what is a histogram

[FLECOM]

as the title sais, ive read countless articles, but i still dont get it

what is it for, how do you read it, what do i use it for, why should i care

winner gets a cookie

 

[torment]

It shows you what your picture looks like. Left side are the darks and right side the lights. If you take a picture of the sky and its highlights are blown out...you'll have a spike on the right side of the histogram. If you take a picture of the dark night sky, you'll probably end up with the majority of the graph on the left side. It helps to show you how your exposure came out and you generally want a the histogram to be spread across the whole spectrum, as far as I understand it.

Try www.luminous-landscape.com for more help on histograms.

 

[FLECOM]

ya i read that last night... but, dunno... dosent make any sence to me... twoards the left is dark, right is light... so you dont want it to be all the way up against either side..?

does it have any other purpose than telling you expose more/less?

then why have all this complicated graphical representation, cant they just put "over" and "under"

all that information in the graph has to be useful for something

 

[PurdueDood]

Imagine if you will...

The X axis (horizontal) shows the colors based on their wavelengths, in order. Red, orange, yellow, green, blue, indigo, violet, and all the colors in between them.

If you were to count all the pixels in the picture and assign them to a catagory, like the bars on your histogram, you can tell how you picture is saturated.

You can correct the photo based on the concentrations of color, i.e. shift the colors on way or eliminate a color all together.

Same goes with the histogram of each alpha chanel, you can pick a chanel and see how many pixels are dark or how many are light. This can help you correct saturation, contrast or brightness.

-PD

/eats cookie.

 

[FLECOM]

ok but the histogram in the camera is just the alpha channel right?

the 1D mark II is the only one i know that does individual RGB histograms

 

[Stereodude]

Originally posted by FLECOM
ok but the histogram in the camera is just the alpha channel right?

It's actually the luminance information for the picture.

 

[Stereodude]

For example from the following histogram

I can tell that the picture doesn't have much dynamic range. I can also tell that there aren't any blown highlights, or lost shadow detail. Sure enough if you look at the picture.

You can see exactly what I mean.

However, you can also see that the exposure isn't exactly centered. Had I exposed the picture a stop more you would have gotten this histogram, which is more centered and a better exposure.

Again, there are no blown highlights or lost shadow detail. Look at the picture and you can see it is a better exposure than the first.

 

[Stereodude]

An over exposure would look like this

where the picture looks like this. You can see it's cooked.

An underexposure would look like this

You can see it's way too dark.

 

[Stereodude]

Now, by manipulating the histogram in PS with the levels command I can get this

which give us this very nicely exposed picture with good contrast.

 

[Stereodude]

Now, you're still asking what good does this do me?

Well, lets take an outdoor picture with very high contrast. It has a histogram like this.

Now you can see that the picture really exceeded (or came really close to) the dynamic range of the sensor in the camera.

Sure enough.

This was shot in jpeg, and I was watching the histogram, so I knew to put the camera in RAW and try again (RAW has wider dynamic range), and I was able to get this.

 

[Stereodude]

Now, lets say you wanted to get a little bit more artistic.

If you wanted to capture the steeple of the church with a nice proper exposure you'd want to target a histogram like this

which gives you a picture like this

Note the camera will likely not give you that exposure on it's own. It would tell you that's an overexposure.

Say you want the inverse. Where the sky stands out against a silloutte of the steeple. You'd want to target an exposure like this

which gives you a picture like this

Note the camera will likely not give you that exposure on it's own. It would tell you that's an underexposure.

But because of the histogram you can see what's going on and get what you want and make sure you have good exposure over the part of the picture you want.

Make sense?

 

[Gondi]

A post worthy of a sticky. Thank you, SD.

 

[blip]

Great thread.

The best way I've seen to learn about histograms is to take say 10 well exposed images and 10 poorly exposed images and mix them up in whatever software you use to look at histograms on your PC. (I use the software that comes with canon cameras and it is surprisingly useful for cataloging and checking histograms.)

Now as you scroll through each picture look only at the histogram at first and form a mental picture of what the exposure will look like (obviously it helps if all 20 of the images are of the same object maybe even from the same angle etc.) Then after building the mental image look at the picture and see how close you were. Do this enough and you will have a pretty decent idea of how a picture will look from the histogram alone.

 

[jmroberts70]

Thanks Stereodude for that visual lesson. I've never made use of the histogram features because I never quite understood the effectiveness of this tool. Thanks also to PurdueDood and torment for their help with this. Definetly worthy of a sticky thread.