Members: 77,681   Posts: 1,715,782   Online: 844

1. ## Lens geometry question

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

6. 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. 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. 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. Originally Posted by couldabin
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
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. 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

Page 1 of 2 12 Last

 APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY: