ghostbase
Posts: 343
Joined: Fri Nov 26, 2004 4:49 pm

Dslr Histogram - What Am I Looking For?

Sun Oct 16, 2005 7:08 pm

I have noticed that there have been several references here to using the histogram when taking photos with a DSLR and that this facility can be used to get better exposure results in difficult conditions such as dark hangars or poor weather.

I have started reviewing each photo taken when using my Pentax *istD but I am not sure what I am looking for in terms of a 'good' histogram and I am not really sure what the histogram is telling me. I managed to take some scenery photos in North Devon over the last few days which came out really well when viewed on my PC but when I go back to the histograms of the best ones I still cannot really see any pattern or consistency that would tell me at the time that they were good photos.

Can anyone here explain what I am looking for or point me to somewhere which would help shed light on this?

Thanks

  

[Edited 2005-10-16 12:09:37]
"I chase my dreams but I never seem to arrive"
 
mrk25
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RE: Dslr Histogram - What Am I Looking For?

Sun Oct 16, 2005 8:28 pm

The histogram needs to be balanced from left to right, peaks on the left will mean the exposure will be darker, high peaks on the right, to bright.
Check out this link for a better explanation.

http://www.digitalkb.com/digital_photography/knowledge_base/histogram/

Regards

Mark
 
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JeffM
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RE: Dslr Histogram - What Am I Looking For?

Mon Oct 17, 2005 12:18 am

The histogram, lets you evaluate the distribution of tones in an image. Each pixel in an image can be set to any of 256 levels of brightness from pure black (0) on the left to pure white (255) on the right. A histogram is a graph that shows how the 256 possible levels of brightness are distributed in the picture.



The horizontal axis represents the range of brightness from 0 (shadows) on the left to 255 (highlights) on the right. Think of it as a line with 256 spaces on which to stack pixels of the same brightness. Since these are the only values that can be captured by the camera, the horizontal line also represents the camera's maximum potential dynamic range.

The vertical axis represents the number of pixels that have each one of the 256 brightness values. The higher the line coming up from the horizontal axis, the more of that color is seen by the camera during that exposure. An all black image would be a vertical line on the left side. An all grey image would be a vertical line in the center, and an all white image would be a vertical line on the right side.

To read the histogram, you look at the distribution of pixels. An image with lots of color and well lit will have a reasonable number of pixels at every level of brightness.
 
corey07850
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RE: Dslr Histogram - What Am I Looking For?

Mon Oct 17, 2005 9:10 am

Quoting JeffM (Reply 2):

To read the histogram, you look at the distribution of pixels. An image with lots of color and well lit will have a reasonable number of pixels at every level of brightness.

So in a perfect world the histogram would be completely flat from left to right meaning every level of brightness is equally represented?
 
ArmitageShanks
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RE: Dslr Histogram - What Am I Looking For?

Mon Oct 17, 2005 11:40 am

Camera meters made to show 18% gray as correct exposure. So, technically, the histogram should be totally flat in the middle.

Is this correct?
 
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JeffM
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RE: Dslr Histogram - What Am I Looking For?

Mon Oct 17, 2005 1:19 pm

Quoting Corey07850 (Reply 3):
So in a perfect world the histogram would be completely flat from left to right meaning every level of brightness is equally represented?



Quoting ArmitageShanks (Reply 4):
So, technically, the histogram should be totally flat in the middle.
Is this correct?

No. A flat horizontal line (across the bottom is the only place the line could be with a one pixel width) would indicate a scene with with all the colors from black to white.

An 18% gray card would likely be a vertical line somewhere in the middle. You can think of the rise from the bottom (vertical axis) as the amount of a color present, though technically that is not exactly correct.
 
GPHOTO
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RE: Dslr Histogram - What Am I Looking For?

Mon Oct 17, 2005 3:09 pm

Quoting Corey07850 (Reply 3):
So in a perfect world the histogram would be completely flat from left to right meaning every level of brightness is equally represented?

No, because each scene does not havea n even distribution of brightness in it.

The histogram shape will vary depending on the scene you are looking at. If everything is bunched over to the left, however, the scene is probably under-exposed. If everything is bunched over to the right, the scene is probably over-exposed. The histogram shown above looks correctly exposed, even though the left hand side of it has more peaks in it than the right. This shows that the scene has a fair amount of dark (but correctly exposed) areas.

Histograms need to be interpreted in relation to the scene, for example, if you were to take a shot of a grey card, or a clear patch of blue sky, the histogram would all be bunched into one little area. This is because the brightness in the scene is uniform - you are looking at a uniform scene. If you were to add a bright white cloud to the blue sky scene, you would end up with two peaks, one for blue sky and one for the (presumably) brighter pixels that represent the white cloud. Does that make sense?!  Smile

Just remember that bunched to the left means under-exposed, bunched to the right means over-exposed. Its all you need to know to start using it. You then adjust your settings (aperture, ISO or shutter time) to move the histogram to the right or left accordingly.

Best regards,

Jim
Erm, is this thing on?
 
martinairyyz
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RE: Dslr Histogram - What Am I Looking For?

Tue Oct 18, 2005 4:57 am

Hello Jim,

May I recommend that you purchase this month's Practical Photography magazine? I subscribe to it and this month they have a 30-page DSLR booklet, with a few solely to the histogram.

Cheers,
mt
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ghostbase
Posts: 343
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RE: Dslr Histogram - What Am I Looking For?

Wed Oct 19, 2005 7:43 am

Thanks Mark, Jeff, Jim and mt for all this information and the links as well, much appreciated.

 ghost 
"I chase my dreams but I never seem to arrive"
 
GPHOTO
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RE: Dslr Histogram - What Am I Looking For?

Wed Oct 19, 2005 3:36 pm

Quoting MartinairYYZ (Reply 7):
May I recommend that you purchase this month's Practical Photography magazine?

Will do so. Thanks for the tip!

Best regards,

Jim
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