Curious about max apertures

[Reds]

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Bit of a random question - but why do some lenses only go to f/16 and others to f/22. No particular reason for asking, other than curiosity. Is it a physics or mechanical thing?

 

To do with performance/diffraction? Just wondering in a idle moment!

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[Paul J]

It's a good question. Only those at Leica could give you a definite answer but i'm mostly convinced it's a combination of both of those things.

 

Smaller formats don't need as much depth of field as larger formats, for one.

 

Diffraction always starts to kick in around f8 and by f16 it's sometimes a problem. Some lenses are better than others, but for an example, I won't shoot my 35 FLE at f16 because the diffraction is a degrading the image.

 

I also suspect the limiting of close focussing is also down to Leica's ultra strict lens performance standards.

[smkoush]

I don't see how this is a physics constraint. It is probably mechanical construction limitations in the design. For example, the 28mm Elmarit-M ASPH goes down to f22, while the 28mm Summicron-M ASPH goes only down to f16. Same focal length, two different minimum apertures.

[michaelwj]

Its purely mechanical. To go to a smaller aperture (f/22) the aperture blades have to be large enough to close down that small (cover a larger area). If you want them big, then its harder to get them out of the way for the larger apertures (f/1.4 for example). Usually for this reason, a lens with go from either f/2 -> f/22 OR f/1.4 -> f/16, its the same range, just shifted.

I'd imagine to go from f/1.4 to f/22 would require a much larger set of blades which would require a larger lens barrel diameter, by a fair bit, but also be a lot more delicate, but thats just a guess and someone with more of an insight into the nuts and bolts will hopefully fill us in.

Michael

[Reds]

Thanks guys - I'll keep myself busy today, so I don't dream up any more silly questions ;-)

[wattsy]

Its purely mechanical. To go to a smaller aperture (f/22) the aperture blades have to be large enough to close down that small (cover a larger area). If you want them big, then its harder to get them out of the way for the larger apertures (f/1.4 for example). Usually for this reason, a lens with go from either f/2 -> f/22 OR f/1.4 -> f/16, its the same range, just shifted.

 

A nice elegant theory that sounds right. Maybe one of the self-appointed optical gurus like Olaf could chime in?

[UliWer]

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Former Leitz lenses with longer focal lengthes also had f/32: 90mm-Elmar or 135mm Hektor are the best known examples.

 

I don't think Leitz did this just out of a whim. I presume it had to do with usage of those lenses for scientific, makro etc. together with the Viso-devices. Therefore the largest depth of field was important.

 

Other old lenses from the same time which never had such a "dual use"-function stopped down only to f/12,5 - like the Summar.

[TomB_tx]

Some Leica lenses also changed the minimum aperture during their production runs - possibly due to customer requests or increasing film speeds. My 1939 Summitar stops to 12.5, but my 1948 stops to 16, and the 3.5 50 Elmar changed over it's long run. Perhaps today's faster shutters will mean the smaller stops will change again.

[jdlaing]

Thanks guys - I'll keep myself busy today, so I don't dream up any more silly questions ;-)

 

There is no such thing as a silly question.

[Peter H]

There is no such thing as a silly question.

 

Is that a challenge?

[jdlaing]

Is that a challenge?

 

Gee.

It could be.

[michaelwj]

A nice elegant theory that sounds right. Maybe one of the self-appointed optical gurus like Olaf could chime in?

 

Hi Ian, all I know is its not physics, its mechanical. Although I don't know the mechanical specifics as to why as I've never designed a lens. I'm an optical physicist by trade, and I always thought it'd be fun and really interesting to design a photographic lens.