Welcome to our community

Be a part of something great, join today!

  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Doubts on swapping mounts... [related to Birger's mount]

Hey Frequent Swappers.

It's going to depend a bit on what lenses you are using and how you are using them, but generally I would not worry so much about flange focal distance. Unlike a film camera if you are having a problem holding focus over a zoom range or lenses are not focusing at the expected distances you will see the results immediatly on the monitor. It will be simple to check this after swapping mounts. Put on your widest-fastest-sharpest lens, shoot a wall wide open with 5 focus targerts equidistant from the camera, does the lens seem to be in best focus on the center target at the distance indicated on the scale?. Look at a frame in 4k - is the center sharp? the corners? If you are on a zoom, zoom in, still looks good ? You're golden! now practice your awards show speech.

The rear of Lens / PL mount areas are subject to many times more wear and abuse than the nice pristine machined surfaces of the sub mount and the surface on front of the camera body. Keeping the mount area and rear of the lens free from dust and nicks should be your priority. It it's set right at the factory and you don't knock it around it should maintain tolerances for the life of the camera.

For still camera mount lenses, really don't worry. Compared to a PL mount The twist lock bayonet mounts found on Nikons and Canon EOS is inherently imprecise, lenses are not very well marked and most zooms don't reliably hold focus over the range anyway. If you are using still lenses you are probably using eye focus more than taped distances. If you are using a birger mount.... we'll find out what works best.

On film cameras there is lots that can go wrong, the spinning mirror shutter / ground glass / mount and filmplane, all had to be aligned to give reliable results. On some cameras due to sound issues there was a lack of a solid metal to metal mechanical link between all these things. Things could look perfect in the viewfinder and be NG on the film. None of this applies to the Red which has only a mount and a sensor connected by ultra burly metal.

Matt Uhry
www.mattuhry.com
 
SLR users swap out lenses all the time without worrying about the precisions involved. There's slop enough that micrometers shouldn't matter too much, unless you're dead set on perfection, in which case you should only shoot footage from a vibration isolated table in a lab.
 
SLR users swap out lenses all the time without worrying about the precisions involved. There's slop enough that micrometers shouldn't matter too much, unless you're dead set on perfection, in which case you should only shoot footage from a vibration isolated table in a lab.

Hi,

SLR users have the advantage of focusing by eye, focusing on a motion picture camera is usually carried out by an assistant who is not looking through the camera.

Stephen
 
,You're forgetting the Graeme's magic focus assist, perhaps?

Hi Gordon,

As I have not seen it in action I don't know how it will work in practice, keeping focus on an actor's eyes as he turns his head, if the focus goes from 1 eye to both without the nose becoming the point of focus it will indeed be 'magic'.

Stephen
 
However, with the birger's mount, why bother with collimation or swapping mount problems? Is it automatic right? Yet, there are no marks on the lenses or can I be wrong?
 
However, with the birger's mount, why bother with collimation or swapping mount problems? Is it automatic right? Yet, there are no marks on the lenses or can I be wrong?

Hi,

If the mount is not collimated you may not be able to focus at infinity. As I have not seen a birger mount I don't know. Is birger mount offering auto focus like a DSLR? if it does, assuming the lens can focus at infinity it should work. If the mount is not flat then focus will not be parallel to the sensor, possibly not an issue just a cool effect.

Stephen
 
Hi Gordon/Stephen

The Birger mount won't offer autofocus, at least not to start with.

M
 
Hi Gordon/Stephen

The Birger mount won't offer autofocus, at least not to start with.

M

Hi Martin,

That's what I thought so collimation is an issue IMHO.

Stephen
 
Hi Gordon/Stephen

The Birger mount won't offer autofocus, at least not to start with.

M

But it does offer programmable focus marks. That's what I meant.

So why collimation?

Yet according the most recent thread currently open, how is it possible to collimate on the Red sensor?
 
But it does offer programmable focus marks. That's what I meant.

So why collimation?

Yet according the most recent thread currently open, how is it possible to collimate on the Red sensor?

Hi Gordon,

Wait & see, it may work for you without collimation.

There will be some trial & error involved in getting the exact FFD, Cine techs who have done this 1000's of times will get a spot on result fairly quickly due to experience. If you have a lens that is known to be 'perfectly collimated' see where infinity focuses, then calculate the error & correct.

Stephen
 
How? Following your formula posted before?

Hi Gordon,

Thats the theory, the problem is that the lens may focus infinity @ 62 feet but there is no accurate mark on the lens for 62 feet, thats the trial & error bit!

Stephen
 
What formula?

(d = f squared divided by a-f ) will give you the distance the lens is out of the mount when the lens is focused at infinity. d = displacement of lens out of mount at infinity. f = focal length of lens. a = distance of sharp focused object. You can use inches or metric measures. Note, d = 0 when a = infinity. Example: for a 50 mm lens (2 inch) set to infinity but is focusing at 10 feet. 2" squared/ 10x12"-2 =4/120"-2"=4/118"=.034" so the lens mount is .034" too far out of the mount. Shorten the mount by .034". Better yet, try .030 and check again

Stephen
 
Can you give us a sketch? Like this one, for example:

http://www.reduser.net/forum/showpost.php?p=67324&postcount=5

Hi Don,

I will photograph the back of 1 of my Fries Mitchell mounts & some shims next week.

Precision grinding it the most accurate way of making something smaller (and totally flat), its like using a file or 'wet & dry sandpaper' just way more accurate. A milling machine is often used as final accuracy can be achieved with shims.

Shims are very fine sheets of plastic or metal. (like sheets of paper, some much thinner than paper others thicker. The final precision required is about 1/10 of the thickness of a human hair. Dirt & dust are clearly an issue & how tight the screws are holding the mount.

Stephen
 
Grinding and shims? Gosh, I would expect both the Red PL mount and the Birger mount to be perfect from the factory, otherwise I'd ship them back and complain about QC!
 
Back
Top