Understanding the histogram

ShoeQueen

ShoeQueen

Suspended / Banned
Messages
2,370
Name
Lynn
Edit My Images
No
Has anyone got any links to sites explaining the histogram in relatively easy to understand terms? I know the very basics but would like to understand it better.
 
Don't forget the blinkies - the highlight warning (in the menus) that flashes white/black on the LCD.

Blinkies are linked to the histogram and give a visual indication of exactly which parts of the image are blown, or are on the brink of blowing. I find they are at least as useful as the histogram.
 
I've never quite understood what it means when it goes off the top? Either side of the histogram yes, but off the top.. err. I'm lost :)
 
MrJaba said:
I've never quite understood what it means when it goes off the top? Either side of the histogram yes, but off the top.. err. I'm lost :)
Click to expand...

It never goes off the top. When any tone busts in the vertical, the whole histogram is automatically rescaled to a lower level.

I think people struggle with histograms becuase they look very technical and it's actually quite hard to clearly identifiy exactly which tones in the histogram relate to which tones in the image*. That's why the blinkies are so useful because they give you a precise fix on the highlights and where they are in the picture.

Highlights are the main danger area, because once they're blown, they're lost and unrecoverable. Sometimes this is unavoidable when you have very bright areas in a scene - the sun, bright areas of sky, specular refections etc. With the blinkies enabled, you can make a decision about which highlight tones are important and need to be retained, and those that you are prepared to let blow because either they're so bright that they're way outside the dynamic range anyway, or because in the inevitable compromise that usually accompanies most exposure decisions, they don't matter so much as other tones - let them blow.

Don't try to analyse histograms in great detail, but in general areas of bright/light on the right, mid grey (think elephant grey) in the middle, and darker tones on the left. With the blinkies giving to a firm fix on the brightest highlights (and you will often have blinkies flashing somewhere on most images, even when correctly exposed) you know pretty much where you are and can then aim to have mid-grey somewhere around the middle. Caucasian skin tone is about two-thirds to the right.

BTW, there is no such thing as the perfect histogram. You sometimes hear it said that you should have a big lump in the middle, but that depends entirely on the subject. If you were shooting a black cat in the snow for example, the histogram would have peaks at either end and nothing in the middle - and that would be entirely correct.

* Try the challenge on p3 here, and see if you can link the histograms to the images. A few people on here tried it a while ago, and nobody got them all right! The point is, the shape isn't important (the subject dictates that) but what does matter is where key tones sit on the scale. Eg, mid-grey in the middle (elephant grey - grass, brick, concrete, most tarmac) and skin about half way towards the right. http://www.sekonic.com/images/files/HistogramsLightmetersWorkTogether.pdf
 
MrJaba said:
I've never quite understood what it means when it goes off the top? Either side of the histogram yes, but off the top.. err. I'm lost :)
Click to expand...

It doesn't mean anything that you need to worry about. Where you have a spike which goes off the top of the histogram, all it signifies is that a particular tone at that point exists in such quantity that it can't be contained within the histogram. It's probably a question of scale - with very high pixel count sensors being common you'd probably need a preview screen about a foot high to contain all the tones within the histogram in some cases.

So it means nothing other than to show the tone at that point is there in such quantities it can't be represented within the histogram - it doesn't signify anything about exposure.
 
CT said:
It doesn't mean anything that you need to worry about. Where you have a spike which goes off the top of the histogram, all it signifies is that a particular tone at that point exists in such quantity that it can't be contained within the histogram. It's probably a question of scale - with very high pixel count sensors being common you'd probably need a preview screen about a foot high to contain all the tones within the histogram in some cases.

So it means nothing other than to show the tone at that point is there in such quantities it can't be represented within the histogram - it doesn't signify anything about exposure.
Click to expand...

Sorry, CT is right. I oversimplified it in my comment above about the histogram rescaling to stop things going off the top of the vertical. It does rescale, but if you have a very large area of just one tone in the image, it will bust, but as CT says, don't worry about it.

Apologies.
 
HoppyUK said:
Sorry, CT is right. I oversimplified it in my comment above about the histogram rescaling to stop things going off the top of the vertical. It does rescale, but if you have a very large area of just one tone in the image, it will bust, but as CT says, don't worry about it.

Apologies.
Click to expand...

No probs Hoppy. To be fair I think it's often more a problem with the histogram in editing packages where they don't rescale which may have been what the OP was thinking of?
 
HoppyUK said:
BTW, there is no such thing as the perfect histogram. You sometimes hear it said that you should have a big lump in the middle, but that depends entirely on the subject. If you were shooting a black cat in the snow for example, the histogram would have peaks at either end and nothing in the middle - and that would be entirely correct.
Click to expand...
Thanks - that's how all this started. I was out with a friend shooting the Red Arrows - against a bright, mostly white, sky. She was trying to get her histogram in the middle as she'd been told that was 'right'. I wasn't looking at my histogram at all.

When we got back we put both sets of images on my laptop - her histograms were more in the middle, and when there were occasional bits of blue sky, it was bluer in her shots. But they looked a bit underexposed.

My histogram was mostly to the right, my exposure looked ok and I had the red of the planes showing up - but not so much blue (though this was partly because I was shooting in a slightly different direction to her.)

That prompted me wanting to understand histograms...

Lots of helpful stuff in this thread - thanks. :)
 
I've never been a great user of histograms but I used it (and the "blinkies") when taking a picture of the moon the other night.

It wasn't quite a full moon so the left hand side had some nice crater detail which was a little on the dark side but the right hand side was very bright.

I set the camera up so the exposure value was 0 initially and then saw that the whole right side was blown by the "blinkies".

I then carried on staking shots, increasing the shutter speed until I got a shot where the blinkies were either non-existent only covering very small areas.

In this situation, the blinkies were more useful than the histogram as a lot of the image was completely black but using either/or or a combination of the two in a given situation is a lot better than guessing or trying to check things on the screen alone :)
 
ShoeQueen said:
Thanks - that's how all this started. I was out with a friend shooting the Red Arrows - against a bright, mostly white, sky. She was trying to get her histogram in the middle as she'd been told that was 'right'. I wasn't looking at my histogram at all.

When we got back we put both sets of images on my laptop - her histograms were more in the middle, and when there were occasional bits of blue sky, it was bluer in her shots. But they looked a bit underexposed.

My histogram was mostly to the right, my exposure looked ok and I had the red of the planes showing up - but not so much blue (though this was partly because I was shooting in a slightly different direction to her.)

That prompted me wanting to understand histograms...

Lots of helpful stuff in this thread - thanks. :)
Click to expand...

If you are post processing and shooting Raw, then the way to go is Expose To The Right technique.

Just like there is no perfect histogram, there is no perfect exposure either! Your Red Arrows example is a good one. If the sky is bright white and the planes back-lit, effectively in shadow and almost a silhouette, you will probably find there's no way you can get a decent exposure both for the sky and the planes so you might elect to let the sky blow and pull the darker tones of the planes to the right and get some colour and life into them. There is at least 10x to 100x more data recorded on the right of the histogram than the left - massively more, and also far less noise.

Or if there is some colour and clouds in the sky you could go the other way, forget the planes and let them go dark and do something arty with what little colour there is in the sky and the smoke trails.

Or you might try and darken a blue sky with a polariser, pulling down the dynamic range so the sensor can cope, and then optimise the exposure carefully to get both good blues and reds, and smoke trails too, for that classic Red Arrows shot.

Lots of options. More info here, and the related article Expose Right on the link http://www.luminous-landscape.com/tutorials/optimizing_exposure.shtml Using ETTR technique with Lightroom and its clever recovery and fill light sliders makes this a doddle - with amazing results :)
 
CT said:
It doesn't mean anything that you need to worry about. Where you have a spike which goes off the top of the histogram, all it signifies is that a particular tone at that point exists in such quantity that it can't be contained within the histogram. It's probably a question of scale - with very high pixel count sensors being common you'd probably need a preview screen about a foot high to contain all the tones within the histogram in some cases.

So it means nothing other than to show the tone at that point is there in such quantities it can't be represented within the histogram - it doesn't signify anything about exposure.
Click to expand...
Ahhh that's good to know, was worried I was losing information in the shot. Thanks! Thanks HoppyUK too for the additional info :)
 
Russ77 said:
I've never been a great user of histograms but I used it (and the "blinkies") when taking a picture of the moon the other night.

It wasn't quite a full moon so the left hand side had some nice crater detail which was a little on the dark side but the right hand side was very bright.

I set the camera up so the exposure value was 0 initially and then saw that the whole right side was blown by the "blinkies".

I then carried on staking shots, increasing the shutter speed until I got a shot where the blinkies were either non-existent only covering very small areas.

In this situation, the blinkies were more useful than the histogram as a lot of the image was completely black but using either/or or a combination of the two in a given situation is a lot better than guessing or trying to check things on the screen alone :)
Click to expand...

Another great example of using these exposure aids, they really are gifts from the Gods of Digital.

TBH I use blinkies far more than the histogram, especially in a hurry. Take a shot, and immediately look at the LCD and see where they are. I want and expect to see some (reflections, a bit of sky perhaps) which shows that the sensor is maxing out on data, then tweak the exposure so they're only flashing on unimportant areas.

This can result in theoretically incorrect exposure, in that a given tone may not end up in quite the right place on the histogram. But that doesn't matter if you're post processing because you can moderate it later on the PC but have ensured the sensor has captured as much data as possible (brighter colours in the shadow end, less noise).

If you want to get into ETTR technique, be aware that the contrast setting in picture styles stretches/compresses the histogram and also affects the point at which the blinkies kick in. And blinkies flash not only on blown stuff, but also subjects that are on the brink on blowing, as a warning. Even when shooting Raw, everything on the LCD is generated from a small JPEG tagged to the Raw file. For the closest match to what you actually have on the Raw, turn the contrast down to min.

Do some tests to see exactly how much headroom you've got with your camera and your post processing regime. With ETTR you can usually squeeze in an extra couple of stops of exposure, above what the meter says or what is technically correct. It doesn't make a huge difference a lot of the time (and there are risks of blowing stuff of course) but if detail in darker tones is important, cleaner colours in the shadows and much less noise, then you'll love ETTR :thumbs:
 
HoppyUK said:
If you are post processing and shooting Raw, then the way to go is Expose To The Right technique.
Click to expand...
I do shoot RAW and post processing (though just started processing from RAW about a week ago). And I've heard of ETTR, which is what I ended up doing but not intentionally (as I wasn't looking at the histogram) but I was aiming to expose for the planes and trails rather than the sky.

HoppyUK said:
Just like there is no perfect histogram, there is no perfect exposure either! Your Red Arrows example is a good one. If the sky is bright white and the planes back-lit, effectively in shadow and almost a silhouette, you will probably find there's no way you can get a decent exposure both for the sky and the planes so you might elect to let the sky blow and pull the darker tones of the planes to the right and get some colour and life into them. There is at least 10x to 100x more data recorded on the right of the histogram than the left - massively more, and also far less noise.
Click to expand...
I was aiming to expose for the planes - though as I'd never shot planes before I put it in shutter priority and just used the exposure lock to get it on the planes. When the sun did shine a bit I also tried to get the planes with the light on them to show the colour better.



HoppyUK said:
Or if there is some colour and clouds in the sky you could go the other way, forget the planes and let them go dark and do something arty with what little colour there is in the sky and the smoke trails.

Or you might try and darken a blue sky with a polariser, pulling down the dynamic range so the sensor can cope, and then optimise the exposure carefully to get both good blues and reds, and smoke trails too, for that classic Red Arrows shot.
Click to expand...
I'll try that next time. And I did think at the time that a CPL would have helped - must get one.


HoppyUK said:
Lots of options. More info here, and the related article Expose Right on the link http://www.luminous-landscape.com/tutorials/optimizing_exposure.shtml Using ETTR technique with Lightroom and its clever recovery and fill light sliders makes this a doddle - with amazing results :)
Click to expand...
Thank you! :D
 
Sounds like you're on the way :)

Be aware that a polariser only works on blue sky, and also varies according to the angle you're standing to the sun. But when they work, the sky can go dark navy blue :eek: It will also reduce reflections on the planes, again if the angles are right. Probably a good thing on balance, as a starting point, but those glints on the planes can look great.
 
Oh, thanks... that's what I was looking for. I've been reading about histrogram for quite a while and couldn't get my head round it... this just amazingly simplifies everything. cheers.
 
Great thread! Thanks to all for shedding some light on this subject
 
Haha! Good test - 4 out of 6 for me (damn balloon) :D
 
Thanks for the link. I'm starting to learn about histograms. :)
 
brman said:
6 out of 6 :p

Probably helped by your balloon comment as it made me think between that and the sunset twice.:thumbs:

Toby
Click to expand...

Haha! Well done :)

But I wouldn't like people to think that, just because they get them wrong it means histograms are hard to read - the point that's being made is that they're all different.

When you have the image and the histogram together, which obviously you always do, it's easy to relate peaks in the graph to tones in the image.

As I said above, I always check the blinkies first and that nails the highlight areas absolutely. Then work back and make sure that in general terms, the light areas, the mid-tones and the shadows are where you want them.

It's easy really.
 
You must log in or register to reply here.
Back
Top