Tips On Exposure...

[Chuck Gardner]

A digital camera histogram, overexposure warning and LCD playback are all valuable tools for evaluating exposure. Making sense out of them is simply a matter of understanding how they work.

A digital sensor is like a bunch of buckets which hold photons instead of water. Some in the brightest highlights get filled up fast like with a fire hose, but at the same time others down in the deep shadows are getting filled with like an eye-dropper. And oh yeah, there's aways dirt on bottom of the bucket -- that rainbow "noise" you see it the dark shadows until the "water level" or brightness is high enough to cover it.

Cameras store brightness level information numerically as a value from 0 (no light - black) to 255 (full bucket- white no detail ). Meter sensors in the camera read the light during the exposure and close the shutter when they detect the first bucket fills up completely. The problem is that a 6 megapixel camera has 6 million buckets and the camera can't watch all of them. So some the meter isn't watching can fill to overflowing, "nuking" all the detail out of the highlights.

Most digitial camera have some sort of overexposure warning in the playback which will black-out or flash in areas that are overexposed. When that happens on a automatic camera the solution is the use Exposure Compenstation which overrides the default metering. Somewhere on the camera back or menus there a button marked +/- or labeled "EC" or "Exposure Comp". If the playback of the first shot is overexposed then to correct it you'd dial in + compenstation.

The most critical area to get correct are highlights with texture, such as a white shirt. When shooting around water the specular reflections can fool the camera metering into thinking the scene is brighter than it actually is. When the camera sees more light it closes the shutter quicker, resulting in underexposure.

Situations like backlighting will also fool the meter in the other direction into closing the shutter too soon and underexposing the foreground. The simple solution is to just leave the flash on all the time outdoors, even at high noon. With the flash on most cameras nowadays will do a pretty good job of balancing the strong backlight and flash. If you have a DSLR it will probably have separate compensation adjustments for flash and ambient labeled EC and FEC (Flash Exposure Compenstation). Use EC to make the background lighter / darker and FEC to adjust the flash intensity and foreground lighter / darker.

For shots where the subject is too far away for flash to be effective the best solution is to change the camera position so the subject is in the sunlight but the background is dark. That will help the camera meter the scene more accurately and make your subject contrast more.

Digital cameras also have a graph called a "histogram" which shows how much of each intensity level of light the camera sees. The horizontal scale represents 255 different levels of light from black ---> white The vertical scale indicates how much of each brightness level there was.

You really only need to be concerned with where the right side of the "bell" curve is hitting the baseline. That right part of the curve is telling you how the brightest things in the scene are being recorded by the sensor. If the photo is overexposed the curve will be running off the right side of the histogram window. If the file is underexposed there will be a big gap between the right base of the curve and right side of the histogram. When the exposure is perfect the right base of the curve will just kiss the right side of the histogram window.

When shooting portraits I have the subject hold a white textured terry wash cloth. It creates a nice big spike on the histogram that is easy to see. Getting good exposure is then just a matter of adjusting the EC until the spike caused by the wash cloth kisses the right edge. Get the white rag correct and everything else will be too...

When in doubt its better to underexpose slightly than overexpose. Its easy to lighten a dark photo but impossible to put detail back into highlights which were blown when captured.

CG

[jdmidwest]

Great article on exposure. I have been playing with my Pentax ist DS since April trying to figure out all of the features. The exposure settings are more complicated than old film SLR that I owned. I did a shoot at a local airshow and found that the black planes were way underexposed. I did a little research and found the light was being metered in the center and not on the whole scene. The Alaska trip was rain and clouds the first 5 days, most of the mountains and glaciers were really grainy. I was able to get a few good shots out of a moving car and the plane.

The nice thing about digital is you can see the shot after it takes and can make adjustments, something you could not do with film. And Photoshop will correct some mistakes later if you don't catch them in the field. And, you can always delete the bad shots without paying for the print.