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Question about Flange Focal Distance & Rear Elements on PL Mount Lenses


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#1 David Fitch

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Posted 16 February 2014 - 12:36 AM

I'm trying to improve my knowledge on the finer points of lens mounts, and I have a couple of questions regarding FFD and how it applies specifically to PL mount lenses.

 

First, I'll mention that I do understand the concept of FFD as the distance between the lens mounting flange on the camera and the film plane.  I also understand that the FFD varies from one lens mount to another, and that the FFD for PL mount is 52mm.

 

Here's where I'm curious.  I've looked at a number of different PL mount lenses, both prime and zoom lenses, and it appears as though there's some variation on the distance between the mount on the lens and the rear element on the lens.  My initial question is this...what's the technical term for the distance between the mount on the lens and the rear element on the lens?  And, if my observations are correct and this distance can vary from one lens to another, then this means that the distance between the rear element of the lens and the film plane can also vary depending on which lens is used.  That said, I'm curious how this doesn't result in a myriad of focusing issues.  I'd think that for proper operation, not only would the FFD need to be consistent, but also the distance from the mount on the lens to the rear element of the lens and, consequently, the distance from the rear element of the lens to the film plane.

 

Along the same lines, if these variations do exist, then do manufacturers specify a maximum/minimum value for their cameras?  For example, I could see a situation where the rear element of a PL mount lens might have plenty of clearance on a digital cine camera, yet it might be too deep for a 16mm Arri or Aaton and could potentially interfere with or crush the rotating shutter.

 

Thanks in advance to anyone who can help with my questions!

 


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#2 dan kessler

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Posted 16 February 2014 - 02:15 AM

I think the term that describes the distance from
the rear element to the film plane is 'back focal length.'

You are correct in observing that back focal length varies from
one lens to another, but your expectation that the distance from
the mount to the rear element should somehow be consistent for all
lenses just means you need to do more research into the whole, vast
subject of optics and lens design.

Practically speaking, the standards only apply to flange focal distance.
Focus calibration is based upon that.

As to shutter clearances, you are right again.  Some lens designs will NOT
clear rotating mirror shutters, and it is therefore important to always check it.
 


Edited by dan kessler, 16 February 2014 - 02:16 AM.

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#3 dan kessler

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Posted 16 February 2014 - 02:50 AM

Just to hopefully clarify a little more...  you're absolutely right in thinking that
there is a critical relationship between lens elements, back focal length and
flange focal distance.  Those who build and/or service lenses make sure that

the lens mounts are correctly attached to lens barrels so that those critical

relationships are maintained.  When that is properly done, the only
distance a user needs to check is flange focal distance.


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#4 David Fitch

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Posted 16 February 2014 - 06:09 PM

Thanks for the reply, Dan. 

 

So, just to make sure I'm understanding this properly, while the FFD for a particular mount (52mm for PL mount, for example) remains fixed, the back focal length can and will vary from one lens to another.  If this is correct, then am I correct to assume that the variation in back focal length is a function of the internal optics within the lens?  In other words, I'm guessing that variations in internal optics are what allow back focal length to vary from one lens to another yet still manage to produce a properly focused image on the film plane.


Edited by David Fitch, 16 February 2014 - 06:10 PM.

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#5 Dom Jaeger

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Posted 16 February 2014 - 07:40 PM

The variation in back focus is due to different lens designs and also the focal length. With a simple lens (a single element) the back focus more or less equates to the the focal length, with compound lenses that relationship becomes more complicated but often longer focal lengths will sit further away from the film plane than shorter ones. Retro-focus (or reverse telephoto) lenses were developed to overcome the problem of short focal lengths sitting too close to the film plane to clear a reflex mirror. Very long focal length lenses are often very long physically because the elements sit far away from the film plane.

 

Most early lens designs had the entire lens move away from the film plane to focus at closer distances, with longer focal lengths needing to move further than shorter ones. But different designs can have a fixed rear element with a moving front group, or floating elements that move at different rates, or a central focussing group with fixed front and rear groups etc. Zooms generally have a fixed rear group with 2 zoom elements moving in different curves and a moving front group for focus. A macro zoom will have an additional movement of the rear group.

 

Lens designs are often influenced by the requirements of the camera mount - Bausch and Lomb's Baltar lenses of the 50s needed to be redesigned when Mitchell cameras evolved a reflex mirror, becoming Super Baltars with a longer back focus. Early 16mm lenses made for the Arri 16S could no longer be used on the later 16 BL and SR cameras because the mirror clearance changed. These days Angenieux's range of DP Optimo zooms can only be used on digital cameras without a reflex mirror - part of the reason they cost less than the film Optimo range is that the designs were less restricted in terms of the back focus and how far the rear could protrude.


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