Saturday, August 22, 2009

Should you expose for shadows or highlights?

The short answer is that it is usually recommended that for digital photography you expose for the highlights as they get easily blown out and for film photography it is easier to lose detail in the shadows and easier to bring back detail in the highlights and therefore you need to expose for the shadows.
The more complicated answer is that it depends on what you want to do, what you are visualizing.

We all know that a strongly overexposed part of the photo turns to white and a very underexposed part of the photo turns to black, cutting off any texture that might have been hidden in those areas. Film and sensors do have a limited ability to record light. It can only capture light between specific intensities and these intensities are defined by the film or sensor you are using, this is the so called dynamic range.

Exposing for shadows

Make sure that the histogram is not cut off on the left side. Use your camera options to overexpose, if needed. You will have a lot of detail in he dark areas of your photo but you might have blown out the highlights.

Exposing for highlights

Make sure that the histogram is not cut off on the right side. Use your camera options to underexpose, if needed. You will have detail in the bright areas, but the darker areas will be clipped to black.

Exposing for the mid-tones

There is, of course, always the alternative to ignore the highlights and the shadows and just expose for the mid-tones. The drawback is also the benefit: clipping will occur on both ends of the histogram, but not as severely as when you expose for highlights or shadows.


This leads nicely to a discussion of HDRI and it is maybe a good idea to revisit the section about bracketing.

Saturday, August 15, 2009


The classical solution to improve your chances of getting the exposure correct has been bracketing: Take multiple shots with different settings to get different exposures of the same scene. Typically you take one shot exposed normally followed by one or more overexposed and one or more underexposed shots. Check your camera manual and it is very likely that your camera offers an auto-bracket option for exposure where you can set the number (usually three or five), order and step interval (1, 1/2 or 1/3 stops). My Pentax K20D has a special exposure bracket button on the outside of the camera to quickly switch between bracketing and non-bracketing, no doubt the other brands have similar functionality to avoid going through long setup menus.

Bracketing has always enjoyed a questionable reputation as it wastes a lot of film when used with film cameras and because of the randomness of the approach. True, without taking steps to correct the lightmeter, you are in fact taking several potshots to see if you can obtain a good result. However, when you take the prize of a correct exposure of a difficult shot, nobody will poke fun of you but applaud you for being careful.
On a Digital SLR, the waste of images is of course less of an issue, especially when you take the quickly dropping prices of memory into account (and you're not on a long trip with just the one memory card.) 

Let it be clear that you don't need bracketing unless you are confronted with a difficult subject. Still, no matter how easy bracketing sounds, you still need to make decisions about the number of exposure and the step intervals. You'll need to learn to judge the subject to make those decisions, but usually three exposures with steps of 1/2 a stop is a save bet. We'll be getting back about how to judge subjects.

Friday, August 07, 2009

Why do lightmeters fail?

To understand why light meters fail, we need to understand what light meters do: No matter if it is a through-the-lens (TTL) meter as found on compacts and (D)SLRs or a handheld meter, they all try to reduce the scene to a neutral tone. This tone is often called "18% gray" but the ANSI standard against which the light meters are calibrated is a luminance value that is closer to the reflectance of 12% gray. 12% gray or 18% gray, we don't really care at this point, but what I would like you to bring back from this is that light meters generate those settings that will render the dominant part of your photo into a neutral colour. It may come as a surprise, but this will do nicely for most photos. It is not until you take a photo of a scene where this doesn't apply that you start to run into problems.

Ever tried to take a portrait on a sunny day where the face came out too dark? If your camera allows it, you could walk up to the subject, take a light reading and lock it, walk back to get the composition you want and the face will be well exposed. By this time it is also be a good idea to see if your camera supports spotmetering.

Saturday, August 01, 2009

Exposure and why do we care?

You bought an expensive (D)SLR with a state of the art through-the-lens (TTL) light meter operating in different modes, so why should you ever want to start thinking about exposure again? Surely those days have gone as it is now so many years after TTL light meters were first introduced? Haven't any bugs been ironed out by now? And I believe you that that advanced weighted multi-segment mode is state of the art.

The truth is that for most shots on all cameras, the built-in light meter does an excellent job that would be difficult to improve upon. However, if you start using your camera more and more, you'll notice that some shots are too dark or too light. The highlights are all blown out, or there is no detail or too much noise in the shadows which may result in a disappointment. This is not a malfunction of the light meter, it does what it is meant to do but recognizing those situations and improving those exposures is one of the subjects of this blog.