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ThePenguin
20-Jan-2013, 06:05
I'm gonna take on a challange:

To build a 8x10 camera with fixed infinty focus för a 210mm lens (wooden box style).

But I have one question, the distance between the filmplane and the lens should be 210mm right? But where on the lens should I measure? The lensplate? Frontlens? Backlens?


Thanks!

sethlatimer
20-Jan-2013, 07:11
I think each lens differs by a few mm. If you had a view camera I would just carefully focus the lens you plan on using then measure it carefull from the ground glass to the lens mount. That way you would know for sure.

Jim Jones
20-Jan-2013, 08:09
It might be more accurate to improvise a ground glass in the film plane, and adjust the focus for maximum sharpness.

C. D. Keth
20-Jan-2013, 10:03
It's really best to have some form of adjustment. If you can lock that adjustment, you can have the fixed focus you want. You can look up a number called the "flange focal distance" of most modern lenses. That is the distance from the film to the front of the lensboard. To build a camera based on just that would be very, very tricky and the tiniest error will screw it up. For example, if you designed the camera with bare wood dimensions and then gave it a few coats of lacquer and flocked the surface where the lensboard sits. That's enough to throw it off.

drew.saunders
20-Jan-2013, 10:35
Ebony Cameras kindly provides this really handy table of most modern lenses that includes the flange focal distance. That's the distance from the front of the lensboard to the film plane, and is what you'll need to use to set your lens at infinity: http://www.ebonycamera.com/articles/lenses.html

For most of the 210mm lenses listed, the flange focal distance is a couple mm less than 210.

andreios
20-Jan-2013, 10:38
Just a question as I've been having similar thoughts - I've been thinking about putting the lens a couple of mm more further from the film plane to use more of the hyperfocal distance - as I would use it most of the time stopped well down - or am I wrong here?

C. D. Keth
20-Jan-2013, 10:45
Just a question as I've been having similar thoughts - I've been thinking about putting the lens a couple of mm more further from the film plane to use more of the hyperfocal distance - as I would use it most of the time stopped well down - or am I wrong here?

You're not at all wrong. Hyperfocal distance at a given stop is an easy calculation.

mdm
20-Jan-2013, 17:51
What I did was work out what distance I need to focus on using a dof calculator. I then made the box with a piece of scrap MDF nested in the front with the lens mounted. Focused at the correct distance, marked the walls, cut it down and made a new permanent front board. It was a cheap way for me to work out I dont like 8x10. I still have the box camera and B&J back, but the lens is doing duty elsewhere.

Cletus
20-Jan-2013, 18:44
Why don't you just use a helical focussing mount on the front? That way, you could be off by a tiny bit in your box dimensions and still be able to focus tack-sharp n the GG. You could potentially use different lenses that way too.

Dan Fromm
20-Jan-2013, 21:08
Guys, the published flange focal distance for a lens are for the ideal example of it. Real ones don't always match up. Calculations are fine for working out roughly what's needed and what can't be done, aren't a substitute for collimating the lens to the body.

Why do you think that "Hollywood" has every lens collimated to the camera it will be used on and recollimates when the lens is swapped from one body to another?

Frank Kistemann
21-Jan-2013, 11:02
Guys, the published flange focal distance for a lens are for the ideal example of it. Real ones don't always match up. Calculations are fine for working out roughly what's needed and what can't be done, aren't a substitute for collimating the lens to the body.

Why do you think that "Hollywood" has every lens collimated to the camera it will be used on and recollimates when the lens is swapped from one body to another?


Well that's why everyone suggested some kind of adjustment, which would take care of those unavoidable variations between lenses. If you know what you want (infinity or hyporfocal distance) and did your homework, you shouldn't need more than a few millimeters of fine tuning to get it right.


Btw motion picture lenses aren't "collimated to the camera"; both are actually calibrated separately. The flange focal distance on the camera is measured with a depth micrometer and , if necessary, corrected in order to be exactly at the specified standard. Independently, each lens is tested on an optical bench or collimator in order to determine wether or not, the light rays entering the lens would be focused at the correct distance. So basically, when a DoP or camera assistant walks in a rental house to put together his kit, he can take any lens and any body he wants. It is only at this moment, that the camera assistant performs filmed tests with the whole setup to be sure that everything is properly adjusted.
Now if you had the rare case that camera and lenses are out of spec, they would be recalibrated to match the official specs, so that they work, not only with each other, but also with every other body/lens on the shelf. So cameras and lenses are actually meant to be interchangeable and that's what happens, if the situation demands it. However, I assume that most assistants wouldn't change bodies or lenses if it's not necessary, just to avoid any possible risks.

The testing procedure is somewhat different for digital cameras of course, but the principle of interchangeability still applies.


Frank

C. D. Keth
21-Jan-2013, 11:45
Guys, the published flange focal distance for a lens are for the ideal example of it. Real ones don't always match up. Calculations are fine for working out roughly what's needed and what can't be done, aren't a substitute for collimating the lens to the body.

Why do you think that "Hollywood" has every lens collimated to the camera it will be used on and recollimates when the lens is swapped from one body to another?

We don't do that, actually. The cameras are, however, maintained with absolutely perfect on-spec flange focal depths so the lenses can be changed among bodies without tape focus problems.