Thank you! Yes, I completely forgot that "equivalent ISO settings are related through R squared."
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mogul wrote:
ISO means nothing as each manufacturers have their own idea of digital ISO.
While there is some variability in ISO, there is not as much as you'd think and definitely not in this case. For example from ruthenium's original post:
300mm, SS (ISO): 1/80 (200), 1/200 (500), 1/400 (1000), 1/640 (1600), 1/1250 (3200)
200-600mm at 600 mm, SS (ISO): 1/30 (200), 1/80 (500), 1/160 (1000), 1/250 (1600), 1/500 (3200)
Properly applying equivalence we know that if we want to think of a M43 300mm lens as an equivalent FF 600mm lens, we have a crop factor of 2 and therefore the Olympus lens is equivalent to a f/8 FF lens and (8^2)/(6.3)=1.61 times slower or 0.69 stops slower. We also know that equivalent ISO ratio is the square of the crop factor, in this case this is 4.
Putting together the above two facts, we expect an equivalent FF shot (i.e. one that captures the same total light for the same shutter speed as a M43 one) to use an ISO 4/1.61 = 2.48 times higher. And what do you know, the ISO ratio between the M43 and FF shots at 1/80 of a second is 2.5; very close to our calculated value of 2.48. Bear in mind that although the FF shot has an ISO 2.5 times higher, this shot actually has noise comparable* to a 500/4 = 125 ISO shot on M43; that is to say 0.69 stops less noise. This is because the 200-600 at f/6.3 collects 0.69 stops more light from its larger relative aperture.
*[This statement assumes two FF and M43 sensors from a similar sensor generation with similar noise behavior. This is not broadly true for the A1 and OM-1 MII to my knowledge, but provides a basis for comparison]
Jun 22, 2024 at 12:42 AM
Previous versions of userjjb's message #16577499 « Sony A1 & 200-600mm against OM-1 II M.Zuiko & 300mm »
