Bulge in the middle of histo?

Hi All,

Trying to figure out how to fix this exposure problem. On some setups, I'm getting a bulge in the middle

of the histogram that is off the charts. In this last occurance, it was right in the middle of the histo. How

do I get rid of that? I tried to adjust exposure, but it just moved up or down the scale. Or is it just that

there is too much of the image area that is that same tonal value, and I need work on getting, say, the

background to be more tonally separated? I wouldn't be concerned except I know that if I'm off the charts

there, I'm losing data.

Thanks!

You cannot lose data if it goes off scale vertically (I've never seen it off scale). The ordinate of the histogram is proportional to the number of pixels at that value, limited only by the number of pixels in the image. Have you looked at the individual RGB values?

If you adjust exposure on a linear scale, the histogram will simply shift right or left. You can, of course, lose data if the curve is truncated at the white or black end. If you adjust exposure against a curve, the histogram will be stretched or bunched up since density is no longer proportional to exposure. ACR uses a curve with RAW files. You can also use curves in Photoshop to accomplish the same thing.

Have you considered posting an example or screen shot of the histogram in question?

OK - i'm attaching a screen shot. Based on the level that I see when putting my mouse over

the bulge, and then comparing that to the info tab when I put my mouse over the gray

background, they're both in the same range (139 to 145). so, should i even be worrying

about this? or just roll with it?<div></div>

"Or is it just that there is too much of the image area that is that same tonal value"

That sounds like it: Say a beach with lots of sand.

OK - so nothing to worry about then? I just thought that since extension over the right or

left meant clipping highlights or shadows, that extension over the top would mean the same

sort of thing - a data loss in some way. So the correct way to interpret that is "there's a lot of

space in the tonal area around the middle of the scale?"

Thanks!

The "bulge" is nothing more than the gray background, which is made up of middle tones and takes up many pixels.

There seems to be a common misconception that there is some kind of "ideal" histogram with a uniform distribution. That's absolutely not the case. If you have a picture with lots of pixels of similar tone (as in your example) you're going to get bulges and peaks even though the exposure is perfectly fine.

A histogram is nothing more than guidance that has to be examined with common sense. A histogram that is bunched up to the left or right may indicate underexposure or overexposure. Or it may be exactly what what you want if your intent is a low-key or a high-key image. You probably want to look out for histograms with a large number of pixels at the extreme ends, indicating clipping. But other than that, you need to decide whether the histogram is appropriate for the image you're after. In your example, the "bulge" is exactly what you'd expect with correct exposure. Underexposure wold shift the "bulge" to the left; overexposure would shift it to the right. But if the background were darker, a "bulge" in the middle would indicate overexposure. Get the idea?