Fresnel Lens Focusing Accuracygreenspun.com : LUSENET : Large format photography : One Thread
I have a Toyo Field 45A which has a camera back with a Toyo fresnel lens placed in front of the ground glass. I used to use the Toyo 3.6x loupe for fast focussing and then I refine the focussing by using a 30x microscope magnifer. I have been quite confident that I can obtain the most accurately focused images possible and in fact I have been quite happy with the results. Today I bought an identical spare ground glass without a fresnel lens. I intend to carry one spare ground glass in case that I break the focussing screen in the camera back, I can have one to replace. I did not buy the fresnel lens because I don't want to spend the extra money. When I got home I became aware that the thickness of the focussing screen with a fresnel would be different from that without a fresnel lens and I suspect focussing discrepancy would result. I then used a standard lens (set to f5.6, focus on a bright object 3m away)to test the focussing with and without the fresnel lens. The result is there is minor shift of focus observed on the screen under 30x maginfication. The observed difference in focussing would only disappear when I stopped down the lens in either case to f11 . I have not had a chance to test the focussing on the film and I am not sure under which scenerio I can have a more accurate focussing. Any comments and suggestion will be appreciated. Thanks in advance. C.W. Lee
-- C.W. Lee (firstname.lastname@example.org), October 06, 2001
First, if your ground glass breaks, the (plastic) fresnel screen likely won't have broken. Just install your identical spare ground glass over the original fresnel.
FYI, fresnels offset the focus plane by 1/3 their thickness. Toyo's factory faxed me a drawing showing that it sets the "actual focal plane" on its 4x5 backs to 4.75mm behind the seating surface. That's where your film emulsion should be. Depending on what holders you're using, some or all of it may or may not be.
Finally, the focusing difference you saw didn't "disappear" at f/11. A combination of increasing depth of field/focus and screen darkness rendered it unobservable. It does have the potential to cause you fits when attempting to maintain sharpness while shooting subjects with significant depth.
-- Sal Santamaura (email@example.com), October 06, 2001.
Thanks for the information Sal, I then assume the original fresnel should always be used together with a GG for my Toyo 45A as it is the design of that model. C.W. Lee
-- C.W. Lee (firstname.lastname@example.org), October 07, 2001.
I'm not sure I understand your last question. If your 45A has the original Toyo screen assembly, the fresnel can't be used by itself. Unlike some other manufacturers' designs where a fresnel and "ground glass" are molded together, the Toyo drawings show two separate parts. If that's the case, the fresnel would have no frosted surface to focus on, so yes, it must alway be used in combination with a ground glass.
-- Sal Santamaura (email@example.com), October 07, 2001.
My Toyo 45A comes with a GG and a fresnel (fresnel on the lens side) clipped together. I bought this used camera 3 years ago and at that time, I did not know there are actual 2 separate elements for that screen system. I tried to clean the stain on the screen (as 1 piece) and could not remove the dirt at all, then I discovered that the 2 elements could be separated. I was careful to mark down the proper sides before I cleaned the inside of each element. I found out then that the sequence of the surfaces, from lens side to the rear, is: 1)smooth side of the fresnel, the concentric ring side of the fresnel, 3) the ground side of the GG and 4) the smooth side of the GG. I believe this combination, GG with fresnel, is the standard feature of the Toyo 45A (even though I have not seen any Toyo specification on this and I have not got confirmation from other sources) . I have a comprehensive Toyo Large Format Camera System catalogue but it does not mention the GG should be used with a fresnel for proper focusing. In spite of the lack of proof, I think the GG should be used together with the fresnel (in my last thread I wrote: fresnel should always be used with the GG - sorry to cause confusion )just because they come together with the camera and that the slides I got back have been perfectly sharp. Now I become aware of the focus shift problem and will therefore not to contemplate to use GG alone for my camera. Of course, the theory of why there is a focus shift is beyond my knowledge.
-- C.W. Lee (firstname.lastname@example.org), October 08, 2001.