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john wilton
7-Aug-2009, 11:24
I am the lucky recipient of a couple of B&L Protar triple convertibles...a small one, 7" combined, and larger, 12" combined.

I've looked through the archives, but have a few questions remaining.

When using only one of the cells (behind the shutter of course), is it sufficient to stop down just one or two stops to get the best performance?

Dealing with focus shift: It's extremely hard to see anything at the smallest stops. (In real life situations there often is no proverbial shiny sunlit fender.) With the BTZS focus hood I can see well enough at f/22 to focus. Has most focus shift already occurred by the time an f/12.5 cell is stopped to f/22? That is, can one focus at f/22 and shoot at say f/45?

EdWorkman
7-Aug-2009, 13:52
It has been a long time
I have an 11 3/8 VIIa, made up of nominal 18 and 24 inch cells
I recently found some oooold notes .
From them I believe I used the 24 cell in front of the shutter and experienced a 3/8 inch shift. I did not record the shooting aperture, but probly 45-64, focussed "wide" open initially.
Single cell infront/inback has been discussed thoroughly hereabouts. I have found older literature by mfgers that insist on one way, or on the other way, same as photographers, so I use the easier way.
This isn't a direct answer, but I hope you find it useful

Vick Vickery
7-Aug-2009, 14:09
I've found that focus shift can happen at any stopped-down point, but often gets worse as you stop down further (of course some lenses don't exhibit any shift at all if you're really lucky), so I make it a habit of refocusing after stopping down all the time in order to be sure to get the best results possible. A flashlight shinning back toward the camera or a lazer pointer can be worth their weight in gold! :)

benrains
7-Aug-2009, 15:05
I've found that focus shift can happen at any stopped-down point, but often gets worse as you stop down further (of course some lenses don't exhibit any shift at all if you're really lucky), so I make it a habit of refocusing after stopping down all the time in order to be sure to get the best results possible. A flashlight shinning back toward the camera or a lazer pointer can be worth their weight in gold! :)

I hope you mean using the reflected light from a laser pointer as a "targeting device" of sorts, and not pointing it back toward the camera like the flashlight suggestion. The way you've written it is a little ambiguous!

Vick Vickery
7-Aug-2009, 20:38
Right, Ben...the flashlight would be placed in the scene pointed toward the camera or the pointer would be pointed into the scene from the camera position. Thanks for pointing that out. :) Wouldn't want to damage any eyes!!