Effective "f" number

[sk66]

An FX lens will not be faster on a DX body.
Yes, it is letting in a larger circle of light but it is not falling on the sensor (the "extra" light is absorbed by the black/textured interior of the camera). To the sensor it is the same.

[lerabu]

Thanks sk66!

here is a cool app with an effective answer!

Depth of Field Simulator

[Thunder_o_b]

Then you have the MP-E65. It is a lens that is a f2.8 lens wide open, but as you increase mag the effective f stop hits f16 at 5:1 mag wide open at f.2.8, and at f.11 where I normally shoot, at 5:1 it is a staggering f. 66

MPE65

[SusanH1970]

Originally Posted by lerabu

Thanks sk66!

here is a cool app with an effective answer!

Depth of Field Simulator

That's cool!