DoF - straight or curved?

[Bend The Light]

Are you sure that wood is not curved. I'll beleive you if you say yes but how is it the wood has field curvature, but the wings are not showing the same bend?

Pretty sure, yes. But i think the wings ARE showing signs of the bend.
The wings on the dragonfly are both pointing forwards...see this image (a different dragonfly, different place):


A four-spotted somethingorother dragonfly by http://bendthelight.me.uk, on Flickr

The wings are actually floowing the curved focus locus, until they go directly behind the head. The focus locus takes in the head, not behind it (obviously that's where my focal point was).

What do you think?

 

[BrianV]

How old is this lens, and has it ever been to a repairshop, or at least show signs of being opened?

I'll try a couple of shots with a Micro-Nikkor to show the flatness of field. I've used them to document construction of circuit boards. The images look nothing like this.

If an optic is inserted backwards into a lens during reassembly, you can get results like this. Center sharp, with high field curvature. Once, someone sent me a Leica Summicron that would not focus correctly. Three elements/groups were in Backwards. That took a while to figure out.

 

[Bend The Light]

How old is this lens, and has it ever been to a repairshop, or at least show signs of being opened?

I'll try a couple of shots with a Micro-Nikkor to show the flatness of field. I've used them to document construction of circuit boards. The images look nothing like this.

If an optic is inserted backwards into a lens during reassembly, you can get results like this. Center sharp, with high field curvature. Once, someone sent me a Leica Summicron that would not focus correctly. Three elements/groups were in Backwards. That took a while to figure out.

I would say not, but could never be sure. It is an old tank of a lens, taking 3 full turns to get to 1:1, and extends to some 2.5x it's length in doing so. Weighs a ton. If it'd have been dropped I expect the floor to yield!

In the end, I could be wrong - the wood could have a curve, and at this magnification, that curve could be unnoticable to the naked eye but picked up easily by the magnification of the lens...

But I still thought it was an interesting phenomenon to explore, and have enjoyed the debate/thoughts.

 

[Helen B]

It might be worth doing a test with a truly plane surface first (at an angle, not perpendicular), and trying a few distances to see how the field curvature changes.

As you have realized, the terms used in casual discussions of lens behaviour are not always the technically correct ones, but most of the time we know what the user intends, and using the correct terminology may end up in unintended confusion or be excessively verbose. If you would like a guide to the technical terms it might be worth starting another thread. Believe it or not, the plane of focus really is a plane, but it may not be where the lens is focused, apart from at the optical axis.

 

[Bend The Light]

It might be worth doing a test with a truly plane surface first (at an angle, not perpendicular), and trying a few distances to see how the field curvature changes.

As you have realized, the terms used in casual discussions of lens behaviour are not always the technically correct ones, but most of the time we know what the user intends, and using the correct terminology may end up in unintended confusion or be excessively verbose. If you would like a guide to the technical terms it might be worth starting another thread. Believe it or not, the plane of focus really is a plane, but it may not be where the lens is focused, apart from at the optical axis.

No, I understand that. I understand that the plane of focus IS perpendicular to the lens axis. I am just being pedantic, as my mathematical brain allows.

 

[BrianV]

Try another test, as Helen suggested, on something that is flat.

Attached: Close up of a lens being rebuilt, Photographed with a micro-Nikkor 60/2.8, showing good flatness of field.

Second image: a Nikon S3 rangefinder camera with a Zeiss Sonnar 50/2. The RF coupling is good, the high field curvature is due to the rear group being in backwards. Fixed it, second roll of film was much better. What's really odd: the lens was misassembled and then Shimmed to work properly with the rangefinder of the camera. I paid $25 for the lens, now- it is quite good.