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sar-photo
08-13-2009, 10:55 AM
What's the widest lens you can use on 6x12 that doesn't need a centre filter?

6x12cm is equivalent to 2.2x4.7 inches.

I'm hoping to get a 6x12 with a Rodenstock 65mm lens which has an image circle of 170mm @ f22. I have done a drawing and I can see from it that you would definitely need to use a center filter with that focal length on 4x5, but it looks as though it should be ok with 6x12 - does anybody have any experience with this format/lens combination?

Is it correct that as you open up the aperture that the image circle gets smaller?

Cheers
Simon

Steve Smith
08-13-2009, 11:00 AM
I have a 65mm Super Angulon for which I am building a 6x12 format body. I asked a similar question and was advised not to worry about a centre filter for it.

It would probably cost more than I paid for the lens anyway!


Steve.

David A. Goldfarb
08-13-2009, 11:14 AM
Is it correct that as you open up the aperture that the image circle gets smaller?

As to whether you need a center filter with a 65mm on 6x12--it's going to depend on how even you want the illumination to be and what film you're shooting. Slide films will show more falloff than neg films in general.

With modern lenses, the image circle stays the same usually but falloff of illumination increases as you open up, so on 4x5" you might find that light still gets out to the corners with the lens wide open, but the corners are darker relative to the center.

If you have a camera with a groundglass that has cut corners, you can see what's going on with this. When the lens is wide open, you might be getting some mechanical vignetting of the aperture from the lens barrel (and shade, if you're using a shade) at the edge of the image circle. Even if there is no mechanical vignetting, the aperture may look like a football (an American football, that is) rather than a circle, and that's the main thing that's causing the corners to be darker. As you stop down, there will be less vignetting of the aperture at the edge of the image circle. When you can see the whole aperture and it looks like a circle, you should have pretty even illumination.

The best solution for that is to use a center filter usually, but sometimes that is going to mean you don't have enough light to make the photograph you want (the CF will cost you about 1-1/2 stops usually, and the lens usually should be around f:16-22 with the CF), so you might shoot without the CF and correct it by burning in the center of the image in the darkroom.

keithwms
08-13-2009, 11:23 AM
Yep it depends on what you're shooting. For b&w and c41, the 65 on 612 probably won't need a CF, but slide might well.

sar-photo
08-13-2009, 11:34 AM
I'll only be shooting negative film. Just to keep my options open I'm also considering a Fotoman 617 with a 90mm Super Angulon XL (259mm image circle) - centre filter needed??

David A. Goldfarb
08-13-2009, 11:38 AM
I shot 6x17 for a few years, and I generally preferred to use a center filter with a 90mm lens, even with B&W neg film, but you could get away with fixing it in the darkroom. With a 75mm lens I thought the CF was a necessity on 6x17.

keithwms
08-13-2009, 11:50 AM
With 617 and neg film and the lenses you mention, with no tilts and shifts, you can easily skate without a CF. Why not just try it and if you disagree then buy the (pricey) CF.