Making a recessed lensboard and figuring out angles,vignetting risk etc

[Nick_3536]

I'd like to make a recessed lensboard but you've likely figured that out. What I'm wondering is how deep I can go without risk of vignetting.

The extreme case would be for a 90mm on a 5x7. I can't find anything on angle of view for 5x7s but assuming it's similar to 18mm on 35mm then I found 93degrees. Does that sound right?

The lensboard is 5" across but between everything I figure about 4" of recessed area.

From my high school math I get the following.

tan angle = opp/adj

angle is half the 93degrees so 46.5 degrees.

opp is the depth of the recessed board and for the moment unknown

adj is half the width. So 2"

That works out to 2" deep and no vignetting far more then my 1" need. OTOH it's only measuring from the middle of the lens. If I use 1" for the adj then I get just over 1".

Am I on the right track? Any obvious screw ups? I know the lens itself will be forward of the recessed 1" so I think I'll have a built in safety factor.

BTW I apologize to all the math teachers that told me I'd use this stuff some day.

[Ralph Barker]

I don't think vignetting is really the issue, Nick. It's more a matter of still being able to access the controls on the lens at the depth necessary to focus the lens on the camera. Most of the commercial recessed boards are around 25mm (about 1") or 12mm, as I recall. So, you're in the ballpark with 1".

[Don Sparks]

Are you sure you need a recessed board? I use a 90mm lens on a flat board with my Burke & James 5x7.

[Michael Veit]

Ralph is correct that access to controls is the big issue, particularly if it's a "big" 90mm. I've got a big 72mm XL on a standard Linhof recessed board and always have to take the lens off the camera and make adjustments to the recessed control levers with car keys and such. A real PITA. Having a 4" recessed area should help.

[Nick_3536]

I actually need a new back for my Ansco. With the current back 150mm is the widest that will fit on a flat board. The current back seems to provide over 50mm of extension.

It'll be an F/8 lens so hopefully not too big.

[Ralph Barker]

Be sure to pay close attention to the coverage of the 90mm lens you choose, Nick. Some just barely cover 5x7, and even then you might not like the image rendition in the corners.

There are several good comparison charts linked from the main page here, including:

http://www.largeformatphotography.info/lenses/LF5x7in.html

[James E Galvin]

Measuring is tricky, as your measure from the front principle plane. Instead look at the diaghram from the front of the lens as you tip it, and see how far forward and obstruction can be. I think only a little past the front of the lens. But that is a board nearly 2 in deep. You WILL have trouble adjusting the shutter. Make the box as large as possible, I made mine about 3 3/4, (for a 4 in board) and it is cramped. With a center filter on, I can't get to the shutter at all.