histograms

[adrian mandea]

can anyone teach me ,or anybody else interested all there is to know about histograms? thank you

[Steve Eisenberg]

That's a big question. Here's what I understand so far:


1) The black & white histogram represents every tone in the photo. From black on the left to white on the right. The X-Axis


2) The vertical (Y-Axis) represents the number of pixels for each tone.


3) There is no such thing as the perfect histogram, it depends what you are trying to create.


4) But for general picture taking: A) A histogram that has a hump in the middle tones and trails off to either end is probably a good sign that you are on the right track. B) If the histogram extends past the extreme left or right sides, you will lose all detail in black and white, respectively. C) If the entire histogram is shifted towards the left or right, the entire image is trending underexposed or overexposed, respectively.


I hope this helps some.

[adrian mandea]

yes it does, thank you steve

[Steve Eisenberg]

Glad I could help!

[Don Burkett]

That's a big question. Here's what I understand so far:


1) The black & white histogram represents every tone in the photo. From black on the left to white on the right. The X-Axis


2) The vertical (Y-Axis) represents the number of pixels for each tone.


3) There is no such thing as the perfect histogram, it depends what you are trying to create.


4) But for general picture taking: A) A histogram that has a hump in the middle tones and trails off to either end is probably a good sign that you are on the right track. B) If the histogram extends past the extreme left or right sides, you will lose all detail in black and white, respectively. C) If the entire histogram is shifted towards the left or right, the entire image is trending underexposed or overexposed, respectively.


I hope this helps some.
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Steve's explanation is an effective one. So with that as the baseline, let me add some more...


His second point is very important. You typically want to avoid bars from touching either end with the right side being more important to avoid than the left. A good healthly spread of the bars across the histogram indicates a lot of tonal quality in the shot, a narrow spread indicates less tonality. Either is ok depending on what you are shooting but the graph should visually represent the scene.

If you're shooting a duck in the snow expect a narrow graph to the right of center. In this example, if the histogram was centered you're shot is underexpose because snow is white and the middle means gray. Hope that makes sense.

If you're shooting a black bear in a cave, expect the histogram to be left of center. If it's in the middle your shot is overexposed because it made the black bear gray.
If you're shooting a black bear chasing a deer, through the snowy woods on a beautiful blue sky day you want a wide histogram graph that peaks in the middle but doesn't touch either side. Why, the blue sky, the deer, the woods are all mid-tones and should represent most of the scene. Yet the bear is black (left side) and the snow is white (right side).



Three other quick points:

The height of the bars mostly just indicates how much of that tonal value there is in the scene and not something I pay too much attention to.
Many people will tell you to expose to the right. Which means, in an average scene, use enough exposure to prevent too much of a left sided histogram. The logic is simple, in post processing if you have a picture that is too dark overall or too dark in the shadows and you increase the brightness it will cause the noise signals in the shadows to show. On the other hand if you shot is a little overexposed (no blown highlights though) and you decrease the brightness you will have no ill effects from noise. So an slightly overexposed shot is more forgiving than an underexposed shot.
The RGB (red, blue, green) channels each have their own histograms. I personally, don't use them much unless I'm photographing a highly saturated or difficult color like red and then it's just to ensure I've got wide enough tone in the color channel the the detail will show up in the final result.

[Steve Eisenberg]

In addition to Don's good comments:


Everyone should have a grey card that's easy to take with them. I like the Lastolite Ezybalance. It folds up, and is weatherproof. Meter off that, and you will be on your way to good exposures in sketchy situations (use the "Exposure Lock" button, the "asterisk"). It's also handy for White Balance. Shoot a picture with the card in the frame, and use it to adjust your white balance later. If I shoot a bunch of bugs, as soon as I'm done I'll toss the card into the frame, and shoot it for later use.

[Daniel Browning]

Another important point: if you shoot raw, your histogram is lying. It is based on the JPEG, which is often one, two, or even three stops over- or under-exposed relative to the true raw data. The reason for this is that the saturation, curves, space conversion, contrast, and especially white balance processing all affect the histogram, even though they don't affect the raw data.


Here's an example:


http://forums.dpreview.com/forums/read.asp?forum=1037&amp;message=26905476 ("http://forums.dpreview.com/forums/read.asp?forum=1037&amp;message=26905476)


Until manufacturers add a raw histogram feature, the only workaround is to build a custom white balance file that reverses the effect of the in-camera processing to give you a *real* histogram. Some call that a "Uni-WB". The downside is that the metadata, preview, etc. are all useless (and very green!), so you can't check an image for color tones *and* histogram at the same time.

[adrian mandea]

thank you very much guys for all the answers, from now on i'll check the histograms ,i've never tought they were important.


thank you

[Flish]

I like to understand more about this...


I think I understand the post processing white balance part as the card become the neutral gray target to balance against but I'm not sure how or why you are using the card relative to setting the exposure.





Thanks in advance for the help...

[Kyle Webb]

Adrian,


Below is a link to the Cambridge in Color website that has a couple ofvery good tutorials on histograms, how to read them,and how to use them to improve your images.


http://www.cambridgeincolour.com/tutorials/histograms1.htm ("http://www.cambridgeincolour.com/tutorials/histograms1.htm)


http://www.cambridgeincolour.com/tutorials/histograms2.htm ("http://www.cambridgeincolour.com/tutorials/histograms2.htm)


By the way this Cambridge in Colour website is an excellent resource.


Kyle

[Don Burkett]

By the way this Cambridge in Colour website is an excellent resource.


Kyle
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It is a very worthy read. Be prepared for disappoint though, becasue when you are done you're going to wish there was more.


He did an excellant job on a number of subjects that helped me understand a number of things much clearer.


Good recomendation Kyle.

[Kyle Webb]

Don,


<span style="font-size: 9pt; color: black; font-family: Verdana;"]As an Opto-Mechanical Engineer I use this <st1:city w:st="on"]<st1:place w:st="on"]Cambridge</st1:place></st1:city> in Colour website recourse all the time. It is an especially good reference source for technical reports and presentation in explaining optical phenomenon related to digital photography as well as optical phenomenon in general. The beauty of it is that they explain theory and show practical applications and results of the phenomenon. I am glad that you can appreciate it.


<span style="font-size: 9pt; color: black; font-family: Verdana;"]Kyle<o:p></o:p>

[Steve Eisenberg]

I like to understand more about this...


I think I understand the post processing white balance part as the card become the neutral gray target to balance against but I'm not sure how or why you are using the card relative to setting the exposure.





Thanks in advance for the help...
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For the extremes (white and black), the meter will over-compensate due to the extreme reflectance of light for white, and the lack therof for black. The meter has a benchmark expectation of 18% reflectance. So if white relects more than that, you will have an underexposure, and if black relects less, you will get an overexposure.


Here's an example where the first photo is the "correct" exposure, and the second is "overexposed" by one stop:


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[Flish]

Ok that makes sense but I think I need help understanding whatconstitutes (histograms excluded)proper exposure.


Your examples seem to show the main subject with significant detail differences. In the first, the background seems natural to my eye and the subject seems to lack detail whereas in the second example the opposite appears to be the case. Shouldn't correct exposure be when the intended subject detail reaches your desired expectation?





I'm not sure if my question even makes sense but what the heck...can't hurt too much to ask.

[Don Burkett]

Now you're getting into territory where one could argue that correct exposure is the one that produces a result you are pleased with. [:D]


A techical step above that, the correct exposure is one where the colors, hue, tone, saturation all reproduce what your eye saw when you took the picture. The use of a grey card in trickly lighting situations can help you adjust your photos in post processing as you use that white balance to apply to your other shots. Many people are happy with a result that is not a true reproduction of the original scene. In fact, many people adjust exposures to affect a particular emotion or feeling. In other applications (documentaries, product photos, etc.) it's critical to exactly reproduce the original scene.

[Steve Eisenberg]

Ok that makes sense but I think I need help understanding whatconstitutes (histograms excluded)proper exposure.


Your examples seem to show the main subject with significant detail differences. In the first, the background seems natural to my eye and the subject seems to lack detail whereas in the second example the opposite appears to be the case. Shouldn't correct exposure be when the intended subject detail reaches your desired expectation?





I'm not sure if my question even makes sense but what the heck...can't hurt too much to ask.
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Interesting. I guess having taken the picture, I would be the only one to know how the background looked in real life. You may think it's supposed to be brownish burlap, when in reality it is bleached. I suppose the most important thing is the subject matter, and if the only way to make the subject appear correctly is to sacrifice the background, maybe that's just the way it is.


There are numerous other ways to take the same photo. A large aperture, with the flash and a high shutter speed would make the texture fade into the background, etc. But, my main point with the images was to show the camera's view vs. real life. In this case, the second photo is closer to reality, and I should have mentioned that...


But back to the ultimate question. It's all subjective. If you are trying to please yourself alone, then the answer is yes. But if you are trying to please others, and strangers at that, you start getting into the deeper question: What is art? In my opinion, a piece of art is successful if it evokes something inside of the viewers. Even better, if it evokes the exact same feeling in others. And even better than that, if it's abstract and evokes the same feelings with all of the viewers!


Your question makes perfect sense, and I have thought alot about it myself. My conclusion is that there is no ideal focus, sharpness, or anything. If it looks good to you, enjoy it. Otherwise, have some friends take a look. But don't give any pre-conditions or apologies regarding the quality of the picture. Get a pure first impression. In fact, post them in the Critique forum, cause we're your friends too!

[Flish]

I suspect it's more important for me to understand first the correct way to interpretexposureand then go toward what pleases me if that my objective. I've up some pretty valuable information from this discussion and it's much appreciated. I intend to start employing a grey card aspart of my shooting workflow in hopes to better understand the entire process.


As always, thanks for youguidance.

[Flish]

Ok, that makes sense. I was beginning to wonder if I was missing some critical element in the whole process. My goal is to first understand the correct process and then explore outward from that point. I suspect I'll be asking more question that may border on the riduclous but that's just my nature I suppose.


Thanks for all your help along the way...

[Steve Eisenberg]

Ok, that makes sense. I was beginning to wonder if I was missing some critical element in the whole process. My goal is to first understand the correct process and then explore outward from that point. I suspect I'll be asking more question that may border on the riduclous but that's just my nature I suppose.


Thanks for all your help along the way...
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I think digital photography has made us all waaaaay too self-critical. When I took guitar lessons, my great teacher told me to first learn to play, then worry about reading music, music theory, and all the rest of the intellectual stuff. Please remember to have a good time, and shoot some nice pictures. You will work out the more technical aspects over time.


And ask all the questions you want!

[Tom Alicoate]

Another important point: if you shoot raw, your histogram is lying. It is based on the JPEG, which is often one, two, or even three stops over- or under-exposed relative to the true raw data. The reason for this is that the saturation, curves, space conversion, contrast, and especially white balance processing all affect the histogram, even though they don't affect the raw data.


Here's an example:


http://forums.dpreview.com/forums/read.asp?forum=1037&amp;message=26905476 ("http://forums.dpreview.com/forums/read.asp?forum=1037&amp;message=26905476)


Until manufacturers add a raw histogram feature, the only workaround is to build a custom white balance file that reverses the effect of the in-camera processing to give you a *real* histogram. Some call that a "Uni-WB". The downside is that the metadata, preview, etc. are all useless (and very green!), so you can't check an image for color tones *and* histogram at the same time.
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I have seen this before too. Usually If I bumped up contrast and saturation I would get a Histogram that was out of whack. Now I reduce the saturation and contrast to zero and the Histograms don't look as incorrect. I only use the black and whitehistogram however. There may be a better way to make this Histogram closer to true. Anybody have any ideas? I don't want to mess with the three color ones if the results are a s bad as you say.


Thanks,


Tom

[Daniel Browning]

There may be a better way to make this Histogram closer to true. Anybody have any ideas? I don't want to mess with the three color ones if the results are as bad as you say.


The only way I can think of would be for a camera manufacturer to add this simple option. Barring that, there is no way you can get close without messing with the colors, because the colors are exactly the reason why it's off in the first place.


Accurate colors or accurate exposure: pick one.

[Stefan Stuart Fletcher]

I've learnt a lot from all the posts in this thread. Thanks to everyone for contributing. One other thing: the histogram in PP software such as ACR or Lightroom is not quite the same as the camera's (especially if you shoot RAW), but can be used to correct overall exposure and tone based on the same principles. Any good book on e.g. Lightroom will provide a thorough explanation. The colour channels are, however, more useful than implied elsewhere. You can use them to check your HSL settings in addition to getting the exposure right.