# Lens geometry question

Discussion in 'Camera Building, Repairs & Modification' started by couldabin, Oct 27, 2006.

1. ### couldabinMember

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I'm building a 4x5 "camera" for a project -- the camera won't use bellows but instead will be a rigid box that simply holds the lens/shutter and filmholder. I plan to put three or four baffles between the lens and filmholder to block stray light.

My question is how large the rectangle immediately behind the lens should be? Would it be a rectangle whose diagonal is the diameter of the rear element? Could it be smaller?

TIA.

duane

2. ### Nick ZentenaMember

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Do you mean the box will be like a > narrow at the front wider at the back? If so I'm thinking the exact size will depend with the lens. Wider lenses needing to be bigger.

That makes sense to me since you need to hit the full size of the 4x5 negative with a shorter gap between the lens and the film.

You could try drawing some triangles and doing the trig. It shouldn't be that complicated.

3. ### couldabinMember

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Nick, I plan to make the box straight sided (= rather than >) to simplify construction. The trig should be straightforward, but the question I have is what portion of the lens is actually used to deliver the final image?

4. ### reellis67Subscriber

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The image circle will be quite small immediately behind the lens, so you should be able to get by with something not much larger than the rear element of the lens. To check, place a sheet of waxed paper over the opening of the front half of the camera, pull a towel over your head and the camera, and look into it. You should be able to see if the hole is large enough.

- Randy

5. ### Nick ZentenaMember

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But if the box is straight then it needs to be the size of the negative.

6. ### reellis67Subscriber

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I'm thinking it is two nesting boxes with baffles around the sides to block light, like sticking two shoeboxes, one inside the other, and that he is asking how big a hole to put in the front box where the lens mounts, but I could be wrong.

- Randy

7. ### Dan FrommMember

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Hmm. You're trying to eliminate glancing reflections from the box's sides. If the box is larger than the gate, a couple of baffles as large as the gate should do.

Build a model. That is, sketch the thing and try to find a path for rays of light that exit the lens and hit the side of the box to get to the film. Then sketch a baffle to eliminate, etc.

8. ### Nick ZentenaMember

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If he's doing sliding boxes usually the two boxes are fairly large. The inner box isn't much smaller then the outer box.

9. ### TerenceMember

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Not really answering your question but, from experience, unless you're using a lens that has MUCH larger coverage than your film format, flat black paint (like Krylon's "Ultra Flat Black") are more than sufficient. Adding something closer to the light path actually increase the chance of reflection into the image area. Picture shining a small flashlight into an empty, dark stadium versus an empty dark closet.

Anything immediately behind the lens is likely to be out of the light path anyway, so a baffle there would be unnecessary, assuming the gap between your boxes is small. If there's too much "play" a very small baffle should take care of it.

10. ### couldabinMember

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Yes, Randy, you guessed correctly that I'll use two boxes (like a shoe box or film/paper box). Originally, I wasn't planning to do that, since this project involves taking a series of photographs from a fixed vantage point over a long period of time so there's not need to focus or have a focusing screen. However, I got to thinking that adding the ability to change the point of focus would be pretty simple. The baffles, of course, would be in the inner box.

I think I'll start by sizing the baffles on the assumption that the one at the rear element will have a diagonal equal to the diameter -- I doubt I'll get any image clipping. I'll post something here about that I find out ...

The lens does have a pretty impressive coverage area -- it's a 210mm from an 8x10 enlarger.

duane

11. ### Joe VanCleaveMember

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You could also consider lining the inside of the box with black adhesive craft felt, available at hobby stores like Hobby Lobby in the States. I've found this works better than flat black paint at absorbing stray light, and there's no waiting time for drying. My experience is that with the inside surfaces of the camera lined with black felt, there's little need for baffles.

12. ### Claire SenftMember

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I would think that the trapping of the light would not so much be determined by the len's angle of view. Rather I believe the need for baffling would be dependent upon the size of the image circle relative to the size of the negative. For example if the lens chosen at the apertures used just barely covered the negative the problem of light bouncing around would be much reduced compared to, say, using a lens that will cover more than 8x10 at any aperture on a 4x5 camera that was of mimimal size.

You can also probably accomplish much by using a adjustable lens shade that cuts off extraneous light before it enters the camera.

13. ### Frank RSubscriber

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Does this help?

The tail board is not necessary (and makes it really difficult to carry)

Just slide the inner box out to focus using a ground glass; keeping the inner box parallel to the outer box.

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14. ### Helen BMember

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If you decide to add baffles then the requirement is that the baffles should not obscure the exit pupil of the lens when viewed from anywhere within the area of the film. If you pointed the lens at a white surface with the aperture wide open (or at the maximum aperture that you are likely to use) you could play around with pieces of card to determine where the edges of the baffles should be. At no time should a baffle obscure your view of the exit pupil when your line of sight is within the area of the film.

I hope that makes sense.

Best,
Helen