[Mikesul]

I have been playing with a Rokinon 14mm. on a full frame A7M2. It is incredibly sharp over most of its width but the corners and extreme sides are distorted. The lens is 2.8 at its widest but it is usually necessary to stop it down to get acceptable corners. By f8 it is usually pretty good edge to edge. Under carefully controlled circumstances it can be good on fairly flat subjects even at 2.8. Is this normal or is it likely that I have a bad copy. I am aware of decentering but this does not seem to be in play here since the lens can be very sharp all the way across at smaller apertures. Objects are usually elongated at bit on the extreme edges. Any knowledgeable thoughts appreciated. Thanks. By the way I am a 40 year Pentaxian playing with the A7II in anticipation of our own FF. Our lenses produce astounding images with the big sensor.

[hjoseph7]

It just amazes me how people come up with these mysterious cameras and lenses that nobody has ever heard from. So what the heck is a A7M2, please ?

[robthebloke]

It just amazes me how people come up with these mysterious cameras and lenses that nobody has ever heard from. So what the heck is a A7M2, please ?

Sony a7 II.

[dms]

All (yes all) very wide lenses suffer from the apparent distortion near the edge--for example circles become oval shaped.

Oh, sorry, you are referring to loss of sharpness--that is because the plane of focus in a (very) wide angle is likely more of a section of a sphere--(and the loss of sharpness/loss of focus) because as the distance from the lens to the center (of the flat area being focused on) is less than the distance to the edges/corners. The flatness of the plane in focus is subject to the lens design, so some are better than others, but very wide angle lens make it very tough to do.

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Last edited by dms; 01-24-2015 at 08:31 PM.

[Digitalis]

All extreme wide angle lenses need to be stopped down to f/5.6-f/11 range to achieve even Image quality across the frame, it comes with the territory. Astigmatism is the principal culprit of corner softness, aspheric lenses can counter it to a degree however they need to be complimented by ED glass or fluorite to reduce dispersion. Fast lenses need large diameter aspherics which are difficult and expensive to produce*. Even in a 14mm f/2.8 lens designed for FX format, the elements are larger in diameter than they would be for a slower zoom lens of comparable focal length. Even the Nikkor 14-24mm f/2.8G has problems with corner softness and distortion.

*though this depends on the method of production and the design of the elements themselves.

[Tom S.]

Moved to proper area.

[Mikesul]

All extreme wide angle lenses need to be stopped down to f/5.6-f/11 range to achieve even Image quality across the frame, it comes with the territory. Astigmatism is the principal culprit of corner softness, aspheric lenses can counter it to a degree however they need to be complimented by ED glass or fluorite to reduce dispersion. Fast lenses need large diameter aspherics which are difficult and expensive to produce*. Even in a 14mm f/2.8 lens designed for FX format, the elements are larger in diameter than they would be for a slower zoom lens of comparable focal length. Even the Nikkor 14-24mm f/2.8G has problems with corner softness and distortion.

*though this depends on the method of production and the design of the elements themselves.

Thanks, Digitalis. This is what I was hoping to hear. I worked with it some more yesterday afternoon taking test shots at a variety of f stops and trying to focus closer to the camera. Most of the distortion could be taken care of that way. I would rather not go through the hassle of returns (or bother dealers) switching copies as some on the net have suggested is necessary. The lens is sharp in all other parts of the frame at any settings. Even at 2.8 a close up of an antique print could be made sharp edge to edge by taking great care with the alignment. So my tests combined with Digitalis' analysis have convinced me to continue working with this copy. Even some of the test shots on LensTips showed the distortion and their reviewer was very pleased with the lens.