250mm C

[stewart_skelt]

Hi all,

I have just bought a 250mm C lens in (apparently) excellent condition. I plan to take it away on a vacation shortly so I will not have had the opportunity to take a lot of pictures with it beforehand. I remember reading somewhere that the performance of this lens is not so good at small apertures - what would people recommend as a good compromise aperture for DoF and sharpness, given that I will be mainly taking landscapes?

Thanks

Stewart

 

[rlwmd1]

Wildi's manual discusses the loss of sharpness due to diffraction at very small apertures but says that the Zeiss lenses control for that by limiting how small the aperture can be. IE, if a lens looses sharpness at f32 it will only go to f22. Having said that, I think I get sharper images one or two stops off the minimum f stop.

 

[qnu]

I always use this lens wide open. It is excellent wide open, and will not show any improvement when stopping down.

You will not gain much DOF by stopping it down either. Nor, i think, will you want to gain very much DOF; that's part of the beauty of long lens photography.

The only time i stop it down is when i run out of shutterspeeds.

Diffraction is one of those unavoidable thingies. Stopping down will always limit the maximum possible resolution. Even in the large aperture range. In fact, resolution is halved every two stops you stop a lens (any lens) down.

It is true, however, that most lenses are not free from lens faults that will limit resolution even more than diffraction will. Especially when used at large apertures. So stopping down moderately will reduce lens faults, while not increasing diffraction too much. Hence the ubiquitous "compromise" advice that says a lens is best moderately stopped down.

That does not apply to this particular lens, the 250 mm Sonnar. Performance will not (!) improve with reduction of the aperture.

So all that happens when stopping down is increasing diffraction. even when going from f/5.6 to f/8.


The Wildi advice is sensible (lenses still perform very well when stopped down, so there's no real need to worry about using the small aperture end of the scale. If only because lenses outperform most film anyway) but must not be taken to mean that image quality will only start suffering due to diffraction somewhere beyond smallest aperture you can set.