APO problem with Fuji lenses and Leaf digital back?

[alan_dale]

Any other photographers using a Leaf back on Fuji GX 680? When I focus

on objects 3-5 feet away, no problems. Larger things--- 15+ feet away

from camera - and I get colorful registration problems --- after

registering one part of image in software, other parts look

unregistered, as if the camera is focussing the different colors at

different distances. Problem is worse with 50mm lens than with 65mm,

80mm or 135mm. Rule of thumb seems to be that the wider the lens, the

greater the problem. Problem dissapears when I focus close. Problem

remains consistent if I switch from 1 specimen of lens to another

(studio camera -- have access to several 50mm lenses, several 65mm

lenses, etc.).<p>

Have switched out cameras, magnifiers, adjusted focus too many times to

count, locked up mirror, loaded camera stand with sandbags to supress

vibration, etc., etc., etc.

Anyone else been here? What gives?

[larry_albert]

Alan,

 

Have you shot any film with this camera? Does film show any signs of color fringing/outlining that would indicate different focal planes for different colors? (chromatic aberrations?)

 

If the film does not show the same problem as the Leaf back; the problem is with the Leaf. It could be that when mounted the image chip is NOT absolutely parallel to the focal plane. (think of film not being totally flat)

 

I have no idea if the problem is in the back-camera mounting

plane or the chip alignment in the back. Would it be possible to try your back on a second camera?

 

Your details indicate that the problem becomes more noticeable when the lens is closest to the focal plane. (shorter lenses and greater subject distances. If there is an error of one tenth of an millimeter in the "film plane" these shorter distances would have a greater percentage of error. (One part in 500 for a 50mm lens - 0ne part in 2000 for a 200mm lens)

 

I reach this conclusion based on experience with TV cameras. The face of the pick-up tube has to be as close to parallel, as possible, to the focal plane so that sharp focus is had across the entire image.

 

 

That is my .02 USD worth.

[steve_rasmussen]

The difference in focus of colors in a lens as short as 50mm will not produce enough color separation to cause your problem. The difference in focus for your 50mm should be around .005mm if it's well corrected.