XF56 f1.2 released and specs

Started 11 months ago | Discussions thread
Krich13
Regular MemberPosts: 466
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Re: XF56 f1.2 released and specs
In reply to malcolml1, 11 months ago

malcolml1 wrote:

Yes, I agree on the noise and detail argument.

If you agree on that, it is not clear to me what are we all arguing about. The angle of view is going to be the same, same DOF, same shutter speed (motion blur) and same signal-to noise... For all protographic intents and purposes, 56/1.2 on APS-C is equivalent to 85/1.8 on FF.

I was just curious about the exposure bit. Another way of posing the question is perhaps in terms of a given field of view of a scene. The 56 mm Fuji lens has roughly the same FoV as an 85mm lens on a FF sensor. If both lenses have their aperture open to give an f number of f1.2, would the exposure time be the same to get the same total exposure on the sensor?

Depends how you define "exposure". In the old film days, it was indeed "exposure PER UNIT AREA" regardless of frame size. Deliver too few photons, and the film is underexposed, milky what not. Deliver too many, chemical changes facilitate turning the entire frame black.

For semiconductor sensors what matters is the total number of photons per photosite, pretty much regardless of the site area. Make two sensors: say an APS-S and FF ones both of say 16 megapixels. Expose them using the same scene for the same duration using 56/1.2 and 85/1.8 lenses respectively. Each photosite will receive the same NUMBER of photons (the number per unit area is 2.25 times larger in the first case, but the area itself is smaller by the same factor). Each photosite (given equal quantum efficiency) will produce the same number of electron-hole pairs (photocurrrent) ergo the same signal and the same shot noise. If you read each sensor with the same preamplifier, equal Read noise will be introduced in both cases, as would be required amplifier gain!

However, a Photomertist would assign the first case say ISO 100, and the second -- ISO 225 based on exposure PER UNIT AREA, even though the physical amplifier gain is exactly the same in both cases.

Must admit to being a bit confused.....

The origin of your confusion is extrapolation of ISO sensitivity concept to modern sensors -- irrelevant one for digital sensors, the proper metric would be "light delivered to the whole sensor")

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