# 60/2 Apo-Lanthar E testing started

Started Sep 20, 2017 | Discussions thread
Re: 65/2 Apo-Lanthar E testing started

JimKasson wrote:

If there are tests anyone would like me to run, please let me know.

Jim

I'm interested in field curvature wide open, perhaps at three distances: closest focus, infinity, and somewhere in the middle. Saying something about field curvature particularly at infinity does not sound easy, but I would love to know if it is possible to investigate, even if it must remain a theoretical question.

Weird idea alert: Since it is difficult to draw a diagram of the plane of focus at infinity, I'm thinking the units could match locations on a lens ring. I think it might be called a harmonic scale, but "harmonic scale" is taken by another discipline. I've never seen this idea tried, but it would seem cool and useful to know that when focusing the center of the image at infinity, the objects off axis are sharpest at particular other distances (as a function of distance off axis).

For example, if at 20mm off axis on the sensor the sharpest distance is 100 feet, then drawing a line in harmonic units (if I'm being clear), we might guess that at 10mm off axis the sharpest distance is 200 feet---the harmonic mean of 100 feet and infinity, if you'll recall from our discussion here:

https://www.dpreview.com/forums/post/57800235

We managed to end up agreeing on quite a few issues in the end.

Anyway, some way of knowing something about the point of sharpest focus anywhere off axis, even if only an approximation, could be quite useful in the field. It might even help you choose your lens, your composition, or where to focus. That kind of information has to be useful, but it seems difficult to come by.

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Jerry Fusselman

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