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05-24-2009, 11:14 AM   #1
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Newbie question about histograms

Recently got myself a K10D as my first DSLR, having been a user of Pentax film SLRs for many years, and am just getting my head around how to use it well. I'm a little puzzled about what my histograms should be telling me. I have a couple of images I took to test lens resolution, featuring a mix of tiled roofs and trees. To the naked eye the exposure looks OK, and in Photoshop when I do Image->Histogram, everything seems fine. But when I go into Adjustments->Levels, the histogram for the RGB channel shows a very sharp narrow spike at the extreme right, the full height of the graph. Is this indicative of the loss of some highlight information? What should I read into it? Sorry if this is more of a Photoshop question, I'm just not sure what belongs where at this stage of the learning process.

PS This is just with JPEG, I've not got on to playing with RAW yet.

05-24-2009, 11:55 AM   #2
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Do a Google search for "histogram tutorial". There is tons of good info out there.
05-24-2009, 01:16 PM   #3
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Yes, I'm sure Google will lead me to plenty of general information about histograms. Actually, I was wondering whether it was something I needed to be aware of with this camera specifically, whether there is something odd about the extreme spike at the right hand of the graph on what appear to be correctly exposed images.
05-24-2009, 05:03 PM   #4
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If the spike is on the very extreme right 255 reading it indicates chanell clipping. You have overexposed and one or more of the red;green;blue chanells is blown. Whether this is relevant depends on the scene...can you post a picture?

If it a small section of white or light reflection it won't really matter.

There is a lot to learn with digital, the histogram being one small part. The "white histogram you see on the K10 is a "luminance" histogram. This takes the combined readings from the seperate 3 colour histograms but "weights" it according to how sensitive the human eye is to that colour. You can also view the red;green ; and blue histograms on your K10 by using the up/down rocker button while looking at the luminance histogram.

A RGB histogram is a simple average of the seperate colour ones and is less accurate than the luminance one for general over/under exposure, but can disguise the fact that one single colour has "blown". This will generally occur when photographing strong solid colours (flowers for example).

As a rule if photographing "general" scenes without very bold colours use the K10 luminance histogram to check for overexposure. If photographing strong colours, pay more attention to the seperate coloured histograms.

Have a look here for some excellent tutorials Digital Photography Tutorials

05-24-2009, 05:13 PM   #5
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A spike on the right hand that is not "cut out" by the edge of the screen simply means that your picture has a lot of light (almost white) colors. If the spike is in the middle or left, it means you have a lot of mid-tones or dark colors. The histogram is NOT a bible that must be followed with strict rules. It takes some experience to interpret it correctly. Unfortunately, they don't sell experience with the camera. When you check your pictures, check the histogram at the same time, so you can learn to interpret it correctly.
05-24-2009, 05:33 PM   #6
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QuoteOriginally posted by J2R Quote
Yes, I'm sure Google will lead me to plenty of general information about histograms. Actually, I was wondering whether it was something I needed to be aware of with this camera specifically, whether there is something odd about the extreme spike at the right hand of the graph on what appear to be correctly exposed images.
Google will lead you to all sorts of very specific tutorials on what histograms are, what they mean, how to interpret them and how to adjust them to get the best picture.
This isn't general information.
05-24-2009, 07:29 PM   #7
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One important point. The histogram is based on jpg processing in camera which is used for the LCD display. If you are shooting RAW, it may well show clipping, when in fact, the file is not clipped. I find if you are shooting RAW only, turning down the saturation (Record menu - jpg) will make the histogram closer to reality.
05-25-2009, 01:39 AM   #8
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QuoteOriginally posted by mithrandir Quote
One important point. The histogram is based on jpg processing in camera which is used for the LCD display. If you are shooting RAW, it may well show clipping, when in fact, the file is not clipped. I find if you are shooting RAW only, turning down the saturation (Record menu - jpg) will make the histogram closer to reality.
I think you may have hit the nail on the head here. Following the recommendations on a review I read, I had increased the JPEG saturation and contrast by +1 from their defaults. When I got around to looking at the converted RAW images side by side with the JPEGs, I found that the histograms of RAW ones were much more as I would have expected. I need to do some more experimenting today with the JPEG settings restored to normal.

05-25-2009, 01:43 AM   #9
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QuoteOriginally posted by flyer Quote
A spike on the right hand that is not "cut out" by the edge of the screen simply means that your picture has a lot of light (almost white) colors.
The spike is cut out by the edge of the screen. It occurs on lots of images, even ones simply of foliage which was what made me wonder whether it was indicative of some camera problem. I'm doing some more experimenting today which should give me some more useful information.
05-25-2009, 01:49 AM   #10
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QuoteOriginally posted by pschlute Quote
If the spike is on the very extreme right 255 reading it indicates chanell clipping. You have overexposed and one or more of the red;green;blue chanells is blown. Whether this is relevant depends on the scene...can you post a picture?
Peter, thanks for your lucid and helpful response.

Further digging on one image revealed that it was the blue channel which was getting blown, and looking in Raw Therapee (at which I am a complete beginner) showed highlights clipped in an area of blue sky. So could it be as simple as that ? Where I have exposed for the foreground, the sky is always likely to show clipping? In which case, would it manifest itself as a very sharp spike in the blue channel at the extreme right of the graph?

I'll maybe post an image later on if today's experimentation doesn't resolve all this for me.
05-25-2009, 04:08 AM   #11
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As a beginner myself, I found the tutorials on this link to be very helpful, especially when switching from film to DSLR.
Ron Bigelow Articles


Thanks,
05-25-2009, 05:04 AM   #12
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Here's a good tutorial.

Understanding Histograms
05-25-2009, 08:46 AM   #13
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QuoteOriginally posted by J2R Quote
Further digging on one image revealed that it was the blue channel which was getting blown, and looking in Raw Therapee (at which I am a complete beginner) showed highlights clipped in an area of blue sky. So could it be as simple as that ? Where I have exposed for the foreground, the sky is always likely to show clipping? In which case, would it manifest itself as a very sharp spike in the blue channel at the extreme right of the graph?
Yes. Any bright object - and make no mistake, the sky *is* bright - will tend to overexpose (clip) if your main subject looks properly exposed. The flip side of this is why some people erroneously think Pentax cameras underexpse - they try to keep highlights from clipping, leading to well exposed skies but underexposed foregrounds, when you probably would have preferred the look you are getting. Presumably, that wasn't the default camera exposure, but you dialed in compensation or exposed off the ground using M mode or AE-L?
05-25-2009, 11:24 AM   #14
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QuoteOriginally posted by Marc Sabatella Quote
Presumably, that wasn't the default camera exposure, but you dialed in compensation or exposed off the ground using M mode or AE-L?
Well, I used an M series lens, which meant that I was inevitably using centre weighted metering, and the sky was not in the centre and was thus not metered for. All this is slowly coming together for me, and I'm very grateful for the help on here to speed the process!
05-25-2009, 12:14 PM   #15
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QuoteOriginally posted by J2R Quote
Well, I used an M series lens, which meant that I was inevitably using centre weighted metering, and the sky was not in the centre and was thus not metered for. All this is slowly coming together for me, and I'm very grateful for the help on here to speed the process!
Center weighted metering includes all of the screen in the metering pattern, it is just biasing the meter somewhat towards what is in the center of the screen.
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