over or under expose?

I have an Olympus SP 350 and a YS -27 strobe. At times I have problems with exposure lighting. I am looking for information about which is better to do, OVERexpose or UNDERexpose on purpose, if I can't get the correct exposure factors. The camera histogram rarely fills the graph and it is usually way in the bottom of the graph.

Thanks to anyone who can help me until I grow up to a DSLR.

Thom

Hello Thom and welcome,

Back in the early days of digital, many people's mantra was "overexposure is death, always underexpose by 2/3 of a stop just in case."

Well, we have found that is just plain wrong! :-)

The best thing to do is expose your photo so the histogram is just about but not quite touching on the right. Then you can adjust the blackpoint and brightness or exposure to make it look right. If you do that you will get the most saturate colors you can get and the least noise.

Cheers

James

I have an Olympus SP 350 and a YS -27 strobe. At times I have problems with exposure lighting. I am looking for information about which is better to do, OVERexpose or UNDERexpose on purpose, if I can't get the correct exposure factors. The camera histogram rarely fills the graph and it is usually way in the bottom of the graph.

Thanks to anyone who can help me until I grow up to a DSLR.

Thom

Let's be clear here about definitions. An under-exposed image is relatively hard to define in absolute terms, as it is subjective. A good working definition is any substantial measure on the left-most edge of the histogram. Over-exposed is a little easier to define -- the presence of blown highlights (substantial measure on the right side of the histogram). Note that by these definitions, it is possible to both over- and under-expose the same image! See: HDR photography. If you have to choose, I would avoid blown highlights (keep the histogram from falling off the right side).

I am assuming you know this, and that you should strive to always have your "image data" reasonably within the range of "not falling off of the left, and not falling off of the right" of the histogram, and you want to know "should I aim to the right or the left?"

The standard answer is, "shoot to the right", meaning expose so that your histogram just misses, or barely touches, the right (highlight) side. The reason for this is simple. Sensor noise -- those colored flecks/dots/blotches/streaks -- are caused by thermally excited electrons activating the sensor and being recorded effectively like "random light". This effect is linear, affecting shadow and highlight areas equally. In shadow areas, on the "left" side of the histogram, proportionately fewer "actual" light photons hit the sensor (that's why it's a shadow, it's dark, right?), so that the signal to noise ratio in shadow areas is lower -- often MUCH lower -- than in highlight areas.

Yes, I'm a technical junkie with a physics background. I hope you're still with me. Here is a practical example you can do in the comfort of your own home.

- Get yourself a medium-tone, monochome target (a "gray card" is perfect, but you can use anything with a reasonably similar tone -- construction paper, a neutral-colored sheet, just something that is a single color, and is not too dark or too bright)

- Set our camera at ISO 1600 (or some high number, we want to emphasize noise here)

- Take an exposure with plain-vanilla in-camera metering (let's call this "exposure 1"). You should get a very narrow histogram that is pretty much dead center, if your camera is working correctly.

- Now, adjust your exposure compensation so that your histogram is almost to the left side. Call this "exposure 2".

- Then, adjust your exposure compensation so that your histogram is almost on the right side. Call this "exposure 3".

Note: By the definitions stated above, none of these three are "over-exposed" or "under-exposed" (the histogram is not off the edges). Others will quibble about these definitions, but let's go with them for now.

Now:

- Take exposure 2 in your favorite photo editor and increase the exposure to match exposure 1's histogram.

- Likewise, take exposure 3 and decrease the exposure to match exposure 1's histogram.

In an ideal world, all three of these should be identical. If the ISO is low enough, it will pretty much be that way. However, with high ISO, the modified exposure 3 should have the least noise, followed by exposure 1; exposure 2 should have a lot more noise.

Does this make sense?

Regards,

Dan

Oh, one more thing -- always shoot RAW. If you haven't made this jump, do it. It's the single most important thing to do, to improve the quality of your images.

Dan

James is, of course, right: but what does it mean in practical terms? I've found that with a DSLR and "spot" metering I need to shoot with no exposure adjustment, or even 1/2 - 1 stop over-exposure when shooting in balanced light, but with strobes at 1/2 - 1/4 power when the subject is within 1 - 2 feet. With a (Fuji) compact I've found that I still need to shoot with about 1/2 stop under-exposure.

It's really important to bracket the exposure of a good subject, if you have the time. It's not unusual for me to shoot 10 frames and recompose, then shoot more if I can. After all, memory is cheaper than diving!

Tim