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Getting the correct exposure.


 

For the most part, cameras do a pretty good job at getting the right exposure. However, they can be fooled, and you may have to take over control of the exposure to get it correct. Whether you are at the beach in the Caribbean, or on a snow-packed glacier in Alaska, or anywhere in-between, you may encounter some difficulty when you are on a cruise or vacation.

The problem. Cameras use a really simple method to determine the correct exposure; they try to resolve the exposure so that the average lighting would be 18% grey. You may have heard of a 18% grey card, and it's sole purpose is to allow the lighting to be adjusted properly. For example, if you point your camera at 18% grey card, the camera will resolve the exposure so that the result is also 18% grey. As you go through different lighting conditions, the exposure will vary to keep the photo the same 18% grey.

Here is a short video to show how cameras can be fooled:


White side of card

Black side of card
If we intentionally over-expose the white side of the card by +2 stops, or under-expose the black side card by -2 stops, we get the correct exposure.

White side of card overexposed +2 stops

Black side of card underexposed -2 stops

Realize that this only works when the scene is predomanently white such as the beach or on snow, or black, such as at night.


How to obtain the correct exposure

Exposure Compensation: Most cameras, even fully automatic compact cameras, have an "exposure compensation" control. Exposure compensation is a method of telling your camera to add or subtract (offset) a certain amount of exposure for whatever the camera decides is the correct exposure. This control may be a dial, combination of dial and button, or menu selection.

Exposure compensation generally works in Auto mode as well as Program, Aperture Priority and Shutter Priority modes in your camera if you have those featuers. However, you won't normally use exposure compensation in Manual mode.

So how do you know how much exposure compensation to dial in? Unfortunately the histogram is of little use here as it already thinks the exposure is OK.

The best solution is trial-and-error. Generally you can consider 2 stops the maximum one way or the other (+2 for white scenes, -2 for black scenes). You can dial the exposure compensation in increments and look at the results.

Or if your camera has exposure bracketing, you can take a series of photos at different exposures and pick the best.

Realize though that with exposure compensation, you cannot just set it and forget it. Anytime you go from the snow, beach, etc. to a "normal" scene, then the exposure compensation will work against you, and you will likely have to reset it back to zero offset.

For those cameras that do not have exposure compensation, your only recourse is to use a "scene mode" if your camera has one, and set the mode to "snow" or "beach" for scenes with a lot of white in them, or "night" mode for those scenes with a lot of black.

Manual Mode: If you are shooting in manual mode, you probably already know what to do. But if not, simply adjust the exposure settings (aperture, shutter speed, or ISO) so that the metering system on your camera reads either over or under exposure.

As shown at the right, the exposure metering system (bar graph) is pointing at "0", which indicates a correct exposure. If the aperture/shutter speed/ISO is changed, the pointer will go to the left when the scene is underexposed, or right if it becomes overexposed. Remember, the camera thinks the exposure is correct, but you are over-riding that assumption.

Exposure Bracketing: A good method, especially if you are not sure what to set the exposure, is to use exposure bracketing - if your camera supports this feature. Typically, there are two settings required for exposure bracketing; the number of photos in the bracket, and the exposure compensation for each. For example; you may be able to setup your camera to take 5 photos, with each photo 1 stop apart.


Bracket -2 EV

Bracket -1 EV

Bracket 0 EV

Bracket +1 EV

Bracket +2 EV

Generally you must depress the shutter 5 times, one for each photo. You simply pick which photo that seems the most natural to you.

And some cameras automatically reset bracketing after the set, but other cameras keep exposure bracketing active. So make sure you return your camera to normal operation, or you may have a few unfortunate surprises when you review your photos.

Spot Metering: Advanced cameras have different metering modes; Matrix, Center-Weighted, and Spot. With Matrix mode, the camera evaluates the entire scene when making a decision for the correct exposure. In Center-Weighted, the camera assigns more signficance (or weighting) to the center of the photo (and you can change the area size with some cameras). And Spot metering uses the selected focus point and only meters that point. Typically the camera must be set to Spot Focus mode for Spot metering to work, but check your manual to be sure.

In the photo to the right, the camera's viewfinder shows the spot the metering is being taken from (Red box). The metering will ignore all of the areas outside of this box. This ensures the hillside is correctly exposed.

Typically in Spot metering, you select the focus point - and metering point with a selector on the camera. You can typically chose any point you wish, as shown to the right. Different cameras have differnt focus points, so your camera viewfinder may or may not look like the one shown.


This photo was metered using matrix metering, which averaged the foreground with the background. This resulted in under-exposure of the foreground.

The camera was put into Spot metering mode, and the center metering point was selected, which was over the fuel pump, properly exposing it.

Recomposing the photo: This technique involves positioning the camera so that it "reads" your desired exposure, then moving the camera to achieve the proper composition. This can be useful when there is a lot of contrast - such as a bright sky vs dark water, and you do not want the camera to read the bright sky, which can over-exposure the photo.


Compose the shot for proper exposure

then recompose the shot for proper composition

In order to achieve this, your camera must have some kind of auto-exposure lock (AEL). This button - when depressed - locks the current exposure reading.

To use this function, you would position the camera to obtain the reading (you may have to depress the shutter half way down, then depress and hold the AEL button. Then while you are still holding the button, reposition the camera to recompose the scene, then depress the shutter to take the photo.

The explicit action required to use the AEL is dependant on your model camera (for instance, you may not have to keep the AEL button held down). so consult your manual.

Some cameras also allow programming the shutter button, so that when you depress the shutter half-way down, the exposure is read and held. This is a bit simpler than using an AEL button as you simply have to point the camera to the area you desire to take the reading, depress the shutter half way, recompose the shot, then depress the shutter fully down to take the photo.

There is a disadvantage to doing this though. If you are taking sports and action photos, you typically set your camera to continuous focus, so that when holding the shutter button half-way down, you pan the camera to match the movement of your subject. If AEL is not coupled to the shutter button, then it will constantly update the exposure as you pan your subject. However, if you program the shutter button to AEL, then the exposure might change as you pan your camera, and it will not update any exposure difference.

Fortunately, these are advanced functions normally only found on DSLRs, so anyone with a compact camera can simply use exposure compensation.

Grey Card: Some photographers carry a "Grey Card" and place it in front of the camera's lens to meter the exposure. This will ensure the exposure is correct for any lighting situation - even if the scene is predomantely white or black. After the metering is recorded, remove the grey card and take the photo.

Even though the grey card does not take up a lot of space in your camera bag, you may find that it is just as easy when on vacation to experiment with different exposure compensation values.

Front-lighting: One of the most common occurances for incorrect exposure is "front-lighting"; when the primary light source is in front of you. This is a bit different than the camera error due to white and black scenes, but rather it has to do with high contrast situations, and is especially noticeable in a strong daylight condition.


Face is underexposed because the camera metered the sky for proper exposure.

Metering on the face results in a properly exposed face, but over-exposed sky.

As you can see here, neither photo is good. The photo could be improved by using a reflector, however, carrying a bunch of photo gear is not always possible or desired on vactation. Therefore, always avoid high contrast situaitons (bright sky-shadows) as well as shooting into the light source.

Position your subject so that the entire scene is in the shade if possible, or position subject so that there are no shadows, and position yourself so that the sun is at your back. You can also try a polarizer filter when you have strong sky light, as it can reduce the contrast somewhat.

Fill Flash: When high contrast situations are unavoidable, fill flash can sometimes be used - provided your subject is within range. Fill flash is typically used in daylight conditions when "help" is needed for balancing the contrast. Fill flash typically requires an external flash, or at least a camera that can set the flash exposure compensation independantly from the cameras exposure.


This photo was metered for the interior area of the bridge, which resulted in over-exppsure of the outside.

This photo had it's exposure set for the outside, which resulted in under-exposing the interior.

In this scenario, neither photo is correct, although in each case, the camera did meter the designated point correctly. The problem is that cameras just do not have enough dynamic range to capture the entire range of light values.

By using a flash, a technique known as fill-flash is used. The camera and flash are both put into manual exposure. The camera is metered for proper exposure of the exterior, and the flash is metered for proper exposure of the interior. Since the flash has a limited distance of only a few feet, it will not interfere with the distant exterior exposure. The result is in a properly exposed photo. In a sense, the flash "filled in" the interior; hence the term "fill flash".

In essence, you set exposure on the camera for the background, and you set exposure on the flash for the foreground.

(Note: depending on your camera and flash, you may not need to go into manual mode. Experimentation is the key).

Summary: You have been given several different methods for obtaining proper exposure when your camera is tricked into mis-behaving. Which one you use is highly dependant on your camera's capabilities and adjustments. However, you should at least know the basics of why a camera sometimes makes exposure mistakes, and you should be able to compensate for them.