Lens adapters and Depth of Field calculations

andrew parrish · October 14, 2011

Hi. Sorry If this is a dumb question, or has been asked before. ( the search engine didn't turn up an answer) I have a H8 rex 4, that I use with a bower c mount adapter, and Smc Takumars. My suspicion is that the adapter is going to through off the DOF calculations, because of the increased distance from the film plane. Is this correct? Is there a way to compensate?

Thanks,

Andrew

Hal Smith · October 15, 2011

I'm surprised no-ones chimed in on your questions so far.

DOF is a function of lens focal length, aperture size (f-stop), and circle of confusion. Format size comes into the equation through required circle of confusion for different formats. The lens back focal distance does not come into the equation. If your adapter is passive (no lenses) all it does is hold the lens at the correct distance from the film plane to be able to focus at infinity, etc.

If your adapter has lenses in it, IE: an optical adapter, it can change focal length so there may be a DOF change with it. That will be a fixed change ratio for all your Takumars and the Bower manual should tell you what that ratio is. If not, contact Bower, they'll know for certain what it is.

PS: It'd be fun to put something like my Pentax SMC 70-210mm zoom on a passive Bolex adapter. It'd be slower than snails but be somewhere around the equivalent of a 35mm still camera 280-840mm, now that's a TELEPHOTO!

andrew parrish · October 15, 2011

Thanks for the Answer Hal,

It has been very helpful, not only in the answer, but to help me smooth out my thoughts on the matter. I thought that the answer might be to adjust the focal length by adding in the length of the adapter. I,m glad you sorted this out for me. With the adapter in place, the f stop marks are face down. Now I know that I can use a spacer of some kind to rotate it up.

Your thoughts about the telephoto lens is one of the big reasons that I went with this camera. The only problem is that I didn't count on it being too much magnification for my poor little tripod.

By the way, and not that I'm not going to cheat and use a calculator, but what is the Circle of confusion for 8mm destined to be telecined?

Thanks Again,

Andrew

Dom Jaeger · October 16, 2011

With the adapter in place, the f stop marks are face down. Now I know that I can use a spacer of some kind to rotate it up.

By the way, and not that I'm not going to cheat and use a calculator, but what is the Circle of confusion for 8mm destined to be telecined?

1/2000 of an inch or about 0.013 mm was a standard CoC value for 8mm intended for projection that I've seen quoted, though it varied between manufacturers - as low as 0.006 mm for some fixed-focus lenses to .017 mm used by Taylor Hobson for some of their 8mm depth of field tables. You should do your own tests and have a look, with 8 mm grain it can be pretty arbitrary, and most of the time the depth of field is very large. A lot of 8mm lenses have depth of field scales marked on the lenses themselves, notably the Kern Visifocus ones.

Someone posted some 8 and S8 tables on this thread, doesn't say what CoC value was chosen but it could be worked out from the figures:

http://www.cinematography.com/index.php?showtopic=28046

If you shim out the mount to better align the lens markings you won't affect the depth of field, but you will change the back focus and the lens won't reach infinity, though longer focal lengths will be less affected. Actually, I'm curious, have you checked focus with that adapter? The reflex H8 flange depth is shorter than standard C mount, you may find the lenses focus at infinity well before the mark, in which case shimming will be necessary to get close focus.

andrew parrish · October 16, 2011

Thanks for another reply Dom.

As you were writing this one, I was trying to get the answer for myself. ( my Granny always said the Lord helps those whom help themseves) Here is what I Dug up:

Postby calgodot » Fri Mar 12, 2004 6:24 pm

Lenny Lipton sez (on page 163 of his classic Independent Filmmaking):

"The size of the circle of confusion that we can accept is dependent on how we view the projected film. ...the choice of an acceptable maximum diameter for the circle of confusion is based upon the projected film as it is viewed by some hypothetical viewer, a certain distance from a screen of a certain size. The choice that has been made for the circle of confusion for 16mm is 0.001 inch, and for 8mm 0.0005 inch. The circle of confusion for super 8-single 8 would be very nearly that of 8mm, actually 0.00065 inch."

He goes on to say that this is operating on the assumption of "essentially aberrationless lenses" and the assumption that "the size of the grain of the film is smaller than the circle of confusion." "With some 8mm or super 8-single 8 films," Lipton adds. "the grain size may approach the size of the circle of confusion, and therefore may set a limit on the actual depth of field."

In therms of the F stop scale being on the bott om, maybe its best just to continuing to twist it up to read.

Thanks again,

Andrew

Hal Smith · October 16, 2011

Thanks for the Answer Hal,

Andrew

Your welcome!

My degree is a Master of Science in the Teaching of Physics. I taught Physics at Sewanee (The University of the South) and the University's Prep School for six years which gave me plenty of practice figuring out how to explain complex subjects to people.

It'll look overkill but ancient top quality tripod heads like O'Connor 50D's can be found pretty cheap from time to time. Another good "oldie but goodie" is the little Miller Professional. If you find one you're thinking of buying one, make certain it's not leaking damping fluid, that's a sign of leaking seals and the drags may not work correctly. Either will work just fine on an equally ancient professional grade wood tripod.

andrew parrish · October 19, 2011

Your welcome!

My degree is a Master of Science in the Teaching of Physics. I taught Physics at Sewanee (The University of the South) and the University's Prep School for six years which gave me plenty of practice figuring out how to explain complex subjects to people.

It'll look overkill but ancient top quality tripod heads like O'Connor 50D's can be found pretty cheap from time to time. Another good "oldie but goodie" is the little Miller Professional. If you find one you're thinking of buying one, make certain it's not leaking damping fluid, that's a sign of leaking seals and the drags may not work correctly. Either will work just fine on an equally ancient professional grade wood tripod.

I had a look on ebay ( not always the best place to see what a good deal would be) at therse two models. It seems like they run around $600. Is that what I can expect to pay?

Thanks,

Andrew

Hal Smith · October 21, 2011

I had a look on ebay ( not always the best place to see what a good deal would be) at therse two models. It seems like they run around $600. Is that what I can expect to pay?

Thanks,

Andrew

Offhand I'd say too high for a Miller Professional but okay for an O'Connor 50 if it's a clean, well maintained one with nothing missing. Having said that, I was GIVEN a pair of O'Connor 50D's by a local college. They were in poor condition, I spent $150 or so coalescing the two into one and buying some parts from O'Connor.