PDA

View Full Version : focusing error with fresnel lens removed



oysteroid
12-Mar-2012, 15:39
Hello! What a wonderful forum!

I hope my question hasn't been answered elsewhere.

I am new to large format photography and new to film after having started with digital cameras (hopefully a growing trend!). I recently purchased an Omega/Toyo View 45F. The Toyo back had a fresnel lens. I removed it because I found the circles problematic. The image on the GG now looks much better. I am mostly using a 210mm Schneider Apo-Symmar, so I don't think the fresnel is necessary, but I do also have an old 90mm Angulon and a 135mm Xenar that I intend to use at some point. The question is whether or not I will get focusing errors now that I removed the fresnel lens. Should I put it back? If it makes any difference, the fresnel lens was between the lens and GG, right against the GG.

Thank you!

Bob Salomon - HP Marketing
12-Mar-2012, 15:59
If the Fresnel was between the gg and the lens you will have a shift. If it is between your eye and the gg you won't have a shift.

Do some tilts and swings with your 210 and you may find dark areas of the gg that will benefit from the Fresnel.
If you are seeing the rings then you have not focused your loupe on the grain side of the gg. If you haven't done that then you should.

oysteroid
12-Mar-2012, 21:21
Thanks, Bob!

I put the fresnel back in. It is a plastic fresnel lens and is slightly bowed. It mounts a slight distance from the GG, but bows in and touches the GG in the center. Does this seem normal? If not, might it cause any problems?

Jim Jones
12-Mar-2012, 21:37
I believe it will be fine.

oysteroid
13-Mar-2012, 08:57
Thanks!

Jim Noel
13-Mar-2012, 09:18
It makes a difference which way the fresnel faces. It sounds like you may have the ridges toward the lens,which is backwards.

Louis Pacilla
13-Mar-2012, 09:24
It makes a difference which way the Fresnel faces. It sounds like you may have the ridges toward the lens,which is backwards.


Not always Jim. Case in point. The clip in Sinar Fresnel which is behind the GG and the groves face the lens.

Maybe it's the other way around if the the Fresnel sits on the other side of the GG?

tgtaylor
13-Mar-2012, 10:06
If it's a Toyo fresnel, then it goes between your eye and the GG with the "Toyo" logo right side up and on your bottom right. The fresnel and GG both fit tightly together inside a spacer set. If you don't have the spacer set you can get one from Toyo repair (Mamiya America) in the US for about $16. I purchased a Toyo 45C from KEH and the one they sent had a fresnel, which is an optional accessory with that model, but it was installed loose behind the GG and there was no spacer set. Toyo repair are good people to deal with - quick and competent service the polar oposite of Manfrotto service which is the worlds worst and slowest.

Thomas

Bob Salomon - HP Marketing
13-Mar-2012, 12:13
Ridges side faces the lens. Smooth side faces the eye.

oysteroid
13-Mar-2012, 14:15
Okay, I got it figured out. Thanks for all your help! It turns out that the GG and fresnel needed to be sandwiched together with the clip holders with the ridges facing the GG and the fresnel between the GG and the lens. This is the only way it naturally goes together. The way it was when I got it, the fresnel was in the clip holders and the GG was sitting on top of them. With them sandwiched together now under some pressure, the fresnel flattens out properly. And the Toyo-View logo reads correctly in the lower right corner.

toyotadesigner
13-Mar-2012, 14:23
Ridges side faces the lens. Smooth side faces the eye.

Always? Even in Horseman mirror viewfinders with ground glass (the single eyed model)?