Increased coverage as you stop down

[Tom Stanworth]

Hi,

this is related to the geronar thread....If the 300mm f9 geronar gives a circle of 340mm aT F22, will this increase much as I stop down further. I would imagine it will be used at f32 more often than not, as f16/22 is where my 5x4 lenses see most use. I know some designs do not increase coverage much below a certain f stop (peak at f22 or so), what about this triplet design lens? Certainly more coverage would be handy, though as I am a field user I dont need to much coverage. 360mm or so at f32 would give more wiggle room.

How can one tell if a perticular lens will continue to pick up coverage as stopped down or not (I have heard that many older deisgns like the dagor/g clarong continue to gain coverage, whereas many more modern ones do not and in fact may decrease???). Are there rules that can be applied to particular designs?

Cheers,

Tom

 

[Ole]

All lenses go "fuzzy" at the edge. As you stop down the size of the acceptably sharp area will increase.

Some old wide-angle lenses, like the Angulon, take this to the extreme: The 90mm/6.8 Angulon covers 9x12cm at f:16, 4x5" at f:32, and if you could stop it down to f:64 it would cover 5x7".

But some lenses are built to physically vignette off the unsharp area, most notably those made to be used at full aperture (the Zeiss Planar 135mm/f:3.5 is the only example in my lineup). Others are vignetted at "optimum aperture", so than no further increase is to be had at some point.

 

[Thilo Schmid]

Tom,
stopping down a lens does increase coverage as long as the barrel vignetts (artificial vignetting). Open the aperture and take a look at the exit pupil from the back. Increase your viewing angle until it is totally obscured by the barrel. Now close the aperture and see what happens. It is also easy to see that this effect decreases with smaller apertures. So f32 won't really help.