Understanding Histograms

[Landis]

In a number of recent posts in various places, it's clear to me there there are members who do not understand how using the Histogram feature in your cameras can really help you attain better photographs that have a proper range of exposure intensities. I shall do my best to shed some light on this.

First of all, you need to understand how digital images can be and cannot be changed. If you blow out a high light by over exposing your shot, you will have pure white areas in your file that will NEVER be able to have any detail in them. On the other side, if you have an area that is too dark, there IS salvagable informating hidden in there.

Just about all digital cameras have somewhere in the Menu a place that allows you to visualize how much light your image has in it. This is called the Histogram, which is nothing more than a graph of your image that shows how many pixels your image has for different intensities of exposure. On every one I have seen, Black is on the left and White is on the right. This is the same information you will see in Photoshop if you go to: Image > Adjustments > Levels. For this discussion, I will show you Levels, as I don't know how to transfer the actual histogram from my camera to this.

Attached is my screen shot of three different Histograms:

Left image - This is a flat image that has some blown out highlights (right side) that peg out on the pure white levels. If your histogram on your camera looks like this, give it less exposure, or close down your aperture about 1/2 stop.

Middle image - This is a proper exposure that shows no pure blacks and not pure whites. This actually could be spread out slightly more, but this will print properly.

Right image - This is an underexposed image. There is too much in the black limit and little in the highlight side. This can be salvaged, but it would be better if it was giving about 1/2 stop more exposure to begin with.

OK, where is your Histogram hiding: It should be a part of your Image Preview. Go there and hunt around for the options like "Priview with Histogram" or something like that. It's been on every digital camera I have seen, so chances are you have that option. My little Canon S400 has it and I know all DSLR's have it. I always have mine set to Preview with Histogram.

When you are outdoors taking, say a landscape shot, in most cases, using Auto Exposure, your camera will hunt for shadows, and expose for them therefore blowing out the highlights. What you think is going to be a nice blue sky turns white.

Everyone has their methods for determining exposures, but as I shot B&W film for over 40 years, I always Exposed for the highlights and let the shadows fall where to may. Just about any normal outdoor image will never have over 7 f stops of variance (not counting spectural highlights off chrome, etc). B&W film has a latitude of 7 stops, so you need to get the highlights in there. Digital has about 5 to 5-1/2 stop latitude, so you have to be a little more exact on where your exposure lands.

You can always run a Shadow Recover Action with Photoshop to bring out the detail that is hidden in the shadows. Overexposed highlights have ZERO in them.......gone.......nothing is there and never will be there.

So, when you are shooting, check that Histogram!!! If you see it's off, use your exposure compensation feature on your camera to compensate and reshoot to get that perfect Digital Negative!!

I hope this helps some of you.

[tspin818]

I would not even consider shooting a picture without using the histogram. My Olympus c-8080 has several different options for viewing the histogram, but I prefer the live onscreen method. The histogram is visible through the viewfinder while composing the shot. In manual mode, it allows you to set the f-stop and shutter speed for correct exposure. I also use it if I want to over or under expose a shot on purpose.

What you see in an evf or on an lcd is great for compostion, but not accurate for determining exposure. This is an essential tool for good photography.

Great tutorial, I'm sure it will help many.

[FL]

A good tutorial, I am sure that it will help those who are lucky enough to have on board histogram facilities.

[aweller]

I have also found the following primers to be worthwhile reading:

http://www.adobe.com/products/photoshop/pdfs/understanding_digitalrawcapture.pdf

http://www.adobe.com/products/photoshop/pdfs/linear_gamma.pdf

Ciao for now,
Andrew.

[grays]

Both Landis' post and aweller's are essential reading. I hadn't seen the Adobe ones before but they are excellent.

Thanks guys!

[mickeybb]

Thanks Landis and Andrew! My manual gives me about two sentences on this subject and one of them is explaining how to use the button to show it on the screen! I really appreciate the info. I'll start practicing with my very next photo, and hopefully you'll start seeing a difference in my posts!

[Landis]

Thanks, Andrew. The second Adobe article really hits the nail on the head. A MUST READ for anyone.

The only thing I differ with on the Adobe article is that on the example given with the Grand Canyon shot is that you have lost your blue sky with it being so high on the exposure curve.

All of this just shows you that there is more than one way to skin a cat, there is No One Right Way, Everyone just hang loose and enjoy yourselves.

Landis lol

[jake_i_am]

Thanks for the excellent reading Landis! I really appreciate the work you put in to these sort of things.
My camera doesn't have the histogram function, it just has a simple light meter. Even when I get the light meter spot on, I am getting washed out skies. Should I always just underexpose a little then? (as far as my light meter is concerned) It really frustrated me last weekend when all but a few of my daytime pictures had washed out skies. I use a Nikon Coolpix 4300 BTW. So if anyone has experience with that sort of thing, please let me know what you've done. ;-)

Jake

[JeffreyK]

This is a great article, I have read many that say you should always try to expose "to the right", meaning most of the contents of your histogram should be to the right, but making sure you don't overexpose. I found it in a tutorial located in

http://www.luminous-landscape.com/tutorials/understanding-series/understanding-histograms.shtml

[JeffreyK]

Here is the actually tutorial--

http://www.luminous-landscape.com/tutorials/expose-right.shtml

[Landis]

That is a good tutorial. Luminous Landscaps is norally pretty good. Here is the best part of it:

"This technique isn't necessarily something that you'll use every day. It requires extra work when shooting and extra work and time when post-processing in the RAW converter. But, if you are interested in ensuring the utmost S/N ratio for a particular shot it can be worthwhile. If though you find this approach to be counterintuitive or somewhat dubious, here's a novel idea — try it yourself. "

The absolute best advice I have ever received from a photographer of great renoun who has since died was this:
"If you have a question you need answered, NEVER ask someone for a quick answer. If you do, you will learn only the anwer. If you try it yourself. you will learn not only what you wanted to know, but probably a dozen other usefull things along the way."

[izadoodle]

Landis,
Thank you very much for writing this, this was something I was not doing. I did check it in my photo editing program, but never on the camera. I have been doing this now, since I have read this, and I have been learning a lot.
So once again thank you.
Stacey

[Throughmyeyes]

Thanks for writing this tutorial on the histogram Landis, I know that there are many here that will benefit from this including myself :-D

[tropidolaemus]

Great tutorial Landis thanks for posting it.

[manojmathur]

Landis,Thanks for the detailed explaination, will try and put to use.