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Goldilocks

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Using Live Histograms?
« on: December 08, 2005, 12:43:02 am »

Hi,
I'm not extremely experienced with histograms. And I've never shot using live histograms. I never shot RAW, but got the general idea, and had the opportunity of Dale sending me some RAW photos and I downloaded Raw Converter Essentials (great results) But I need to know if using a live histogram in the canon s80 camera could give me the quality of raw, in particular to not lose alot of color/value information in the highlights and shadows? Maybe, I don't understand histograms enough. But I do use them on a trial and error basis for post processing.
Thanks,
Goldilocks
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Anon E. Mouse

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« Reply #1 on: December 08, 2005, 01:28:04 am »

The live histogram just shows the LCD monitor values and so is just an approximation of the final exposure. It is a guide just like your metered exposure. Whether it is helpful depends on many things. I find them too unstable while the camera is handheld. When on a tripod, it might be better.
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Anon E. Mouse

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Using Live Histograms?
« Reply #2 on: December 08, 2005, 01:33:24 am »

About RAW. That is simply the unprocessed data from the CCD. You simply process the data instead of the camera aferwards.

If you only have TIFF or JPEG on your camera, you simply need to control exposure for the situation to acheive the best results - practice makes perfect. Some cameras have additional contrast, saturation, and sharpness controls to help as well. Whether TIFF/JPEG can be as good as RAW is a subjective opinion and based on too many factors to make a conclusive answer.
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Tim Gray

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Using Live Histograms?
« Reply #3 on: December 08, 2005, 08:43:07 am »

Short answer is no.  But tou can easily test this - take what looks like a well exposed shot on the camera and take a look in your raw converter or PS - on a view that shows all 3 channels.   In my experience with both P&S with live histos and a 1d2, the histogram is only an approximation for what's really being captured.

Here's a link that demonstrates some of the issues.

(subject to the above example) bottom line is that as long as you have a shot that fits well into the histogram with some breathing room at the bottom and top and it's biased towards the right, you likely won't notice the difference between raw and jpg (ignoring the white balance issues).  For those shots that hit both left and right, and where bracketing isn't practical, raw is the only answer.
« Last Edit: December 08, 2005, 08:46:58 am by Tim Gray »
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Dale_Cotton

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Using Live Histograms?
« Reply #4 on: December 08, 2005, 10:17:50 am »

To fully understand Anon's response I think we have to delve a bit deeper into the subject of exposure. I'm going to get a bit technical so hold on to your hat. I'm going to go into exhaustive detail a) because I've got some time to kill right now, and b) because the question that is vexing you simply won't go away unless you really understand these concepts. I'm an artsy fartsy painter-type, not a techie, so what I'm going to write here will almost certainly cause a five alarm fire to start at Techical Headquarters, but as we used to say after tuning our guitars: "Hmmm, well, I guess that's good enough for folk music." ;)

The imager in a digital camera (the CCD or CMOS chip) has millions of light sensitive spots called photodiodes. OK: I know that word photodiode is scary. But let's take it apart. The photo part is simply the Greek for light. The di part is the Greek for the number 2. And the ode part is simply a form of poetry originally invented by the ancient Greeks to woo handsome athletes. So a photodiode is simply a bit of electronics that converts light into two line poems (poems that only an engineer could love). Here's how that works:

Each photodiode reacts to in-coming light "particles" (photons) by generating electricity. The more light that reaches a photodiode, the more electrical energy the photodiode accumulates, just like a wool skirt building up static cling. The problem is that there is an upper limit and a lower limit to this process. Like a water glass, the photodiode can only hold so much electrical energy. If too many photons hit the photodiode it reaches that limit and if still more photons arrive it cannot deal with that - it gets stressed out and freaks. At the other extreme, the photodiode is never quite an empty glass - it always has a small amount of electrical charge in it; this is called noise. So if too few photons hit the photodiode there is no way to know whether the small amount of electrical charge it contains has got there from light striking it or not. This is called the noise floor. (We can tell that electronics engineers are sloppy house keepers because they would have called this a dirty floor instead of a noisy floor if they had ever used a mop or a broom.)

Now that you understand how a photodiode works, all you need to know is that each pixel in your image file is a number that corresponds to the amount of electrical charge in one photodiode (actually this part is a lie because I'm leaving out how the numbers get translated into colours). If only a few photons hit a certain photodiode, then the corresponding pixel will be a small number, which means a dark colour. If scads of photons hit that photodiode, then the corresponding pixel will be a large number, which means a bright colour. (So now we know the dark secret of digital photographers: all they do is paint by number.)

We can also see that there is a maximum brightness number that a given photodiode can generate. This is pure white. And there is a minimum brightness number that a given photodiode can generate. This is NOT pure black because of the noise floor. If you subtract the noise floor number from the maximum brightness number you get the exposure latitude of the imager, which is usually called dynamic range. The dynamic range of most good digital imagers is something around seven stops. Whether this is enough to capture both the highlights and the shadows of any given scene depends on how contrasty the scene is. If the dynamic range of the scene is greater than the dynamic range of your camera's photodiodes then something will get clipped. If you waste the dynamic range of your camera by setting the exposure too high or too low, then something will also get clipped.

To avoid setting the exposure too high or too low, we have two tools: the exposure meter and the histogram. A histogram of an image shows how many pixels are dark, how many are white, and how many fall in the large grey area in between those extremes. My Pentax DS, like most dSLRs, can show me a histogram of a picture after I've taken the picture, IOW, a dead histogram. I can see how the pixels spread from black to white and if they are bunched up at either end I know I need to re-take the picture at a different exposure. Of course, sometimes that's not possible. So if I have a camera with a live histogram I can see the pixel spread just before I take the picture and adjust my exposure accordingly. And again sometimes that's not possible: you can't follow the action in a ball game and fiddle with the exposure at one and the same time.

But as Anon points out, there is another issue with the in-camera histogram that affects its usefulness - it is based on the JPEG version of the image whether you are shooting JPEG or shooting RAW. To create a JPEG the camera has to make certain guesses as to how you want to interpret the scene. As I'm writing this Tim Gray has just posted a link to an example of why this can be a problem. I haven't seen this particular problem in non-Canon cameras, but what I have seen is similar. To create a JPEG in the camera most manufacturers cut out the darkest stop or so of photodiode data to eliminate a good chunk of shadow noise. They also don't try to extrapolate the brightest stop of data when one or two colour channels is clipped like a good RAW converter will. This knocks your dynamic range down from seven or eight stops to six. They also apply an aggressive tone curve to boost contrast, although some cameras allow you to control how aggressive that curve is.

Now, none of this matters if you are dealing with a low contrast scene. None of this matters if you are dealing with a static subject, bracket your exposures, then re-combine them in Photoshop. But all too often in outdoor photography, if you take JPEGs with a compact camera, you are going to find either your highlights have been clipped, which often equates to a pure white sky; or you are going to find your shadows are inky ... or both in the same shot.

A live histogram can be a useful tool to help you make sure you get the best exposure for a contrasty scene, but it can't overcome the built-in clipping limitations of an in-camera JPEG. What might be better here is if your camera can do automatic exposure bracketing without filling up the image buffer.
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Goldilocks

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Using Live Histograms?
« Reply #5 on: December 08, 2005, 04:44:25 pm »

Quote
To fully understand Anon's response I think we have to delve a bit deeper into the subject of exposure. I'm going to get a bit technical so hold on to your hat. I'm going to go into exhaustive detail a) because I've got some time to kill right now, and  because the question that is vexing you simply won't go away unless you really understand these concepts. I'm an artsy fartsy painter-type, not a techie, so what I'm going to write here will almost certainly cause a five alarm fire to start at Techical Headquarters, but as we used to say after tuning our guitars: "Hmmm, well, I guess that's good enough for folk music."

The imager in a digital camera (the CCD or CMOS chip) has millions of light sensitive spots called photodiodes. OK: I know that word photodiode is scary. But let's take it apart. The photo part is simply the Greek for light. The di part is the Greek for the number 2. And the ode part is simply a form of poetry originally invented by the ancient Greeks to woo handsome athletes. So a photodiode is simply a bit of electronics that converts light into two line poems (poems that only an engineer could love). Here's how that works:

Each photodiode reacts to in-coming light "particles" (photons) by generating electricity. The more light that reaches a photodiode, the more electrical energy the photodiode accumulates, just like a wool skirt building up static cling. The problem is that there is an upper limit and a lower limit to this process. Like a water glass, the photodiode can only hold so much electrical energy. If too many photons hit the photodiode it reaches that limit and if still more photons arrive it cannot deal with that - it gets stressed out and freaks. At the other extreme, the photodiode is never quite an empty glass - it always has a small amount of electrical charge in it; this is called noise. So if too few photons hit the photodiode there is no way to know whether the small amount of electrical charge it contains has got there from light striking it or not. This is called the noise floor. (We can tell that electronics engineers are sloppy house keepers because they would have called this a dirty floor instead of a noisy floor if they had ever used a mop or a broom.)

Now that you understand how a photodiode works, all you need to know is that each pixel in your image file is a number that corresponds to the amount of electrical charge in one photodiode (actually this part is a lie because I'm leaving out how the numbers get translated into colours). If only a few photons hit a certain photodiode, then the corresponding pixel will be a small number, which means a dark colour. If scads of photons hit that photodiode, then the corresponding pixel will be a large number, which means a bright colour. (So now we know the dark secret of digital photographers: all they do is paint by number.)

We can also see that there is a maximum brightness number that a given photodiode can generate. This is pure white. And there is a minimum brightness number that a given photodiode can generate. This is NOT pure black because of the noise floor. If you subtract the noise floor number from the maximum brightness number you get the exposure latitude of the imager, which is usually called dynamic range. The dynamic range of most good digital imagers is something around seven stops. Whether this is enough to capture both the highlights and the shadows of any given scene depends on how contrasty the scene is. If the dynamic range of the scene is greater than the dynamic range of your camera's photodiodes then something will get clipped. If you waste the dynamic range of your camera by setting the exposure too high or too low, then something will also get clipped.

To avoid setting the exposure too high or too low, we have two tools: the exposure meter and the histogram. A histogram of an image shows how many pixels are dark, how many are white, and how many fall in the large grey area in between those extremes. My Pentax DS, like most dSLRs, can show me a histogram of a picture after I've taken the picture, IOW, a dead histogram. I can see how the pixels spread from black to white and if they are bunched up at either end I know I need to re-take the picture at a different exposure. Of course, sometimes that's not possible. So if I have a camera with a live histogram I can see the pixel spread just before I take the picture and adjust my exposure accordingly. And again sometimes that's not possible: you can't follow the action in a ball game and fiddle with the exposure at one and the same time.

But as Anon points out, there is another issue with the in-camera histogram that affects its usefulness - it is based on the JPEG version of the image whether you are shooting JPEG or shooting RAW. To create a JPEG the camera has to make certain guesses as to how you want to interpret the scene. As I'm writing this Tim Gray has just posted a link to an example of why this can be a problem. I haven't seen this particular problem in non-Canon cameras, but what I have seen is similar. To create a JPEG in the camera most manufacturers cut out the darkest stop or so of photodiode data to eliminate a good chunk of shadow noise. They also don't try to extrapolate the brightest stop of data when one or two colour channels is clipped like a good RAW converter will. This knocks your dynamic range down from seven or eight stops to six. They also apply an aggressive tone curve to boost contrast, although some cameras allow you to control how aggressive that curve is.

Now, none of this matters if you are dealing with a low contrast scene. None of this matters if you are dealing with a static subject, bracket your exposures, then re-combine them in Photoshop. But all too often in outdoor photography, if you take JPEGs with a compact camera, you are going to find either your highlights have been clipped, which often equates to a pure white sky; or you are going to find your shadows are inky ... or both in the same shot.

A live histogram can be a useful tool to help you make sure you get the best exposure for a contrasty scene, but it can't overcome the built-in clipping limitations of an in-camera JPEG. What might be better here is if your camera can do automatic exposure bracketing without filling up the image buffer.
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Goldilocks

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« Reply #6 on: December 08, 2005, 06:30:14 pm »

Thanks everyone.
Dale, you educated me while making me laugh. You're a great teacher. I never really did understand noise in low light, just took it for granted since higher ISO film speeds were grainer. It appears there is no connection from what you've told me. (or maybe there is and I don't get it. I bet Tim could correct me on that if my interpetation is wrong.  Sorry to the forum when I accidently quote an entire essay, in trying to reply. But Dale, keep a copy of that last post, unless you already teach and have it memorized.
Both cameras have the Automatic Exposure Bracketting. dpreviews shots look pretty good to me in darks and lights. But I remember one person once wrote, she liked raw because sometimes she's a little sloppy in her exposures. According to dpreviews gallery the s80 has less fringing problems than the s70's.
 

I'm definitely becoming mentally challenged. This website challenges the mind. I'm glad for that. My business, non-artsy friends just think I'm crazy.

Thanks again everyone. Oh, and Tim (while I am nowhere's nearly as accomplished and intelligent like you) wouldn't different histograms give different recordings, just like in the medical field different machines that test the same thing, blood sugar, MRI's ect., they never all come out the same  either.  So, is Canon really lieing about that camera (which I think was your point, that their technology could be better) or do we just assume that measuring devices can't be compared from one to the other. My friend was going crazy wondering why his blood sugar on the same sample, with two different brand meters were sometimes 25% off. Doc says expect it. Pick one and use that one only.

Sorry for going off on a tangent.

Linda (Goldilocks)
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Eric Myrvaagnes

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Using Live Histograms?
« Reply #7 on: December 08, 2005, 09:50:46 pm »

Quote
To fully understand Anon's response I think we have to delve a bit deeper into the subject of exposure. I'm going to get a bit technical so hold on to your hat.
<snip> 
A live histogram can be a useful tool to help you make sure you get the best exposure for a contrasty scene, but it can't overcome the built-in clipping limitations of an in-camera JPEG. What might be better here is if your camera can do automatic exposure bracketing without filling up the image buffer.
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Wow, Dale! That is the best technical writing I have read in many, many years! I hope you are planning to write a book of some sort. I'll buy a copy  

As far as I can see, you are pretty accurate, too (especially about the origins of the word "photodiode").

Thank you!

Eric
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Dale_Cotton

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« Reply #8 on: December 09, 2005, 08:06:36 am »

Eric: go outside and face due east. The red glow you see on the horizon is me blushing here in southern Ontario. ;)

Sorry: no plans to try to enter the saturated photography instruction book market - I know just enough about the technology to operate a camera on a good day.
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Tim Gray

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« Reply #9 on: December 09, 2005, 08:17:16 am »

Quote
wouldn't different histograms give different recordings, Linda (Goldilocks)
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I think it relates to several factors: the resolution of the lcd display; the fact that the display is based on jpg, not raw; and the associated software design.  I think it also illustrates the importance of knowing your equipment. Managing noise at the point of capture is all about optimizing the exposure, so understanding what the histo on your particular camera is telling you is important.  I just got a sony R1 as a "carry around" - it's certainly easier to have with me than my 1d2 and 24-70 - but it's histo blinks for overexposure at about 1 stop under where actual clipping occurs in a raw file - so I know that and adjust to where the display just starts to blink (zebra stripes actually) and add one stop.

So to recap, once you understand the personality of your own equipment, a live histo can help you optimize exposure, but it's no substitute (at least in high dynamic range situations) for RAW.

In re-reading this reply, I'm not sure I've responded to the point you made...
« Last Edit: December 09, 2005, 08:18:34 am by Tim Gray »
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Goldilocks

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« Reply #10 on: December 13, 2005, 03:26:43 am »

I don't think I was really trying to make a point. Just trying to understand the net effect of the two methods of photographing. Thanks for your explanation. Never having shot in raw and needing to buy a compact camera, wanting 28mm wide angle lens I somehow have to figure out what is best for my situation. I'm tossing between the Canon s70 and the Canon s80. the s80 has made improvements but also dropped the raw. I guess you could say I got a problem not a point.
Thank you all.
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bcf

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« Reply #11 on: December 13, 2005, 06:03:18 am »

Quote
As far as I can see, you are pretty accurate, too (especially about the origins of the word "photodiode").
Well, not quite... ;-)

In "diode", the "ode" part comes from the Greek "odos", which means "way", "road". Nothing to do with poetry I'm afraid (except perhaps the poetry of the road...).
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Eric Myrvaagnes

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« Reply #12 on: December 13, 2005, 08:58:12 pm »

Quote
Well, not quite... ;-)

In "diode", the "ode" part comes from the Greek "odos", which means "way", "road". Nothing to do with poetry I'm afraid (except perhaps the poetry of the road...).
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Here's the smiley I shoulda put on that last message:  
I really liked the way the "photo" and "di" were accurate and the "ode" was clearly a joke (at least that's the way I took it). I guess I found the combined "definition" quite poetic in its own way.  

Eric
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