There is no magical size/weight advantage

Started Mar 5, 2014 | Discussions thread
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Re: you shouldn't - smaller formats typically have bigger lenses
In reply to razormac, Mar 6, 2014

F-stop = is the ratio of the lens's focal length to the diameter of the entrance pupil.

F-stop = FL / Diameter

Diameter = FL / F-stop

The lens FOV is approximately equal to the ratio of the sensor size to the lens's focal length (using a trig approximation for narrow angles.)


To compare equivalent systems, we would want to keep the FOV the same.


Putting it all together:

Diameter = SS / (FOV * F-stop)

(or F-stop = K * SS / Diameter, which is what I meant by F-stop being scaled to SS, sensor size.)

So, for the same field of view, the lens diameter is proportional to the sensor size and inversely proportional to the FOV and the F-stop.

The same F-stop will give the same light density, and the same noise per area. When enlarged to the same print size, the smaller sensor will have more visible noise and more DOF unless, instead of keeping the F-stop constant, you keep the lens's entrance pupil diameter the same.

In short, for smaller systems, everything scales to give the same quality per unit sensor area, but because of greater enlargement, the final print quality is lower. The only way around this is to use a F-stop that decreases in proportion to the crop factor (equivalence.)

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