A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: The role of ISO

Great Bustard wrote:

Dr_Jon wrote:

The only issue I have with all the equivalence talk, other than that it is the the topic that "keeps on giving" is that when I put a 70-200 f.28 lens on my FF camera my first thought isn't "Ah, the sensor is twice the area of my GH3 so I should immediately crank up the ISO two stops".

Although it is fair to say GH3 ISO is really equivalent to a bigger number than FF ISO in some ways (i.e. noise) and just reflects the smaller sensor needs more "gain" to get the exposure and so the ISO is just what makes the maths work. (People do seem to see ISO as some thing that is equivalent between systems, rather than being implemented as a fudge-factor to make exposure maths work).

First of all, please take a read of gollywop's outstanding post on exposure to see what ISO is all about:


Secondly, if "when I put a 70-200 f.28 lens on my FF camera my first thought isn't 'Ah, the sensor is twice the area of my GH3 so I should immediately crank up the ISO two stops'." then you are thinking wrong.

Instead, you should be thinking that FF will use two stops higher ISO than mFT for the same DOF and shutter speed as mFT.  But often, when there is ample light, FF will be able to shoot whatever DOF they want and use a lower shutter speed than mFT.

For example, while the FF photographer might shoot 200mm f/5.6 1/400 ISO 1600 as opposed to 100mm f/2.8 1/400 ISO 400 on mFT, they might also shoot at 200mm f/5.6 1/400 ISO 100 as opposed to 100mm f/2.8 1/1600 ISO 100 on mFT, light permitting.

Not if you have this:


By such means the MFT photographer in the second of your two examples would be able to lower the shutter speed to the same level as the FF shooter (1/400), lower the effective ISO to two EV below that used by the FF photographer (ISO 25 versus ISO 100), and reap at least some of the potential benefits of the increased exposure. That is the point of the magenta filter that you were wondering about.

That said, I am looking forward to arbitrarily low camera ISOs (without read-noise penalties). That would effectively eliminate any FF advantage in ample-light shooting (except when DoF is in very low demand). There are some hopeful signs on the horizon here:



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