What does the Y Axis on a Histogram represent?

I have been using the historgram as a tool for determining proper

exposure (Over / Under Exposure), but several of my historgrams

don't generate much information in the y-axis. The historgram is

evenly distributed, but the graph is almost flat. What is that

telling me? The image looks OK on the monitor, so I'm not sure what

the Y-Axis indicates. Thanks in advance.

The Y axis represents the number of pixels of a particular intensity (the intensities are on the X axis). So a flat histogram simply means that your image has a very equal distribution of grey values (or intensities, let's forget color for the moment). This usually means a rather low contrast image - but that doesn't have to be a bad thing.

 

Since you use the histogram, you have probably already noticed that when you have no values (no bars) on the high or low end of the histogram, or if all the values pile up on on end, that means that the exposure was not perfect. So this is what it tells you: too many white/black pixels compared to all the pixels in the image. At least for "normal" images. If you do high-key, that might be what you aimed for.

<i> This usually means a rather low contrast image</i><P>

No, it doesn't. It means the image has a full range of tonal

values (black to white) with all values appearing an equal

amount. The histogram of a low-contrast image would have a

significant amount of one or both ends of the X-axis showing

zero because those tonal values are not present in the image.

The histogram is a global measure and contrast is a local property so the only strong relation between the histogram to the contrast is that it is a prerequisite for a high-contrast image to have both light and dark values represented. An image with peaks at the high/low end tends to have high contrast. Midtone expansion (pushing values towards this bimodal distribution) is a way to enhance contrast, midtone compression (the opposite) tends to reduce contrast.

I'm self taught, but this is how I deal with histograms: each image is different, and you can't assume there is such a thing as a "normal" or "correct" histogram. Example: a night shot of a person leaning against a light post in the distance. Suppose I want all that dark empty space for the "mood" of the image (isolated, lonely) The histogram will show lots of dark pixles, maybe a peak in the bright pixles (the light on the post) and very little in between. Here's some other examples... like this waterfall. Notice the peak in the highs (the falls)<div></div>

Here's an average shot of some flowers. Lots of mids and a peak in the shadows (some of the dark background areas.)<div></div>

This is a night shot using "zoom blur" on a campfire. Lots of dark areas, some bright areas, and not much in the mids.<div></div>

I forget where I read this and unfortunately don't have it bookmarked.

 

In PhotoShop, the Y-axis is adjusted to that the highest peak is always the full height of the graph, and the other values are adjusted relative to it. This allows you to compare tonal distribution of two histograms by comparing the areas under the curves. (A histogram with exactly equal distribution of every tone would be a solid black rectangle.)

 

If you are seeing flat histograms, look closely... there's a full-height spike at one end or the other.

Mike Dixon is right...

The histogram is a "pixel popularity poll". If you divide all the shades from black to white

into 256 levels, the relative number of pixels in the image representing each of the 256

tones is displayed on the y axis.