Dr Peter Gilbert <peterg@xxxxxxxxxxxxx> moved upon the face of the 'Net and
spake thusly:
> Firstly, I know that a 1.4x TC will effectively reduce the light falling on
> the film by 1 stop ( and a 2x TC by 2 stops). Many people say "it reduces
> the aperture" but is this technically correct? Does it increase your depth
> of field to that which would be obtained at the smaller aperture?
A f=x lens at aperture N with a 2x TC behaves in all respects like an
f=2x lens with aperture N*2.
That is, the actual f-stop is 2-stops down from whatever the lens
says.
The "aperture number" (aka f-stop) is the diameter of the aperture
expressed as a fraction of focal-length.
If your lens has aperture set to f/4 (i.e. diameter of the iris
diaphragm is one quarter of the focal length) and and you double the
focal length, you now have an aperture of 2f/4, or f/8.
> If I add a 2x TC to get effectively a 500F4, and leave it set at F4 on the
> lens, do I still get y depth of field? Or do I get the depth of field as if
> I am using the lens set at F8?
If you put a 2xTC on a 250,f/4 lens, you now have a 500,f/8 lens (that
probably happens to focus closer than most "real" 500s).
cjb.
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