stopping down - coverage?

[sammy_5100]

hi, the xl 72 has 226mm @ f22 , would the nikon 75 sw (200mm f/16) have the same if it were stopped
down to f/22 also?

[george jiri loun]

No, no way!

[sammy_5100]

what would the coverage of the 75 sw be at f22?

[Brian Ellis]

The 200mm of coverage for the Nikon is at F16. I doubt that stopping down one stop would gain an additional 26mm of usable coverage but I don't know for sure. I vaguely recall seeing a math formula for making that kind of calculation, if there is such a thing maybe someone here could provide it.

[Glenn Kroeger]

Most probably, if the Nikkor had significantly greater coverage at f/22, Nikon would have given that in the specs. The fact that coverage is quoted at f/16 probably implies that the lens reaches its maximum coverage at that wider f/stop, which is generally a good thing! The coverage angle for the Schneider is given as 115 degrees at f/22 while the coverage for the Nikkor is 106 degrees which is typical for 8 element lenses before the advent of the XL series.

[george jiri loun]

For such a kind of calculations you can roughly use a simple extrapolation. Between f4.5 and f16 you have app. 4 stops. The Nikon 75mm has a coverage of 126 mm at f 4.5 and 200mm at f16 i. e. 74mm for 4 stops which gives an average of 18.5mm per stop. Add less than 18.5 mm to 200mm and you getles thna 218mm at f 22. Even if the function is not strictly linear you're not far from the correct dimension.

[Ralph Barker]

My skeptical mind is whispering to me that the marketing folks at Nikon saw the Schneider specs at f/22, so they decided to use f/16 so Nikon specs couldn't be directly compared. ;-)

[Nick_3536]

The faster Nikon's are supposed to be optimized for F/16. So the image circle is listed at F/16. That coupled with the larger wide open F/stop. But that's the marketing speak. Anyways I asked about the same lens last month and every answer was it didn't cover 5x7.

[Glenn Kroeger]

Ralph:

Since the Nikkor specs were published by 1980, over 15 years before the 72XL was introduced, I hardly think it was a marketing ploy.

[Brian Ellis]

Just as a matter of interest, image circles for the Nikon 65, 75, and 90mm SW lenses are specified at F16, coverage for the 90, 120, and 150 lenses are at F22. Presumably that's because the different focal length lenses have maximum coverage at different apertures but I don't know enough about optics to know why the shorter lenses would be best at F16 and the longer at F22.

[Nick_3536]

It's not the focal length but the design. Compare the F/8 90mm speced at F/22 to the F/4.5 90mm speced at F/16. Supposedly Nikon is claiming with the faster lenses you don't need to stop down to F/22.

[Bob Salomon]

"why the shorter lenses would be best at F16 and the longer at F22"

diffraction

[Ralph Barker]

Glenn - my comment was intended as a joke, hence the "winkie".

[N Dhananjay]

Trade-offs in design. Note that the ideal in any case is a lens that is diffraction limited wide open but these would presumably be very expensive to design and manufacture. For taking lenses, the lens designer makes the reasonable assumption that a longer focal length will be stopped down a little more to compensate for the lower DOF from the longer focal length and therefore tries to optimize (make the lens diffraction limited) at that f-stop. That presumably frees up degrees of freedom. Thus, an effort to optimize shorter lenses at larger apertures and longer lenses at slightly smaller apertures. You could try to make a lens of any focal length diffraction limited at larger apertures but it can become expensive - the f/4.5 90mm (optimised at f/16) is bound to be more expensive than the f/8 90mm (optimised at f/22) - and if the work you are doing utilizes smaller apertures, both lenses will provide similar performance (i.e., the cheaper one will perform as well as the more expensive one). This can be contrasted to the situation for enlarging lenses where the lenses need to be diffraction limited as close to wide open as possible (to keep printing times short and reduce diffraction loss). Good quality enlarging lenses will typically be fast (large wide-open apertures) and will be diffraction limited wide-open or almost wide open. Cheers, DJ