No, the focal ratio. From Wiki on diffraction:
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Diffraction-limited imaging
The ability of an imaging system to resolve detail is ultimately limited
by diffraction. This is because a plane wave incident on a circular lens
or mirror is diffracted as described above. The light is not focused to
a point but forms an Airy disk having a central spot in the focal plane
with radius to first null of
d = 1.22λN
where λ is the wavelength of the light and N is the f-number (focal
length divided by diameter) of the imaging optics.
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If you're using a 50mm lens in macro work at 1:1 the lens is extended to
100mm or twice its focal length. A marked focal ratio of f/16 is only
valid at infinity or when the lens is extended to its focal length of
50mm. 50mm/16 gives a physical aperture of 3.125mm. If the lens is at
1:1 magnification then the extension is 100mm and the effective aperture
is 100/3.125 = f/32. That's why a 1:1 magnification ratio requires a 2
stop exposure increase.
Perhaps complaints about using the Zuiko 135 macro lens at f/45 showing
considerable diffraction blurring is because the effective focal ratio
in macro work may be much larger than the marked f/45.
Chuck Norcutt
On 6/1/2013 3:09 PM, Mike Lazzari wrote:
>> And that's only the labeled f/stop. The actual f/stop is only valid at
>> infinity. The effective f/stop get numerically larger as the extension
>> increases.
> Isn't it just the physical aperture which is at issue with diffraction?
>
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