Exposure latitude

A measure of a film's performance and a problem for digital sensors.

The range between the lightest and darkest areas of an image is called its Dynamic Range. Also known as Tonal Range. A picture containing very bright areas and very dark areas has a "wide" dynamic range. In a black and white image, dynamic range refers to the various shades of gray between solid black and absolute white.

The dynamic range - the range of brightness, including shadow detail, that a film or digital sensor can record in a single image before the highlights wash out or the shadows become muddy - is that film's or sensor's exposure latitude.

Exposure latitude is defined in ƒ-stops, and varies according to the type of film more than to individual brands. For instance, most negative films (regardless of brand name) have an exposure latitude of five to seven stops, whereas most transparency (slide) films have less exposure latitude — in the range of three to five ƒ-stops.

Unfortunately, there is almost no exposure latitude when shooting digitally. (Scroll down to read "What about digital photography and exposure latitude.")

EXPOSURE LATITUDE CAN COVER EXPOSURE MISTAKES

You could say that exposure latitude provides the photographer with a margin of safety to cover mistakes in exposure setting. A film's exposure latitude determines how much it can be under-exposed or over-exposed and still produce usable images. If proper exposure calls for an aperture setting of ƒ/8 at a given film speed, you will still get an acceptable exposure at either ƒ/4, ƒ/5.6, ƒ/11 or ƒ/16 with most color negative films.

USE FAST BLACK & WHITE FILM FOR THE MOST-FORGIVING EXPOSURE LATITUDE

Black and white films generally have more exposure latitude than color film. Exposure latitude also increases with film speed. If your light meter conks out and lighting conditions make it difficult to guess at an acceptable exposure, your best bet to get an acceptable exposure (in terms of exposure latitude) is a fast black and white film — something around ISO 400. You could use an even faster black and white film, but your results may be grainier than you would like.

TEST YOUR FAVORITE FILM'S EXPOSURE LATITUDE

It is a good idea, particularly in studio photography, to test a particular film to learn its exact latitude, especially if you plan to make it your film of choice for most of your shooting.

You will then be able to precisely match your lighting adjustments with the film's latitude to capture detail in both shadow and highlight areas.

BRIGHT SUNLIT SCENES ARE THE REAL TEST OF A FILM'S LATITUDE

High contrast scenes that are lit by bright sun and that have deep shadow areas nearly always contain a greater dynamic range than most film can handle.

The amount that you can vary from the ideal exposure and still have an acceptable picture depends on the film's exposure latitude.

WHAT ABOUT DIGITAL PHOTOGRAPHY AND EXPOSURE LATITUDE?

Digital photographers will find that their image's highlights are often overexposed, a common problem where there is an object in much brighter light than the overall scene. That brighter object will have little or no detail, sometimes ruining the image.

Digital cameras have little to no tolerance beyond proper exposure, perhaps a third of a stop at most. (A stop is one full change in aperture, say ƒ/4 to ƒ/5.6, or shutter speed, such as 1/60 second to 1/125 sec), after which detail is lost due to overexposure or underexposure. Compare that with some black and white films, where exposure can vary by five to ten stops among objects in a scene and still achieve detail in them.

What can you do about digital's limited exposure latitude?

When you are shooting a scene that has high contrast or that has a particularly bright object in darker surroundings, you can usually avoid loss of detail in the brighter objects (the highlights) by purposely underexposing the overall scene.

High end digital cameras may do this for you, automatically darkening the overall scene when they recognize that it contains a very bright area. Some photographers think their camera has made an error when this occurs, underexposing the picture. They don't realize that the camera is trying to properly expose the highlight and needs to darken everything else to do so.

When a highlight is overexposed and therefore contains no detail, it can't be recovered, but underexposed (darkened) shadow areas can be salvaged. Because there will be detail throughout all of the image, the shadow areas can be brightened later on your computer, using image-editing software like Adobe PhotoShop, to achieve the balance needed.

A particularly helpful PhotoShop tool is the Shadow/Highlight tool, with which you can incrementally brighten shadow detail without significantly affecting highlights.

When you shoot in RAW format, your images will usually have more detail to edit in very high contrast scenes than when you shoot in JPEG format.

Another solution is to move in close to your subject to take a light meter reading of the brighter highlight area, then back up to take the picture using the exposure settings for properly-exposed highlights. In effect, this is a more reliable way of underexposing the overall scene to ensure that the highlight areas aren't washed out. Shadow areas will be darker, and may need brightening on your computer to show adequate detail.

If your camera has spot metering capability, use it to take an exposure reading from the object that is brighter than the rest of the scene. That object will then be properly exposed, but other areas of the scene will be darker than you might like. Bracketing exposures will provide you with a choice of differently-exposed pictures.

A BETTER SOLUTION

The best results when shooting a high contrast scene (digitally or using film) are achieved by altering the scene's lighting ratio before you take the picture so that there is more light to reveal detail in shadow areas or less light on the highlights so they won't be blown out.

Diffusers, reflectors, light blockers and fill flash can be used to change the lighting in specific areas of the scene so that overall exposure is acceptable.

Taking a well-illuminated photograph in the first place is a far better solution than relying on software after the fact to correct exposure latitude problems in an image.