Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions
Martin.au
Martin.au Forum Pro • Posts: 11,953
Re: Exposure basics, lesson two point one (& ISO)

Jack Hogan wrote:

Ulric wrote:

Jack Hogan wrote:

He wouldn't be naive to believe that he saw more noise (he would). He would be naive in thinking that to capture the same tonal range (shadows and highlights) of the same scene with the same lens and Exposure (ss and f/n) as his friend's EM5 he would have to use the same ISO in the G3. His naivete would cost him a ton of IQ: one stop of highlight headroom and 1 stop of SNR, and he possibly would lose his National Geographic desk

I give up; that is, I don't get it. Given the same exposure, why would the g3 on iso 3200 produce more noise than the em5 on iso 3200, but the g3 on iso 1600 would not?

The short answer: Because ISO labels are arbitrary, and in the case at hand Olympus called 6400 what Panasonic calls 3200, so if you set the G3 at 3200 you need to set the EM5 at 6400 to capture the same image information all else being equal. Pehaps an example would help?

You are an intermediate photographer and you have been given an assignement: shoot a game indoors with the same 50mm lens and two cameras, an EM5 and a G3. The desk tells you that anything other than a dof of f/6.3 and the blur stopping power of 1/800s will be accepted, and you need to get the highest IQ possible without blowing the highlights of the brightest object in the scene, the white helmets of the players. Easy, you say. You switch both cameras to full manual mode in order to set the mandated shutter speed and aperture on both - Exposure has therefore been fixed equally on both and will remain set there for the entire game. Then the issue of ISO comes up.

Quick like a fox you remember various DPR threads where this was discussed ad nauseam. You therfore take a sample shot at a guesstimate ISO of 800 on the G3 and see by looking at the hystogram* that the white helmets are more than 1 stop below clipping. You therefore dial in 1600 ISO take another sample shot, verify that the helmets are now all the way to the right of the hystogram just before clipping, and you are all set. You can now fire away, forgetting completely about Exposure and ISO and metering potentially getting fooled by lights or reflections and concentrate solely on getting the shot. You get some great shots.

At half time you move the lens to the EM5 and go through the same set up procedure. You verify that ss and f/n are the same mandated values of f/6.3 and 1/800s (therefore Exposure is the same as on the EM5) and you go through the exact same ISO setting procedure as for the G3. This time though, in order to ensure that the white helmets end up just before clipping you need to dial in a different ISO value. No sweat, the sample images from the two cameras look about the same in terms of noise and eDR as set up (the EM5 a tad better as DxO's graphs suggested) so you fire away forgetting that Exposure and ISO even existed. You get some even geater shots.

Same scene, same light, same lens, same Exposure (ss and f/n), same tonal range (image information) captured. Why are the ISOs different? You then remember DPR threads that mentioned that in-camera ISO labels are not consistent from camera to camera or from manufacturer to manufacturer. So what one calls 1600 the other can call 800 or 3200, the standard allows them to do it. Does it matter? No, as long as you remember that in the same situation when the G3 calls for ISO 1600 that is instead called 3200 on the EM5.

HIH
Jack

*This is a simplification. To do it properly one would need a Raw hystogram or to learn how the sRGB hystogram of one's camera typically maps to the Raw one in a given light.

Hey Jack, is the SOS measurement from the ISO standard an arbitrary value?

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Jack Hogan Veteran Member • Posts: 6,207
Re: Exposure basics, lesson two point one (& ISO)

Ulric wrote:

Right, I'm willing to accept that increasing the iso setting on the g3 from 800 to 1600 will reduce noise.

I am not sure I understand.

What I wondered about was this exchange:

8<---

Case A

Jack Hogan wrote: It appears that if one wanted to capture the very same tonal range from a scene with the EM5 and the G3, same exposure and all, one would have to set the EM5 at in-camera ISO 3200 while the G3 at in-camera ISO 1600, with roughly similar resulting SNR (EM5 slightly better as per DxO).

Case B

On the other hand, if one set both at 3200 maintaining the same exposure they would no longer be directly comparable, but the G3 would appear noisier to a naive observer.

8<---

Perhaps a different tack would help: the speedometers of two different cars have been set by their respective manufacturers to read about 5% and 100% more than the actual speed the cars are travelling, so when the first car is showing 100km/hr on its speedometer it is in fact travelling at about 95 and when the second car is showing 100 it is in fact travelling at 50km/hr. They both have the same performance and fuel consumption at a real speed of 100 Km/hr .

A gentleman who doesn't know this but would like to decide which car to buy on the basis of fuel consumption performance decides to take each on a 100km highway test drive and measure how much fuel each consumes during the trip. Day one he takes the first car and drives it for the duration of the trip at a speedometer-indicated 100km/hr, taking just over an hour to complete it. He consumes about 10 liters of gasoline. The next day he takes the second car on the same trip. Because it's a nice day for a drive he casually notices that it has taken him longer to travel the 100km, but he is pleasantly surprised that he has cosumed only 7 liters of gasoline to cover the same distance at the same speedometer indicated 100km/hr. Convinced by the second car's fuel performance evidence, he naively buys the second car without hesitation and drives away happily. This is case B.

The next week, an expert friend of the first gentleman goes through the same buying process. However he understands that speedometers in this country are not labelled consistently because the relative ISO standard allows manufacturers a lot of latitude in speedometer display correlation to actual speed, so he brings his own GPS. He makes the two trips on the same two cars but this time following the real speed indication of the GPS. It takes him exactly the same time to complete the journeys on both cars and his fuel consumption is exactly the same, even though the in-car ISO speedometers showed 105km/hr in one case and 200km/hr in the other. Happy that both cars perform about the same, he makes his buying decision based on price and other features. This is case A.

Case A: you toss ISO out of the window because you know it is unreliable and simply set up the two cars to drive at the same Speed by another mean (ETTR in my previus post is one of them) - who cares what in-car ISO speedometers show, we know they are not correct.  The important thing for a fair fuel consumption (SNR) comparison is that they are driven at the same Speed (S) in the same conditions (Exposure).

Case B: you take in-car ISO speedometers at face value, but you end up comapring the fuel performance (SNR) of a car, in the same conditions (Exposure) that is going at a real Speed (S) of 95 to one that is going at a real Speed (S) of 50km/hr, while naively thinking that you are actually going at the same speed because that's what the ISO speedometer shows

Jack

bobn2
bobn2 Forum Pro • Posts: 53,614
Re: Exposure basics, lesson two point one (& ISO)

Mjankor wrote:

Approximately.

They're equal brightness at 1/4 and 1/5, so 1/3 of a stop. I didn't really think 1/3 of a stop would matter to much to this discussion.

1/3 stops seems a lot for cameras of the same make - you would expect that Olympus would want its cameras to perform consistently - inconsistency within a brand puts the wind up people, but they get used to the foibles of the evaluative metering on different brands. For instance, you frequently read that Nikon 'tends to overexpose'.

-- hide signature --

Bob

Jack Hogan Veteran Member • Posts: 6,207
Re: Exposure basics, lesson two point one (& ISO)

Mjankor wrote: Hey Jack, is the SOS measurement from the ISO standard an arbitrary value?

Yes, but don't take my word for it, check it out yourself.  Meter off of a gray card in a few different modes, and see if middle gray comes out to 118 in sRGB. Does it ever?

Martin.au
Martin.au Forum Pro • Posts: 11,953
Re: Exposure basics, lesson two point one (& ISO)

bobn2 wrote:

Mjankor wrote:

Approximately.

They're equal brightness at 1/4 and 1/5, so 1/3 of a stop. I didn't really think 1/3 of a stop would matter to much to this discussion.

1/3 stops seems a lot for cameras of the same make - you would expect that Olympus would want its cameras to perform consistently - inconsistency within a brand puts the wind up people, but they get used to the foibles of the evaluative metering on different brands. For instance, you frequently read that Nikon 'tends to overexpose'.

-- hide signature --

Bob

I did the same test outside a week ago. The difference there was pretty well unnoticeable, so I suspect it's more a quirk with white balance or something. It was on auto.

In practical usage there's pretty well no difference between these two cameras. (Which could be partly why Oly went with such highlight room on the OM-D).

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dosdan Contributing Member • Posts: 509
Re: Exposure basics, lesson two point one (& ISO)

Yes. Ideally it would be blinkies related to the Raw data as opposed to those related to an sRGB rendered image.

Jack, are you aware of any cameras that offer a raw histogram display option?

Dan.

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Martin.au
Martin.au Forum Pro • Posts: 11,953
Re: Exposure basics, lesson two point one (& ISO)

Jack Hogan wrote:

Mjankor wrote: Hey Jack, is the SOS measurement from the ISO standard an arbitrary value?

Yes, but don't take my word for it, check it out yourself. Meter off of a gray card in a few different modes, and see if middle gray comes out to 118 in sRGB. Does it ever?

Can you please explain how they can be arbitrary, with respect to the ISO standard?

Can you also please tell us what DPR are doing, when they do their ISO tests to the 12232 standard?

"If you want to compare ISOs between cameras from our reviews, look at the ISO test section (on the Noise and Noise Reduction page). This is where controlled test results are reported, essentially using the SOS method of ISO 12232:2006. The rest of the testing is based on knowing this SOS ISO calibration, as all test scenes have specific grey patches that are white-balanced and exposed as closely to L=50 as possible (i.e. within 1/6 stop)." - Andy Westlake

And, finally, can you please tell us how every camera I've looked at is consistent in this test? Why do all the cameras agree, if it's an arbitrary standard (oxymoron, I know)?

I assume this is an experiment you've done. Please tell us your results and methodology.

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Martin.au
Martin.au Forum Pro • Posts: 11,953
Re: Exposure basics, lesson two point one (& ISO)

Jack Hogan wrote:

Ulric wrote:

Jack Hogan wrote:

He wouldn't be naive to believe that he saw more noise (he would). He would be naive in thinking that to capture the same tonal range (shadows and highlights) of the same scene with the same lens and Exposure (ss and f/n) as his friend's EM5 he would have to use the same ISO in the G3. His naivete would cost him a ton of IQ: one stop of highlight headroom and 1 stop of SNR, and he possibly would lose his National Geographic desk

I give up; that is, I don't get it. Given the same exposure, why would the g3 on iso 3200 produce more noise than the em5 on iso 3200, but the g3 on iso 1600 would not?

The short answer: Because ISO labels are arbitrary, and in the case at hand Olympus called 6400 what Panasonic calls 3200, so if you set the G3 at 3200 you need to set the EM5 at 6400 to capture the same image information all else being equal. Pehaps an example would help?

You are an intermediate photographer and you have been given an assignement: shoot a game indoors with the same 50mm lens and two cameras, an EM5 and a G3. The desk tells you that anything other than a dof of f/6.3 and the blur stopping power of 1/800s will be accepted, and you need to get the highest IQ possible without blowing the highlights of the brightest object in the scene, the white helmets of the players. Easy, you say. You switch both cameras to full manual mode in order to set the mandated shutter speed and aperture on both - Exposure has therefore been fixed equally on both and will remain set there for the entire game. Then the issue of ISO comes up.

Quick like a fox you remember various DPR threads where this was discussed ad nauseam. You therfore take a sample shot at a guesstimate ISO of 800 on the G3 and see by looking at the hystogram* that the white helmets are more than 1 stop below clipping. You therefore dial in 1600 ISO take another sample shot, verify that the helmets are now all the way to the right of the hystogram just before clipping, and you are all set. You can now fire away, forgetting completely about Exposure and ISO and metering potentially getting fooled by lights or reflections and concentrate solely on getting the shot. You get some great shots.

At half time you move the lens to the EM5 and go through the same set up procedure. You verify that ss and f/n are the same mandated values of f/6.3 and 1/800s (therefore Exposure is the same as on the EM5) and you go through the exact same ISO setting procedure as for the G3. This time though, in order to ensure that the white helmets end up just before clipping you need to dial in a different ISO value. No sweat, the sample images from the two cameras look about the same in terms of noise and eDR as set up (the EM5 a tad better as DxO's graphs suggested) so you fire away forgetting that Exposure and ISO even existed. You get some even geater shots.

Same scene, same light, same lens, same Exposure (ss and f/n), same tonal range (image information) captured. Why are the ISOs different? You then remember DPR threads that mentioned that in-camera ISO labels are not consistent from camera to camera or from manufacturer to manufacturer. So what one calls 1600 the other can call 800 or 3200, the standard allows them to do it. Does it matter? No, as long as you remember that in the same situation when the G3 calls for ISO 1600 that is instead called 3200 on the EM5.

HIH
Jack

*This is a simplification. To do it properly one would need a Raw hystogram or to learn how the sRGB hystogram of one's camera typically maps to the Raw one in a given light.

Hey Jack, is the SOS measurement from the ISO standard an arbitrary value?

You then have the fun task of dropping all the EM5 shots 1 stop in post.

So, what was the benefit in shooting at 3200 again?

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Martin.au
Martin.au Forum Pro • Posts: 11,953
Re: Exposure basics, lesson two point one (& ISO)

Actually, there is an advantage in some situations to shooting at an ISO above what the camera recommends, however I think it only applies in certain situations, and I don't think I've seen you mention it yet, Jack.

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olliess Senior Member • Posts: 1,349
Re: Exposure basics, lesson two point one (& ISO)

Actually, there is an advantage in some situations to shooting at an ISO above what the camera recommends, however I think it only applies in certain situations, and I don't think I've seen you mention it yet, Jack.

What did you have in mind, and does this apply to in-camera JPEGs or the raw capture?

OP texinwien Veteran Member • Posts: 3,326
Re: Exposure basics, lesson two point one (& ISO)

texinwien wrote: Could you clarify a couple of points for me?

Jack Hogan wrote: It appears that if one wanted to capture the very same tonal range from a scene with the EM5 and the G3, same exposure and all, one would have to set the EM5 at in-camera ISO 3200 while the G3 at in-camera ISO 1600, with roughly similar resulting SNR (EM5 slightly better as per DxO).

I'm following your example, so far.

On the other hand, if one set both at 3200 maintaining the same exposure they would no longer be directly comparable, but the G3 would appear noisier to a naive observer.

Would it appear noisier, and would the observer be naive to believe what he saw?

He wouldn't be naive to believe that he saw more noise (he would).

I do not think that this is a given. Exposure and ISO were kept the same in both photos for the E-M5. Exposure was kept the same in both photos for the G3, but ISO was doubled in the second photo.

I think there are at least two scenarios in which the observer would not see more noise in the second G3 photo than he saw in the first.

Scenario #1: We achieved perfect ETTR on the G3 sensor with our choice of ISO and exposure parameters in the first photo, and our entire scene was a uniformly-colored and uniformly-lit, textureless gray card. The second G3 photo will be completely blown out, since we doubled camera ISO, practically halving the saturation point but did not concomitantly half the exposure. All (or at least most) pixels will be blown out, hiding any increase in noise. Note: We could have also taken a picture of something other than a gray card and only blown out some large portion (that I haven't bothered to calculate) of pixels in the second photo - enough to cancel out any noise increase due to bumping up ISO. Additionally, if we were able to salvage some pixels and wanted to use the image, the steps in Scenario #2 would apply to those pixels.

Scenario #2: We achieved something less than perfect ETTR on our first photo, and our scene was of some arbitrary subject (doesn't have to be a gray card). Assume we are shooting RAW and want a uniform brightness in the 4 output images we generate. We are using RAWTherapee to convert RAWs and always start out by setting exposure settings to neutral (backing out any automatic tone or gain adjustments applied by RAWTherapee upon opening our RAWs). Assume that the group of 3 similar photos (the first G3 photo and both E-M5 photos) have the 'correct' brightness. Our 2nd G3 photo will be one full stop too bright, since it received the same exposure at twice the ISO. We will have to adjust the gain of this image downwards one stop in order to equalize its brightness with the other 3 images. My understanding (and feel free to correct me if I'm wrong) is that the noise inherent in an image becomes more apparent as we brighten that image, and becomes less apparent as we darken the image. I think (but have not done the math yet) that darkening our 2nd G3 image by one stop in order to match the brightness of our other images will almost exactly cancel out (at least in terms of appearance or what one can see) the amount of visible noise in this photo.

I believe you have made an error in logic here, and that is that you relied on the DxOMark graphs as predictors of camera behaviour in a test you have set up that, unfortunately for you, does not follow DxO's testing protocols. That is, DxO adjusts exposure along with measured ISO as they test, and their various graphs are based on that. In your test, you have adjusted measured ISO, but have not adjusted the exposure, in sync.

He would be naive in thinking that to capture the same tonal range (shadows and highlights) of the same scene with the same lens and Exposure (ss and f/n) as his friend's EM5 he would have to use the same ISO in the G3. His naivete would cost him a ton of IQ: one stop of highlight headroom and 1 stop of SNR, and he possibly would lose his National Geographic desk

As well he probably should. Any photographer naive enough to think it was safe to just copy the settings from his friend's camera over onto his own camera (of a different sensor generation and made by a different manufacturer) probably shouldn't have a National Geographic desk

The same could be said of a film photographer who copied a friend's camera settings onto his camera, not caring that he (our photographer) was using a different brand of film than his friend, and that he (our photographer) had no knowledge of the characteristics of the film his friend was using. Naive, indeed.

On the other hand an intermediate photographer who understands this subject, would know that to capture the specified tonal range at the given ss and f/n with a G3 he would need to set his camera on ISO 1600 independently of what others need to do to capture the exact same scene and tonal range subject to the exact same artistic constraints. Given the same objectives, his friend with the EM5 and the same lens would need ISO 3200 at the same exposure.

That's correct - perhaps he'll get lucky and be asked to take over the National Geographic desk that was vacated by our hapless naive photographer

What effect does bobn2's statement that "the bigger exposure will win" have in scenario #2, or does it have any effect at all?

It doesn't apply, because exposure stays the same in every scenario here, only ISO changes.

The fact that the exposure stays the same while the camera ISO (and DxO "measured ISO) changes is the very fact that invalidates the predictions you've made regarding your hypothetical experiment.

I look forward to your reply. Perhaps I've overlooked something - it's certainly possible. I'm fallible, after all

tex

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Anders W Forum Pro • Posts: 21,466
Re: Exposure basics, lesson two point one (& ISO)
1

Ulric wrote:

Jack Hogan wrote:

He wouldn't be naive to believe that he saw more noise (he would). He would be naive in thinking that to capture the same tonal range (shadows and highlights) of the same scene with the same lens and Exposure (ss and f/n) as his friend's EM5 he would have to use the same ISO in the G3. His naivete would cost him a ton of IQ: one stop of highlight headroom and 1 stop of SNR, and he possibly would lose his National Geographic desk

I give up; that is, I don't get it. Given the same exposure, why would the g3 on iso 3200 produce more noise than the em5 on iso 3200, but the g3 on iso 1600 would not?

The short answer: Because ISO labels are arbitrary, and in the case at hand Olympus called 6400 what Panasonic calls 3200, so if you set the G3 at 3200 you need to set the EM5 at 6400 to capture the same image information all else being equal. Pehaps an example would help?

You are an intermediate photographer and you have been given an assignement: shoot a game indoors with the same 50mm lens and two cameras, an EM5 and a G3. The desk tells you that anything other than a dof of f/6.3 and the blur stopping power of 1/800s will be accepted, and you need to get the highest IQ possible without blowing the highlights of the brightest object in the scene, the white helmets of the players. Easy, you say. You switch both cameras to full manual mode in order to set the mandated shutter speed and aperture on both - Exposure has therefore been fixed equally on both and will remain set there for the entire game. Then the issue of ISO comes up.

Quick like a fox you remember various DPR threads where this was discussed ad nauseam. You therfore take a sample shot at a guesstimate ISO of 800 on the G3 and see by looking at the hystogram* that the white helmets are more than 1 stop below clipping. You therefore dial in 1600 ISO take another sample shot, verify that the helmets are now all the way to the right of the hystogram just before clipping, and you are all set. You can now fire away, forgetting completely about Exposure and ISO and metering potentially getting fooled by lights or reflections and concentrate solely on getting the shot. You get some great shots.

At half time you move the lens to the EM5 and go through the same set up procedure. You verify that ss and f/n are the same mandated values of f/6.3 and 1/800s (therefore Exposure is the same as on the EM5) and you go through the exact same ISO setting procedure as for the G3. This time though, in order to ensure that the white helmets end up just before clipping you need to dial in a different ISO value. No sweat, the sample images from the two cameras look about the same in terms of noise and eDR as set up (the EM5 a tad better as DxO's graphs suggested) so you fire away forgetting that Exposure and ISO even existed. You get some even geater shots.

Same scene, same light, same lens, same Exposure (ss and f/n), same tonal range (image information) captured. Why are the ISOs different? You then remember DPR threads that mentioned that in-camera ISO labels are not consistent from camera to camera or from manufacturer to manufacturer. So what one calls 1600 the other can call 800 or 3200, the standard allows them to do it. Does it matter? No, as long as you remember that in the same situation when the G3 calls for ISO 1600 that is instead called 3200 on the EM5.

Let's assume, as in the above example, that the G3 and the E-M5 are shot at the same exposure (same light, same lens, same f-stop, same shutter speed). Then consider the following questions and answers.

Question: If the cameras are set to the same camera ISO, say 1600, will the G3 clip the highlights at a lower light level than the E-M5?

Answer: Yes. It follows directly from the DxO "measured ISOs" of 1481 and 782, respectively, that the G3 will start clipping about one EV before the E-M5 does.

Question: Is it, in this particular scenario, important to ETTR for optimal image quality, i.e., make sure that the brightest hightlights of the scene (in this case the white helmets) are brought right up to the clipping point of both cameras, which in this case could be accomplished by keeping the G3 at ISO 1600 and moving the E-M5 up to ISO 3200?

Answer: No. The implication of keeping both cameras at ISO 1600 is that the G3 will express the light levels on an ADU-level scale from 0 to about 4000 whereas the E-M5 will express them on a ADU-level scale from 0 to about 2000. The loss of numeric precision (amount of quantization error) by using 0 to 2000 instead of 0 to 4000 is in this case of little importance since the noise levels at the chosen exposure (i.e., an exposure such that it won't clip the highlights at ISO 1600 on the G3) is too high for that to matter. The read noise (as measured in electrons) is constant on both cameras from about ISO 1600 on so there is no advantage from that point of view to increase ISO on the E-M5 any further. If anything, the E-M5 would enjoy a slight benefit by keeping ISO down to 1600 because it reserves one EV worth of extra highlight headroom, e.g., for specular highlights on the white helmets.

Question: When, then, is it important to ETTR, if, as in this scenario, ETTR is accomplished by raising the ISO rather than by increasing the exposure?

Answer: When raising the ISO brings substantial benefits in terms of reduced read noise (as measured in electrons). Suppose that instead of the choice between ISO 1600 and ISO 3200 on the E-M5, we were considering the choice between ISO 200 and ISO 400. In this case, it would bring substantial benefits in terms of shadow noise to ETTR at ISO 400 rather than to remain at ISO 200 and stay one EV shy of the clipping point. The reason is that read noise is nearly cut in half by going from ISO 200 to ISO 400 on the E-M5.

Question: How, then, can we know whether read noise is reduced or not by increasing ISO at a certain point on a particular camera?

Answer: By checking out the read-noise figures reported by Sensorgen (bobn2) here

http://www.sensorgen.info/

Click on a specific camera model to see its read-noise values for different ISOs. Note that small differences may be due to measurement error and that the values become inreasingly unreliable at higher ISOs due to problems of measurement. In general, you should look for the point at which the read-noise values no longer tend to fall significantly when doubling the ISO.

If results for a certain camera are not (yet) available at Sensorgen but reported by DxO, you can find the essential information via the DxO DR curve. If the loss of DR is less than one EV when you double the ISO, read noise is still falling. When the loss reaches one EV when you double the ISO, it is no longer falling.

While Sensorgen and DxOMark both report the results for the main ISOs (100, 200, 400 and so on), they don't report them for the intermediate ISOs (except for the base ISO). As a rule, that doesn't matter much but exceptions exist. With the E-M5, for example, it is advisable to skip the intermediate ISOs between 200 and 400 since these don't bring the same benefit in terms of read-noise reduction as ISO 400. See here for details:

http://forums.dpreview.com/forums/post/41988325

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Jack Hogan Veteran Member • Posts: 6,207
Don't listen to me, try it for yourself - it's easy

Mjankor wrote: Can you please explain how they can be arbitrary, with respect to the ISO standard?

Hey Mjankor, SOS depends on in-camera processing. No two cameras process an image the same way, in fact there are very large variations, even from mode to mode within the same camera. But don't take my word for it, check it out yourself. Set your camera to a Neutral mode (say Portrait), meter off a uniform wall and take a shot. What value does the wall show in sRGB?

Now change the camera mode to Landscape, meter off the same wall and take a second shot.
What value is the wall now in sRGB? Is it 118? is it the same?

Try both scenarios with your other camera. How consistent are the four figures?

Can you also please tell us what DPR are doing, when they do their ISO tests to the 12232 standard?

"If you want to compare ISOs between cameras from our reviews, look at the ISO test section (on the Noise and Noise Reduction page). This is where controlled test results are reported, essentially using the SOS method of ISO 12232:2006. The rest of the testing is based on knowing this SOS ISO calibration, as all test scenes have specific grey patches that are white-balanced and exposed as closely to L=50 as possible (i.e. within 1/6 stop)." - Andy Westlake

I am not an expert on DPR testing procedures, so I am not the right person to ask. But the first question that comes to mind is 'Do cameras set up as above capture a consistent tonal range, or merely present output images of the same average brightness/contrast' for a given exposure?

And, finally, can you please tell us how every camera I've looked at is consistent in this test? Why do all the cameras agree, if it's an arbitrary standard (oxymoron, I know)?

I assume this is an experiment you've done. Please tell us your results and methodology.

I am starting to get the impression that you are not genuinely interested in my answers but that your questions have an agenda behing them, and that you are never going to be satisfied with any answer I give unless itìs what you want to hear. My opinions have been made clear. Perhaps now you can ask other knowledgrable contributors like bobn2 what they think. Or report back with your findings for the exercise above and answer your own question.

Cheers,

Jack

Ulric Veteran Member • Posts: 4,452
Re: Exposure basics, lesson two point one (& ISO)
1

Ulric wrote:

Right, I'm willing to accept that increasing the iso setting on the g3 from 800 to 1600 will reduce noise.

I am not sure I understand.

Depending on implementation details of the g3, increasing iso may reduce read noise. I don't know enough about the g3 to say if it does or not, that's why I write "I'm willing to accept".

What I wondered about was this exchange:

8<---

Case A

Jack Hogan wrote: It appears that if one wanted to capture the very same tonal range from a scene with the EM5 and the G3, same exposure and all, one would have to set the EM5 at in-camera ISO 3200 while the G3 at in-camera ISO 1600, with roughly similar resulting SNR (EM5 slightly better as per DxO).

Case B

On the other hand, if one set both at 3200 maintaining the same exposure they would no longer be directly comparable, but the G3 would appear noisier to a naive observer.

8<---

[car analogy edited out for brevity]

I don't think the speedometer analogy applies, because the manufacturer iso only applies to ooc jpegs and that's not what we're discussing here, is it?

My question is as follows: Is my understanding correct that in your opinion, case A will result in roughly similar signal to noise ratio, while case B will result in a noisier appearance for the g3?

And if the answer to that question is yes, why would that be?

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Jack Hogan Veteran Member • Posts: 6,207
It's about fair comparisons - not which is better

AndersW wrote: Let's assume, as in the above example, that the G3 and the E-M5 are shot at the same exposure (same light, same lens, same f-stop, same shutter speed). Then consider the following questions and answers.

Question: If the cameras are set to the same camera ISO, say 1600, will the G3 clip the highlights at a lower light level than the E-M5?

Answer: Yes. It follows directly from the DxO "measured ISOs" of 1481 and 782, respectively, that the G3 will start clipping about one EV before the E-M5 does.

OK, but in fact the G3 was the one properly set up for the assignement. So the EM5 recorded 1 additional stop of highlights that were not called for at the expense of 1 stop of shadows that instead were called for (ok 2/3 of a stop to be precise). You are fired

And so with the other questions. AndersW, if you've followed my posts you know that they are only about two issues: that manufacturers label in-camera ISOs inconsistently and that therefore it is often misleading to compare two cameras' performance at the same in-camera ISO aotbe. I did not bring up the EM5-G3 example, ultimitsu did in a thread titled 'DxOMark's measured ISOs vs. manufacturer ISOs', four threads ago.  I merely chimed in with my opinion and for some reason texinwein and mjancor have been chasing me ever since.

I don't care about the EM5 vs the G3 (I am a DSLR guy), what I care about is fair comparisons.

Jack

OP texinwien Veteran Member • Posts: 3,326
Re: Exposure basics, lesson two point one (& ISO)

Ulric wrote:

Right, I'm willing to accept that increasing the iso setting on the g3 from 800 to 1600 will reduce noise.

I am not sure I understand.

Depending on implementation details of the g3, increasing iso may reduce read noise. I don't know enough about the g3 to say if it does or not, that's why I write "I'm willing to accept".

You'll find the answer here: sensorgen.info Panasonic G3 sensor report.

tex

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Jack Hogan Veteran Member • Posts: 6,207
Re: Exposure basics, lesson two point one (& ISO)

texinwien wrote: ...

I do not think that this is a given.

I am glad we agree on most issues.  On the others we'll have to agree to disagree.

Cheers,

Jack

bobn2
bobn2 Forum Pro • Posts: 53,614
Re: Exposure basics, lesson two point one (& ISO)

Jack Hogan wrote:

Ulric wrote:

Jack Hogan wrote:

He wouldn't be naive to believe that he saw more noise (he would). He would be naive in thinking that to capture the same tonal range (shadows and highlights) of the same scene with the same lens and Exposure (ss and f/n) as his friend's EM5 he would have to use the same ISO in the G3. His naivete would cost him a ton of IQ: one stop of highlight headroom and 1 stop of SNR, and he possibly would lose his National Geographic desk

I give up; that is, I don't get it. Given the same exposure, why would the g3 on iso 3200 produce more noise than the em5 on iso 3200, but the g3 on iso 1600 would not?

The short answer: Because ISO labels are arbitrary, and in the case at hand Olympus called 6400 what Panasonic calls 3200, so if you set the G3 at 3200 you need to set the EM5 at 6400 to capture the same image information all else being equal. Pehaps an example would help?

You are an intermediate photographer and you have been given an assignement: shoot a game indoors with the same 50mm lens and two cameras, an EM5 and a G3. The desk tells you that anything other than a dof of f/6.3 and the blur stopping power of 1/800s will be accepted, and you need to get the highest IQ possible without blowing the highlights of the brightest object in the scene, the white helmets of the players. Easy, you say. You switch both cameras to full manual mode in order to set the mandated shutter speed and aperture on both - Exposure has therefore been fixed equally on both and will remain set there for the entire game. Then the issue of ISO comes up.

Quick like a fox you remember various DPR threads where this was discussed ad nauseam. You therfore take a sample shot at a guesstimate ISO of 800 on the G3 and see by looking at the hystogram* that the white helmets are more than 1 stop below clipping. You therefore dial in 1600 ISO take another sample shot, verify that the helmets are now all the way to the right of the hystogram just before clipping, and you are all set. You can now fire away, forgetting completely about Exposure and ISO and metering potentially getting fooled by lights or reflections and concentrate solely on getting the shot. You get some great shots.

At half time you move the lens to the EM5 and go through the same set up procedure. You verify that ss and f/n are the same mandated values of f/6.3 and 1/800s (therefore Exposure is the same as on the EM5) and you go through the exact same ISO setting procedure as for the G3. This time though, in order to ensure that the white helmets end up just before clipping you need to dial in a different ISO value. No sweat, the sample images from the two cameras look about the same in terms of noise and eDR as set up (the EM5 a tad better as DxO's graphs suggested) so you fire away forgetting that Exposure and ISO even existed. You get some even geater shots.

Same scene, same light, same lens, same Exposure (ss and f/n), same tonal range (image information) captured. Why are the ISOs different? You then remember DPR threads that mentioned that in-camera ISO labels are not consistent from camera to camera or from manufacturer to manufacturer. So what one calls 1600 the other can call 800 or 3200, the standard allows them to do it. Does it matter? No, as long as you remember that in the same situation when the G3 calls for ISO 1600 that is instead called 3200 on the EM5.

HIH
Jack

*This is a simplification. To do it properly one would need a Raw hystogram or to learn how the sRGB hystogram of one's camera typically maps to the Raw one in a given light.

Hey Jack, is the SOS measurement from the ISO standard an arbitrary value?

You then have the fun task of dropping all the EM5 shots 1 stop in post.

No need to do it in 'post', just process for the brightness you want. No extra step needed.

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Bob

Ulric Veteran Member • Posts: 4,452
Re: Exposure basics, lesson two point one (& ISO)

Thanks.

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Jack Hogan Veteran Member • Posts: 6,207
Re: Exposure basics, lesson two point one (& ISO)
1

Yes. Ideally it would be blinkies related to the Raw data as opposed to those related to an sRGB rendered image.

Jack, are you aware of any cameras that offer a raw histogram display option?

Unfortunately not, Dan, I've been waiting patiently for years and I know that, at least for me, it would be a very good incentive to buy a new camera (are you listening manufacturers?). This is an issue that has only become relevant in the last few years (since Exmor, really) with PDRs that jumped from nineish (which typically fit within most output devices) to elevenish (which don't). Before most manufacturers squeezed highlights above middle gray in order to capture shadows.  Now they no longer have to as much, so intermediate photographers have choices that are harder to make without a proper raw histogram/blinkies and two separate input referred (a 'gain' of sorts) and output referred (average image brightness of sorts) 'ISO's.

Jack

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