Reading Histograms

[smac]

Does anyone know of a good tutorial that deals with reading and understanding of Histograms? I understand what a histogram represents (a graph of the tonal range) but want to learn how to interpret the histogram and adjust it to maximise the tonality of my images.

Stuart

[Onyx]

This is a very involving topic, and it's great that you already know what a histogram represents. However to understand it fully, it will drive you insane.... it's like trying to summarise quantum mechanics - there are simple explanations which may be easy to learn but strictly speaking incorrect, or there's the really accurate and involved ones that require a degree in histogramic studies from the university of NipponKogaru.

Starting point: Michael Reichmann's article.
http://luminous-landscape.com/tutorials ... ight.shtml

Ron Reznick's workflow as written in his ebook (he is IMHO one of the foremost persons on image post processing techniques) he deals with white balance using the histogram, among other things (Yes, WB can be optimised by looking at the histogram). I would suggest obtaining a copy of his ebook Image Acquisition and Digital Processing Techniques. It goes well beyond histograms, but is an invaluable resource IMO for all digital photographers.

[smac]

Onyx,

Thanks for your suggestion, I just hope that I haven't "bitten off more than I can chew". I have great difficulty comprehending abstract explanations and really just want to know what a "good" histogram should look like so I can improve my tonal range......

Cheers

Stuart

[Aussie Dave]

Hi Stuart
as long as you don't have peaks that run off either side of the histogram, you're fine. I guess the classic mountain peak right in the middle is ideal, but not always what you get under all circumstances. If the peak is more to the left of the LCD screen, the shot is a little UNDER exposed. The more the peak moves to the right, the more exposed (or brighter) it is.

Therefore, if you take a photo and view the histogram - to see that it is OK but a little to the left, make some settings adjustments (Aperture, Shutter or EV comp) and take another shot.

You should also note that the histogram on the D70 is really reading only the Green channel, so depending on what you're shooting, it can sometimes be a little misleading (but is often a very good guide).

Hope this helps...

[sirhc55]

Why not try an experiment - take some control pics and download to PS or ACR. Check the histogram in PS/ACR and compare to the histogram shown on the D70 LCD. In this way you will be able to discern the discrepancies and correct for everyday shooting

[Glen]

Stuart, I will be so bold as to say I don't beleive there is any such thing as a good or bad histogram. A histogram should reflect the image you have captured which should reflect the image you wanted to capture.

Eg A man in a black suit on a grey road will have a histogram different from a man in a white suit standing on snow. Are either histogram wrong? No, each should be a reflection on the image. A warning would be when the histogram for the snow shows it at full tonal range on the right indicating blown highlights. A histogram reflects the image, it can't be right or wrong. Would love to hear other opinions on this.

[MCWB]

A histogram reflects the image, it can't be right or wrong.

Spot on, I agree completely. Just because the histogram seems to be weighted to the left or the right doesn't mean it can't be a great image! In any case there are situations where the dynamic range of the subject exceeds that of the sensor, and in these situations you'll always have a bunching up at one end or the other (bits of the image under/overexposed). Also keep in mind Dave's statement that the D70 shows the histogram for the green channel only...

[wendellt]

Hi

this tutorial is the simplest way to explain histograms
I used a monotone image for example because it is easy to see the highlights midtones and shadows



Original image shown does not look high contrast, it's washed out many would say it's not a nice image

The histogram in photoshop interprets the image as shown 'Levels1'
look at the bottom where the triangles are
Black triangle = Shadows (represents the darkest parts of the image)
Midtoned Triange in the middle = Midtones, you should not change the position ofthis triangle
White triangle = highlights

now in theory you should slide the shadow triangle to the right aligning it to the first line that hits the top as shown in 'levels2' this simple means photohsop is darkening the image to the point that the tones in the shadows are black not a dark grey. The way to read the histogram is on the left you have the shadows when the lines are low in this area it means there are less dark tones when the lines are high and dense that means there is alot of black or dark tones

The histogram curve between theyellow lines(levels2) marks the midtones
grey tones around the 50% range. The curve looks dens and has a nice curve with a few spikes so photoshop is saying these midtones are right, in most cases the grey triangle slider should stay put. The spikes are high tones(ligh greys) within the midtone range so in the dark greys there are some light grey specs, these are not highlights as they are just under the highlight tonal range.

The white triangle should be moved to the blue line see(levels2) to make the highlights white, but depending on the image you may not want to do this, for fear of blowing out the image. In (Levels2) the blue line marks the start of the visible highlight tones of the image, this means very light greys. if you move the white triangle slider to align with the blue line that means you are telling photoshop to get rid of these light grey highlight tones and increase their tone so they turn white. In this image i wanted to retain the clouds which have light grey tones so i moved the slider just a little towards the blue line(levels2) this increased the highlights on the pylon and window.

see end result after shadows and highlights have been tweaked (levels2):


usually for an optimum image the shadow tones in the image are black at their darkest point and the highlights are white at their brightest point
the midtones should stay where they are, a nicely tones image will have dens black smooth and evenly spaced histogram a rugged image with too much going on highlights and shadows will have a messy histogram lots of thin uneven lines across the curve

Photoshop histograms are not that great it's better to use Curves in a RAW editor to maniplulate and image or interpret tones.

Later i may post more examples but thtere is no easy way to explain histograms. They are relative to the image.

if anyone disagress with this rushed explaination let me know.

[Killakoala]

Great explanations above. Well done to all.

But to summarise it simply.

1. Avoid peaks that 'Cutoff' at the top of the histogram. That signifies you have overexposed.

2. Don't worry about peaks way either side of left or right. It's just telling you that you have lots of highlights or shadows in your photo. (Or both)

3. Peaks around the middle just means you have lots of midtones, (A grey card should give you this), but will also give you a flat image with little contrast difference.

4. As Glen said, there is no such thing as a bad histogram.

[smac]

Thanks to everyone for the fantastic responses, the time and effort some of you have put into replying has been amazing........I will take on board all of the recommendations and my histogram issues will hopefully become "history" (pun intended).

Stuart

[Glen]

Stuart, also remember as you have a D2X you can read individual channels on your histogram

[Onyx]

While Glen's statement is factually correct - there is no good or bad histogram on a final image, in the context of Stuart's request to optimise tonal range utilising its histogram, it's fair to say even the black suited man on the grey road should be shot with the majority of luminosity values towards the right (overexposed) as far as practical (ie. don't blow highlights), and then pulling them back down during post processing. This maximises the range of tonal values available to describe the scene, and also minimises noise as you're enhancing signal to noise ratios by shooting to the right.

/pedant mode off

[robboh]

Ron Reznick's workflow as written in his ebook (he is IMHO one of the foremost persons on image post processing techniques) he deals with white balance using the histogram, among other things (Yes, WB can be optimised by looking at the histogram). I would suggest obtaining a copy of his ebook Image Acquisition and Digital Processing Techniques. It goes well beyond histograms, but is an invaluable resource IMO for all digital photographers.

Do you feel that its worth the US$75 +P&P that he is charging?? Seems a lot for an eBook since you dont have a hardcopy you can take to bed and read.

[birddog114]

Ron Reznick's workflow as written in his ebook (he is IMHO one of the foremost persons on image post processing techniques) he deals with white balance using the histogram, among other things (Yes, WB can be optimised by looking at the histogram). I would suggest obtaining a copy of his ebook Image Acquisition and Digital Processing Techniques. It goes well beyond histograms, but is an invaluable resource IMO for all digital photographers.

Do you feel that its worth the US$75 +P&P that he is charging?? Seems a lot for an eBook since you dont have a hardcopy you can take to bed and read.

Robboh,
I have both edition (1st + 2nd).
Let me say: That's the only book which I found lot of value in digital photography, and helped me a lot with my fruits after the harvest , Ron worked so hard on his book and he deserve to earn such $$$.
A hard copy, same as TH, you can print it out and have them in your bag to read on the the train, plane, ship, bed, dinner table, couch chair at your leisure.

[robboh]

Well Birdy, I havent seen you enthuse over anything that much for a while, so it must be good!! I'll have to have a look at getting it as im pretty challenged in the PP area.

As for printing it out, BAH!
They are usually formatted for letter for starters (so have to zoom them). Then to get it even remotely compact, you have to doubleside print them (which is a PITA on my deskjet which has a habit of picking up 2 pages occasionally) and it costs a huge amount in terms of ink!

Id print stuff like that here at work on the lasers, but you loose a lot by not printing them in colour and even on a laser printer, its still ~$50 to print out a 500 page book by the time you take into account toner and paper and maintenence costs per page.

[birddog114]

Well Birdy, I havent seen you enthuse over anything that much for a while, so it must be good!! I'll have to have a look at getting it as im pretty challenged in the PP area.

As for printing it out, BAH!
They are usually formatted for letter for starters (so have to zoom them). Then to get it even remotely compact, you have to doubleside print them (which is a PITA on my deskjet which has a habit of picking up 2 pages occasionally) and it costs a huge amount in terms of ink!

Id print stuff like that here at work on the lasers, but you loose a lot by not printing them in colour and even on a laser printer, its still ~$50 to print out a 500 page book by the time you take into account toner and paper and maintenence costs per page.

I've done one at one of The Officeworks in colour, in a small size book same as the size of the D70's manual, binding, all cost $35.00, 1 day waiting.

Save all your pennies and fly to Sydney, then your saving will be worth it

[robboh]

*grin*
Thats a good idea actually, I hadnt thought of trying on the copyshops. I know they used to be quite expensive, but may be more realistic nowdays.

[radar]

HI Robboh,

another option, in case you have a PDA, is to load it on your PDA, you can take it anywhere. That's what I did with TH, works pretty good on my Palm. I also have the D70s manual on there, a few of the tutorials, etc.

HTH,

André

[Andoru]

dpreview has an excellent tutorial on histograms:

http://www.dpreview.com/learn/?/Glossar ... ram_01.htm

[digitor]

Another good tutorial is here: http://www.rags-int-inc.com/PhotoTechStuff/Histograms/

One point worth noting is this one: The maximum vertical value does not indicate any clipping has occurred. which is a common misinterpretation when first learning about histograms.

Cheers