Where are lens boffins when U need them??

[Tom Stanworth]

Hi,

No insult intended. I posted a question about that famous photo of Ansel A, Moonrise. In his notes he stated that he advanced the focus by x amount to take account of the single component of his cooke lens at f32 (or words to that effect). I would love for somebody to explain what he was talking about and whether one would have to 'compensate' for different apertures using single element of the new very expensive and much applauded Cooke XVa triple convertible. If it does apply and nobody knows about it, there will be a lot of blurry images being produced by these lenses

 

[David A. Goldfarb]

I guess he was compensating for "focus shift" when stopping down, which is a characteristic of some lens designs.

"Focus shift" is a bit of a misnomer, because it seems to suggest that the focal length of the lens changes as it is stopped down. What's really happening is that as one stops down and spherical aberration is corrected, the point of sharpest focus might appear to change. If I'm using a lens that is considered to have focus shift (like a Dagor, for instance), I might check focus at an intermediate stop and the shooting aperture to be sure.

As to whether the new Cooke convertible exhibits focus shift--I'd ask over in the Cooke thread on f32.net, since Barbara Lowry monitors that board, and there may be one or two folks who actually own the lens and can report back.

 

[Alex Hawley]

Quoting from "The Negative, page 127, "I focused and composed the image rapidly at full aperature, but I knew that because of the focus-shift of the single lens component, I had to advance the focus about 3/32 inch when I used f/32".

And that's our Gospel quote of the Evening.

David's explanation is spot on. Adams was compensating for the focus shift that he knew occurred due to the change in spherical aberration.

This example also illustrates just how adept Adams was at handling his equipment. He also states that these adjustments were done intuitively. So was the exposure setting and he rattles of a luminance number for a full moon that he uses to calculate the exposure at f/32. Regardless of whether he ran through all these numbers at the time or whether he filled them in when writing the explanation, it still shows there is a lot to be said for being familiar with one's tools, so familiar that it becomes intuitive.