# Distance between lens and film?

Discussion in 'Camera Building, Repairs & Modification' started by TomMessenger, Jun 30, 2010.

1. ### TomMessengerMember

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Quick question I've wondered about for when building a camera...

How do you know what distance the lens should be from the film to be able to focus it, and how far the bellows should move in and out?

Tom

2. ### Steve SmithSubscriber

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Most lens manufacturers publish a figure known as the flange distance for their lenses. This is the distance from the front (emulsion) surface of the film to the front of the lens board or rear surface of the lens/shutter assembly. In theory when set at this distance the camera will focus distant (infinity) objects. For closer objects the lens needs to be further away from the film. This can be worked out mathemtically or by using a ground glass to focus with.

Here is an example from Schneider's data pages: http://www.schneideroptics.com/info...rge_format_lenses/angulon/data/6,8-120mm.html

Steve.

3. ### wiltwSubscriber

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Very tricky question! Lenses can be designed to have different distances which might be equal to the FL or less than the FL (so-called 'telephoto' design), or more than the FL ('retrofocus' design). Unless you have a spec sheet to refer to, the only dependable way to figure it out is to use the lens to focus an image of the sun onto a brick and measure that distance (don't focus on a piece of paper unless you intend on starting a fire!)

4. ### Ian GrantSubscriber

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LF cameras used to be called Double & Triple extension, that's the amount of bellows extension compared to a standard lens. At double extension a lens will focus 1:1, but this was more important for using longer focus lenses (non telephoto).

If building a 5x4 camera then decide what's the longest lens you want to use, typically most will just accept a 300mm focussing to about a metre. The bellows need to close a close as possible if you want to use a wide angle lens.

Ian

5. ### TomMessengerMember

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Thanks for the replies. I've just looked up a lens I would probably like to use, the Helios 44M-4. I found this page, with a diagram. http://www.zenit-camera.com/lens-helios-44.htm

It says the distance from rear element to image plane is 38mm. I'm assuming the image plane is 35mm film. Say I wanted to use medium format film, is there an easy way to work out that distance? I'm assuming the lens would be further from the film.

Thanks again,
Tom

6. ### Ian GrantSubscriber

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The focus, so film distance, is the same regardless of the format.

You need to find a lens with it's own shutter ideally, this Angulon is an example.

What sort of camera are you planning on ? Have a look at Steve's 6x12

Ian

7. ### Ian CMember

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Calculating Bellows Requirements

There's a simple calculation that lets you do this fairly easily based on the lensmakers equation

1/f = 1/i + 1/p

where f = focal length of the lens, i = image-to-diaphragm distance, and p = subject-to-diaphragm distance.

Knowing any two of these three variables allows calculating the third. f is known and we choose p, so all we need is

i = pf/(p-f)

At infinity focus the image-to-diaphragm distance is i = f.

You have to decide how close the camera must focus.

Example: You want to design a camera to use a 300mm lens from 1 meter to infinity. Well suppose that youve measured the distance from the mounting flange to the diaphragm at 15mm.

The film to diaphragm distance at infinity is i = f = 300mm.

At p = 1 meter = 1000mm

i = 1000*300/(1000-300) = 428.6mm

Taking the 15mm flange-to-diaphragm distance into account your camera must have a film plane to front of lens board range of at least 315mm to 443mm.

You can repeat the calculation for the shortest and longest lenses you think you might use on the camera to determine the range of bellows extensions necessary. Dont be surprised to find that the shortest lenses might require a dedicated short bellows or a recessed lens board. Thats because the longer bellows required for the other lenses might be fully compressed and the short lens is still too far from the film to focus at infinity.

Thats why camera makers supply short or bag bellows and recessed lens boards for very short lenses.

8. ### TomMessengerMember

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With regards to the distance I was thinking if that distance was set for 35mm, would the image from the lens then cover a full 6x6 format right to the edges or just the center portion?

The shutter was another thing I was wondering about. I have a Kodak Junior 620 medium format bellows camera which has it's own shutter in the lens, but then I would be worried about the quality of the glass. If I used the Helios, I was thinking I would more or less chop up a Praktica SLR I have no use for so I also have the ready made M42 mount attached with shutter, and could use that as the front part of the camera that moves when focussing. All just for experimentation purposes - whether it would work or be worth it is another matter.

The type of camera I would like to make is something like that link to Steve's 6x12 (I have 2x tinny old 6x9's) but I would love bellows, but just from an aesthetic point of view. I'm thinking now I might just build a new body for the Kodak as a cheap experiment and probably convert it to 6x6 as I love square images.

9. ### ic-racerMember

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You will likely get a circular image with that lens on a 6x6 segment of film. It might be interesting if that is what you want.

10. ### Steve SmithSubscriber

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Probably not.

You could certainly use that lens but all it will gain for you is the knowledge and experience of building your own camera. This is obviously worthwhile but it may make more sense to get this camera working if possible and get hold of something beyond repair to use the lens from.

Steve.

11. ### Q.G.Inactive

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What you do not know is where the principal planes are, relative to the film, to the subject and to the diaphragm.
So don't use the simple infinitely thin lens formula for anything but an approximation. Results including fractional millimeters will be off the mark by a factor at least ten times the precision suggested by the fractional part.

12. ### Dan FrommMember

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The Helios 44-M is made for M42 cameras, flange-to-film distance 45.5 mm. The OP's link says "Working statement, mm: 45,5 "; a poor translation ...

I don't know what the OP means by "if that distance was set for 35mm." If he means setting the lens 35 mm from the film, whatever that means, he'll get no usable image.

The lens is made to cover 24x36, i.e., a 43 mm circle. Until proven otherwise, that's all it will cover. So it will be useless on a larger format.

OP, its time to acquire the concept "coverage."

QG, you should be ashamed of yourself.

13. ### Q.G.Inactive

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I should?

15. ### Ian GrantSubscriber

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Isn't that the size of the girls swimming costumes on the beach at this time of year

Ian

16. ### Q.G.Inactive

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I thought it was the number of pages available for an assignment for, say, the Swim Suit Special.

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18. ### Ian GrantSubscriber

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That's a useful page, thanks.

Ian

19. ### Dan FrommMember

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Absolutely.

The OP's question was fairly specific. You replied with vague generalities. I'm sure that if you'd have stopped to think for a moment, as you usually do, you'd have posted an appropriately specific response.

Cheers,

Dan

20. ### Q.G.Inactive

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???

"Vague generalities"???

What i posted was a warning, that using the formulae suggested by someone else could not lead to the desired result.
That was very specific.

So what was really bothering you, Dad, uhmm... Dan?

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21. ### Dan FrommMember

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QG, I gave the relevant answer, viz., the lens in question's flange-to-film distance. Your post was correct, as nearly all of your posts are, but irrelevant.

Is that clearer, son?

22. ### Q.G.Inactive

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It was as (ir)relevant as the thing it was in reply to.

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23. ### Dan FrommMember

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Son, if I posted what I thought about the responses to this thread I'd be banned. Permanently. I banned myself from posting APUG a while ago to avoid the pain of resisting the temptation to take bait as offered here, should probably revert to lurking and thinking evil thoughts. The worst of it is that most of the posts that provoke evil thoughts were put up with the best of intentions.

I think I answered the OP's question directly and tersely. It was about a specific lens' flange-to-film distance when focused at infinity. You're right, though, the question wasn't as clear as it could have been. Perhaps I read too little into it.

Cheers,

Dan

Thinking about bait and temptation, during the spring trout season I spend several mornings trying to catch fish from Stony Brook where it crosses US 206 south of Princeton, NJ. As water temperature rises the panfish come out and start to beat the trout to the bait. This year I paid close attention to the small Rock Bass I released; many had partially-healed hook damage. Slow learners, I guess.

24. ### Q.G.Inactive

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You should indeed consider banning yourself. It's best for all of us.

Your remark was and is way out of line.
I had attributed it to a temporary lapse in your straight thinking abilities, but you haven't taken the opportunity to set that straight. Given what you write above, i'll have to conclude that my assumption was partly right, partly wrong. The wrong bit being in the "temporary" part.

Let me act like you for a bit, and point out to you that you should be ashamed of yourself for even thinking that a "Perhaps I read too little into it" would even fool the idiots you apparantly think inhabit APUG into believing it to be a reasonable explanation.
If you really think we're that dumb, please do ban yourself for life.

25. ### Dan FrommMember

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Well, m'boy, you're not that dumb, as shown by my failure to fool you. Wicked, yes, dumb, no.

You're mistaken, I don't think straight. Never have. Clearly, yes, straight, no.

I'll consider y'r well-meant sage advice, for which much thanks.

26. ### nick mulderMember

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Q.G.

Why is it that about maybe 2/3rd's of every thread that you post in ends up being some argument involving you ?

I'm sure you'll have an appropriate response and point out my error, which begs the question why I even bother asking such questions, idle times call for it is about all I can muster...

:rolleyes: