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02-24-2010, 04:00 PM   #1
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ISO and inverse proportions?

I'm assuming that 400 ISO on a dslr isn't the same as 400 ISO on a smaller P&S sensor in terms of the IS of the image, but the question: is there a way of comparing /contrasting different size sensors to find "equal" ISO readings?

For example, is ISO 80 on a 1/1.6 compact sensor the same as (app) 400 ISO on an APS-C sensor in terms of noise and IQ?

I found this quote @ Digital Camera Sensor Size Comparison Chart by Jeffrey Sward that All other things being equal, the amount of noise in an image is inversely proportional to the sensor area. The amount of noise in an image is also directly proportional to the ISO setting. But being even less a math guy than a photography guy, I'm lost on how to apply it. I dimly remember the high school formula of x = y/z for inverse proportions, but I'm not sure if that's the right formula and what I'm plugging into where. Any help would be appreciated,
Brian

02-24-2010, 04:05 PM   #2
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QuoteOriginally posted by FHPhotographer Quote
I'm assuming that 400 ISO on a dslr isn't the same as 400 ISO on a smaller P&S sensor in terms of the IS of the image, but the question: is there a way of comparing /contrasting different size sensors to find "equal" ISO readings?

For example, is ISO 80 on a 1/1.6 compact sensor the same as (app) 400 ISO on an APS-C sensor in terms of noise and IQ?

I found this quote @ Digital Camera Sensor Size Comparison Chart by Jeffrey Sward that All other things being equal, the amount of noise in an image is inversely proportional to the sensor area. The amount of noise in an image is also directly proportional to the ISO setting. But being even less a math guy than a photography guy, I'm lost on how to apply it. I dimly remember the high school formula of x = y/z for inverse proportions, but I'm not sure if that's the right formula and what I'm plugging into where. Any help would be appreciated,
Brian
I Think that what the author is referring to is the pixel density on the sensor. That is, on that tiny sensor, where they are trying to squeeze 10mp, there is going to be a lot more random noise due to crosstalk (among other things). There is a lot more room on the DSLR sensor for that same 10mp. Both will show more noise at higher ISOs but more pronounced on the smaller sensor. That's RAW by the way. As we know, a lot of cameras have put noise reduction in place and the effect of that (IMO) is sometimes worse for the image in loss of detail than some noise. How bad, depends on how agressive they get with the NR.
02-24-2010, 05:14 PM   #3
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ISO is ISO. It's an agreed upon standard of assigning a value to the light sensitivity of the media.
02-24-2010, 06:23 PM   #4
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wheat, you missed the point of how do you correlate ISO with sensor size. For another example, if you want to compare noise & IQ of the the K7 and the Canon G11, is there a way to calculate which ISO image for the Canon is comparable to an ISO on the K7? At the base ISO of the Canon is 100 but the sensor is only one-sixth the size, is a comparable ISO image (under the same conditions of course) for the Pentax say 600? If so, then you get the best possible compare/contrast under like conditions based on ISO. That's the question,
Brian

02-24-2010, 07:18 PM   #5
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QuoteOriginally posted by FHPhotographer Quote
wheat, you missed the point of how do you correlate ISO with sensor size. For another example, if you want to compare noise & IQ of the the K7 and the Canon G11, is there a way to calculate which ISO image for the Canon is comparable to an ISO on the K7? At the base ISO of the Canon is 100 but the sensor is only one-sixth the size, is a comparable ISO image (under the same conditions of course) for the Pentax say 600? If so, then you get the best possible compare/contrast under like conditions based on ISO. That's the question,
Brian
I think you are confusing ISO and noise.

in many ways the issue is the same in digital as it was in film.

remember 16mm instamatic cameras. 100 ISO film was the same in both, BUT when you enlarge the i6mm frame by a factor of 4 times larger than a 35mm frame, the grain size of the film becomes more dominant and the picture appears to have poorer resolution.

the same sort of thing happens in digital, but for different reasons.

In digital, to get the same MP count on a smaller sensor the individual cell size is reduced, causing electrical noise to be increased and more apparent when enlargeed to the same final image size as a DSLR.

the same is true going the other direction. comapre an SMR and a mediam or large format camera. Same ISO, the print from the larger film will be better because the grain size is much less important the larger the negative.
02-24-2010, 08:22 PM   #6
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lowell, I don't think I'm confusing the two. There is an inverse relationship between sensor size and noise, i.e., smaller sensor produces more noise at a given ISO level than a larger sensor, correct? Then I was wondering if there is a table/algorithm/whatever to correlate these things.

So, let me try rephrasing the question with an assumption that 400 ISO on a DSLR produces a lower level of noise (a relatively cleaner image) than does ISO 400 on a P&S 1/1.6 sensor. If there is an image quality difference (noise) between the the P&S and the DSLR @ 400 ISO, at what ISO levels will the DSLR have a comparable image to the P&S?

All I want to do is find a quick technique to estimate IQ using the relationship between two sensors using ISO as the variable , rather than having to take two sets of photographs running through the ISO range for each camera and then printing them for visual comparison. Are you saying that can't be done?
Brian

Last edited by FHPhotographer; 02-24-2010 at 08:27 PM.
02-24-2010, 08:38 PM   #7
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The term you are looking for is "equivalence". It's a pretty straightforward calculation that actually relates focal length, field of view, DOF, and noise. See any number of threads in these forums (including one called "low light advantage of FF = zero" or something like that for long argumentative discussion of the topic.
02-24-2010, 09:53 PM   #8
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QuoteOriginally posted by FHPhotographer Quote
wheat, you missed the point of how do you correlate ISO with sensor size. For another example, if you want to compare noise & IQ of the the K7 and the Canon G11, is there a way to calculate which ISO image for the Canon is comparable to an ISO on the K7? At the base ISO of the Canon is 100 but the sensor is only one-sixth the size, is a comparable ISO image (under the same conditions of course) for the Pentax say 600? If so, then you get the best possible compare/contrast under like conditions based on ISO. That's the question,
Brian

Too many variables. Sensor size is not the only criteria for noise characteristics.
I suspect this is something that would have to be visually evaluated using a test target.

02-24-2010, 09:59 PM   #9
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It's true that sensor size isn't the only determinant of noise, but it's a big part of it. the equivalence deals only with this part of it, of course.
02-24-2010, 11:45 PM   #10
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marc, thanks for the term. Using that search term I came up with this site that describes the author's use of the term ISO compatibility which " ...is calculated by dividing the area of the larger sensor with the area of the smaller one." In his example, a Nikon300s to a Canon G10 gives a factor of 8.6x. All things being equal, as I read it but somebody please correct me if I've got this wrong, a G10 image @ 100 ISO should be noise-comparable to a Nikon300s @ 860 ISO ?
Brian

Last edited by FHPhotographer; 02-24-2010 at 11:46 PM. Reason: typo
02-24-2010, 11:53 PM   #11
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QuoteOriginally posted by FHPhotographer Quote
marc, thanks for the term. Using that search term I came up with this site that describes the author's use of the term ISO compatibility which " ...is calculated by dividing the area of the larger sensor with the area of the smaller one." In his example, a Nikon300s to a Canon G10 gives a factor of 8.6x. All things being equal, as I read it but somebody please correct me if I've got this wrong, a G10 image @ 100 ISO should be noise-comparable to a Nikon300s @ 860 ISO ?
Brian
It's the all things being equal part where it falls apart.
There is really no point in theoretical pap when other variables take any legitimacy out of the comparison.
02-25-2010, 12:02 AM   #12
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wheat, you think maybe you could occasionally respond when you disagree without being dismissive? There's no need to refer to the discussion focus as "theoretical pap." You don't agree, fine, but language like that adds nothing to the discussion,
Brian
02-25-2010, 12:26 AM   #13
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QuoteOriginally posted by FHPhotographer Quote
wheatif you want to compare noise & IQ of the the K7 and the Canon G11, is there a way to calculate which ISO image for the Canon is comparable to an ISO on the K7?
No.

The operative word here is "calculate". You are treating the question as one of fundamental physics and it is not.

This is not an all else being equal situation.

The two sensors are proprietary implementations and one may be a better implementation with respect to noise relative to it's size than the other. For instance the sensor on the Kx may be inherently quieter than that of the K20.

You could, perhaps, answer your question but not analytically by simple calculation but rather empirically through tests done in a lab on the two specific sensors in question.

On the other hand I could be completely misunderstanding your question too.

Last edited by wildman; 02-25-2010 at 12:56 AM.
02-25-2010, 01:29 AM   #14
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QuoteOriginally posted by FHPhotographer Quote
wheat, you think maybe you could occasionally respond when you disagree without being dismissive? There's no need to refer to the discussion focus as "theoretical pap." You don't agree, fine, but language like that adds nothing to the discussion,
You're asking someone to go against his nature... not gonna happen.

In any case, he's right. Adding a bunch of theoretical assumptions to this debate doesn't help, it just gives you unrealistic results. In most cases, "all things being equal" is a gross mis-statement, because when it comes to camera sensors, nothing is ever equal. You won't be able to determine the noise characteristics of a sensor by looking at its size, type or native ISO rating. There are too many other variables involved. The only way you'll get real results is by testing for yourself.
02-25-2010, 06:39 AM   #15
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QuoteOriginally posted by FHPhotographer Quote
wheat, you think maybe you could occasionally respond when you disagree without being dismissive? There's no need to refer to the discussion focus as "theoretical pap." You don't agree, fine, but language like that adds nothing to the discussion,
Brian
I did. Please re read post #8.
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