# XF56 f1.2 released and specs

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

Yes, I follow your argument apart from one step which actually represents my original question:

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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).

I agree with the area and scaling factor part, but my original question was whether it is true that the 56/1.2 will deliver the same number of photons per unit area on the APS sensor as the 85/1.8 lens on a FF sensor. I'd like to understand this part - Of course I understand that on an APS sensor the 56 lens results in an image with the same field of view as an 85 lens on a FF sensor.

Looking at the 85 mm lens and your statement that the 85/1.8 is equivalent to the 56/1.2:

Suppose we are looking at a uniformly lit scene.  Assume the 56/1.2 lens receives a total of N photons at its aperture.  These N photons are then spread over the APS sensor (ignore any losses at this point).  On the 85 lens, to get the same physical aperture area I agree that you need to stop down the lens to f1.8.  This will also capture N photons (uniformly lit scene, same number of photons per unit area).  However, in this case, the sensor area is now a factor of 2.25 larger, so those N photons result in fewer photons per unit area on the FF sensor (by a factor of 2.25).

To me, the only way to get the same number of photons per unit area on the FF sensor is to open up the 85 lens to f1.2........

The next stage - how those photons are transformed into an electrical signal is not something I know much about.  Maybe the FF sensor is so much more efficient than the APS one - but that is not an argument about the number of photons captured by different lenses any more.

Malcolm

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