be careful in regard to color filters for B&W photography. For example: my EOS 5 is relatively blind for reds and using a red filter with this camera will end in underexposed pictures. This is no big problem though. Just meter a scene with and without filter and check the difference with the correction factor given by the filter manufacturer. Compensate the difference between these values and life is cool again.

If we've only an intergral metering sensor or we simply cannot use spot metering for whatever reason we run into problems in scenes with an average reflection rate which differs significantly from 18% gray - resulting in under- or overexposed pictures. This is where manual exposure correction enters the game.

Table with typical exposure correction directions for difficult light situations:

Typical under-exposed Scenes

[ many bright spots (>18% reflektions)]

Exposure correction direction
Contra-light or related light situations Plus (+)
dominant white or yellow areas 
Sunset/Sunrise

Typicalover-exposed Scenes
[ many dark spots (<18% reflektions)]
Exposure correction direction
Scenes with dark green like a forest Minus(-)
dominant shadows
dominant dark objects

Example: Object with many bright spots (> 18% Reflection)
GT1dunkel GT1gut
without exposure correction + 1 EV correction = longer exposure 

Example: dark main object(< 18% Reflection)
F4hell F4gut
without exposure correction - 1 EV correction = shorter exposure

Reflections of the metered main subject

                      more than 18%: under-exposure --> Plus (+)
                 •--- corrections ( snow pics, close ups of white/yellow  
                 |    subjects, contra-light, etc. )
                 | 
Reflection rate  |
  of the     ----•--- 18%: optimal exposure 
main object      |
                 |
                 |    less than 18%: over-exposure --> Minus (-)
                 •--- corrections ( dark greens (forest), shadows, etc.)

The right Amount of Exposure Correction

It's a bit tricky to choose the right amount of exposure correction - finally nothing else than experience will help here. The "severity" of the problem is also dependent on the chosen film type. Slide film is extremely sensitive, a difference +/-1 EV off the exact exposure is already over the edge for most situations here. Print film is much more tolerant. It (or to be precise: the lab) can easily compensate up to +-2 EV without any big impact on the final result. However, playing around with light moods is much more difficult - especially because the labs always try to compensate exposure variations.
Here some rules of the thumbs for a couple of difficult light situations based on a normal center-weighted metering system (does not apply for matrix or spot metering):

  • Bright scenes at a sunny day: 0 to +3 EV
  • Snow or shining water surface: +2/3 to +3 EV
  • Close-ups of bright subjects: +1/3 to +1 2/3 EV
  • Dawn: 0 to +2 EV
  • Landscape with lots of shadows or direct light:-2/3 to 0 EV
  • old steam trains (black): -1 1/3 to -2/3 EV

Several cameras show the decimal value of correction on an external LCD display and/or the viewfinder. These values are equivalent to the following correction factors in EV:

displayed values  -1 -0.7 -0.5 -0.3 0 +0.3 +0.5 +0.7 +1
corresponding EV -1 -2/3 -1/2 -1/3 0 +1/3 +1/2 +2/3 +1


No clue ? Try Exposure Bracketing

Usually the direction for the exposure correction is pretty easy but setting the right amount is often difficult. Just take a simple sunset - finding the exact setting requires some experience here and even then you may like to take some additional pictures with different EVs just to see which exposure shows the best mood.
This can be done manually or a bit faster and more convinient via auto exposure bracketing by the camera. "ABC" (Auto Bracketing Control), "AEB" (Auto Exposure Bracketing) is featured by many mid and upper class cameras. Usually you just need to activate this feature and set a certain shutter speed/aperture combination and your camera will automatically take 3 to 5 pictures with defined exposure variations.
There aren't too many magic moments out there and better invest in burning film via bracketing instead of wasting one shot with an incorrect exposure.

Paris1 Parisgut Paris3
-1 EV 
(under-exposure, here: skyline effect)
no correction applied +1 EV 
(over-exposure)


Available Light

 Night or available light situation usually suffer from extreme contrasts between dark and very bright spots (like street lights). These spot lights often confuse the metering sensor because it only takes an averaged sample of a certain picture area. A correct exposure is pretty tricky here and often quality comes only with experience.

Based on an ISO 100 film at f/2 the following shutter speeds may offer some rough rules for some available light situations.

Theater, football stadion (w/spot lights)................:  1/60s
Circus...................................................:  1/30s
Museum, good illminated expositons.......................:  1/15s
In-door shots (artifical light source), camp fire .......:  1/8s
Candle light, christmas tree.............................:  1/4s
weak illuminated town/street scene (at night)............:  2s
Snow landscape at full moon..............................:  10s
Normal landscape at full moon............................:  20s

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