Help me understand the physics - F-stop and FOV

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MarkG Regular Member • Posts: 399
Help me understand the physics - F-stop and FOV
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Something that has been bothering me for awhile:

Let's say we have a FF camera with a 600mm lens with a 95mm front element diameter (ignore F-stop for now).

Let's also say we have a m4/3 camera with a 300mm lens with a (hypothetically) 95mm front element, as well.

AOV is the same. The light collected by both lenses should be the same.

Let's ignore the transmission index through glass and say that it's 100% in both cases.

The image circle on FF sensor is twice as big as on m4/3 sensor, however the total *illumination* on FF and m4/3 sensor *should* be the same.

So, if total illumination (amount of light) is the same on FF and m4/3 sensors, then why should we see any low-light-related differences in IQ?  It should be the same IMO.

Where is this logic broken?

(Now, Oly 300mm F4 is actually 77mm front element so we can expect to see some loss of light vs 600mm/95mm FF lens, in this case~1-stop (95/77 ^2 =1.5))

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