Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
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Re: Exposure basics, lesson two point one (& ISO)
In reply to Jack Hogan, Mar 19, 2013

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.


*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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