Back by popular demand: Nex-7 vs OMD

Started Dec 4, 2012 | Discussions thread
jtyoung Contributing Member • Posts: 526
Re: Back by popular demand: Nex-7 vs OMD - Part II

Promit wrote:

CosmoZooo wrote:

At ISO 6400 on OMD you're not getting ISO 6400 your only getting 3200, so if there is not enough light the photo will be too dark, the NEX will perform better if ISO 6400 is actually needed. And if there is enough light for 3200 then you can actually drop the NEX to 3200 and again it will perform better, because the OMD will again be darker at 3200 - Considering that shutter and F-stop are the same.

You've critically misunderstood how the measured vs actual ISO works.

The OMD will not be too dark at 6400. The file will be exposed at a physical level that is equivalent to ISO 3200, and then it will be pushed up by one stop when the RAW is converted. That conversion might be in-camera JPEG, it might be ACR, it might be DXO, whatever. Doesn't matter. The final photo will not be too dark. It will show lots of shadow noise due to the pushed processing, it will show reduced DR, but it will look exactly like an ISO 6400 exposure ought to look.

The measured ISO that DXO computes is relevant to the final dynamic range and noise level of the image. It is NOT relevant to exposure or image brightness. "Extended ISO" settings on many DSLRs work exactly the same way.

That's not entirely correct. What you're describing is the linear response curve that a lot of sensors exhibit -- a response curve that argues strongly that it only really matters to take photos at the base ISO.

Regardless, the point of the prior discussion is that the current standard for exposure measurement at a given ISO level has a great deal of latitude resulting in some cameras exhibiting greater sensitivity at a stated ISO level than others. Thus, if you really want to test the same scene at the same apparent exposure you may have to adjust other factors (like shutter speed) to match the variations in true sensitivity at a given stated sensitivity. In the case of the NEX 7 vs OMD, the factor that the OMD required twice the shutter time as the NEX indicates that at the given stated sensitivity level, the OMD is only half as sensitive.

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