thank you all for your kind comments and some interesting history information. Its nice to learn things (even non photographic things) in the course of discussion.
Tom, please feel free to post here if you wish. The thread has created some interest in this small and favoured lens of Mongo.
Akira, re the 20mm f3.5’s use on FX and the question of field curvature:-
Mongo used this lens a lot with film cameras and had no complaints. Mongo had little reason to try and use it with his first digital camera (D200 being DX) because it would not really give a full 20mm wide image. So, he did not really use this lens during that period. However, when he went to a D800 and other FX bodies, he uses it extensively , again, without any problems - including field curvature that would be noticeable enough to require post processing correction. In fact, non of the images ever taken by Mongo with this lens on film or FX have ever needed correction.
If you look at the water line of image #1, you will notice that while Mongo has made an error in tilting the camera slightly downward to the right, the line is nonetheless straight. Mongo also noticed a small post processing error on that waterline also when looking back at this.
By comparison, a lot is made of the qualities of the Nikkor 14-24mm f2.8. Mongo had borrowed this lens for one corporate shoot of Board Members for their annual publication and found the lens to give atrocious results in relation to field curvature. The image was shot at 24mm using Mongo’s D4s. The horizontal timber beam across the top of the veranda and the brickwork of the of the lovely building used as the background for the group Board photo looked bent like a banana. Mongo was shocked and stuck with having to drastically correct the curvature in post processing. Needless to say, Mongo would never consider buying the 14-24mm f2.8 after that experience.
So, in simple examples when comparing a 30 year old lens (20mm f3.5) with much more modern and corrected lenses (e.g 14-24mm f2.8 ), Mongo can honestly say to you that the older lens left the modern more expensive lens for dead in relation to field curvature.
Bjorn’s observations are no doubt correct also and he has infinitely more experience than Mongo over infinitely more equipment. Bjorn also has a finer eye for fine detail when assessing a lens and comparing it to others.
Mongo can only tell you his experiences with various equipment he has used and tell it to you exactly the way he sees it without fear or favour. Equipment will only gain Mongo’s respect if it does well that which it claims to. Granted, this will only ever be relative in relation to equipment as there are no absolutes. Hope this is helpful to you and happy to try and answer any other questions if he can.
Akira, Mongo would suggest you borrow a FX body just to try a few frames on subjects that are likely to clearly indicate the extent of any field curvature using this lens. If you were not so far away, Mongo would gladly lend you one of his bodies.
PS - for landscapes, Mongo uses f8 and sometimes even f11. This is only because the MTFs for this lens show optimal performance at f8. Not sure whether this has any effect on correcting any field curvature or not.
