Question about histograms

[cdm45]

I am new at this and don't understand the behavior of the histogram..

I have noticed that when I am setting up to shoot an insect that has no white on it anywhere, at say, 1:1 with a totally black background that the histogram will not cover the entire range across the bottom of it's scale...When I shoot the image however, it comes out with decent detail. If I increase the light to push the histogram across the bottom the image is tremendously over exposed.

I am assuming that if there is no white in the subject frame the histogram is correct. Is this right? Should I even worry about the histogram for macro?

Thanks

[Bushman.K]

Histogram should help you to evaluate lightness distribution in scene. But it doesn't have any own independent value. In the same time, if you have only Lightness histogram (single graph) it's barely usable, because Reds and Blues have very small impact on Lightness.

The best way to set the exposure is to take some sort of test shot and to check histograms of Red, Blue and Green channels. Green channel is the most sensitive one, while Red and Blue are much less sensitive. So, if an object has blue or red parts in shadow, watch for clipping on the left of Blue or Red channel histogram respectively. You need that, because otherwise darkest Reds and Blues will be mixed with sensor noise - their level will be too low.

It's hard to say you don't need to check histogram at all when shooting macro, but it depends on your lighting setup, properties of object surface and many other things.

[Chris S.]

Should I even worry about the histogram for macro?

Definitely! The histogram is one of our most important tools. However, a "proper" histogram for the kind of shot you're doing will not look like a histogram for a typical, well-exposed landscape, portrait, or snapshot.

I have noticed that when I am setting up to shoot an insect that has no white on it anywhere, at say, 1:1 with a totally black background that the histogram will not cover the entire range across the bottom of it's scale...When I shoot the image however, it comes out with decent detail. If I increase the light to push the histogram across the bottom the image is tremendously over exposed.

As an example of a very good image of the sort you're describing, take a look at Orionmystery's Backlit Leaf-mimic katydid. It's the second picture in that post.

Note that the image is perfectly exposed, with good detail and the highlights holding well. The background is black, as in the situation you describe. Now let's look at the histogram for Orionmystery's image:



You'll notice that there is a big peak on the left side of the histogram, and that this peak appears to be cut off by the left side of the histogram. This peak indicates that the sensor is recording some areas as deep black. Since your scene includes black areas, this is just what we want to see on the histogram.

So the general advice to avoid a left peak on the histogram does not apply when you have a black background.

Bushman's point about checking the histograms for the individual color channels is important, though it doesn't have all that much to do with the question you asked here. But for future reference, you do want to check all three of your color channels to make sure you aren't burning one of them out (which, as you likely know, would be indicated by a peak on the right side of the histogram). In Orionmystery's image, he has chosen to let a very few pixels in the red channel blow out, as represented by the tiny spike on the right side of the red histogram. The part of the image that this represents is the leaf's bright edge. The decision to let these pixels go is, in my opinion, a wise one, as it gives the image sparkle, provides information about the texture of the leaf, and visually sets the leaf apart from the background.

Cheers,

--Chris

[cdm45]

Thanks for this detailed lesson Chris...I think I get it now. You are of course right that I was expecting to see histo's that resembled standard photography.

Since I just started I am mostly stacking dead critters I find laying around so don't have a lot of color; trying to understand light. But even so, by applying these concepts to even a coastal cicada, not an exciting subject, I saw a marked improvement in correctness of textures and colors.

I am looking forward to getting something worthy of posting in the beginners area...

Thanks again,

cdm

[Chris S.]

Cdm45, I'm glad you found my post useful.

Also, welcome to the forum as an active poster. (You've had membership for almost a year, but this thread represents your first posts. I look forward to seeing more from you. )

Cheers,

--Chris