Zoom lens focal length and back focal length relation

[raghavsol]

Hi to All,

Can any one help me in finding that what will be the back focal length (BFL) of a zoom lens of 400mm focal length? By BFL I mean the distance from last surface of lens to senor/image plane distance. And what will be the zoom lens size possible? Actually my concern is how big will be my total system. I want to get 2 degree to 8 degree FOV, the sensor size is 13mmX13mm. The focal length for 2degree FOV will be around 400mm. For such a lens of 400mm focal length what will be the BFL? I think they call BFL as Flange Focal Length (FFL) in optics terms. How these two are related? Is there any definite relation between the two? How much least BFL we can acheive for 400mm focal length zoom lens? All these are my queries...puzzling my mind...

thanks and regards,
raghav

[raghavsol]

Hi again,

I would like to put the same problem in this way what is the mount type for 400mm focal length zoom lens? Will a C-mount or CS-mount will work? upto what focal length the C & CS mounts will work?

raghav

[Leigh]

I think they call BFL as Flange Focal Length (FFL) in optics terms. How these two are related?
BFL and FFL are not the same thing. They're related, but not in a predictable way. The correlation varies with the lens design.

FFL (flange focal length) is the distance from the mounting surface (the front of the lens board on a view camera) to the film.

BFL (back focus length) is the distance from the outer surface of the rear element to the film

For example, for the Nikkor T 360mm/f8 the FFL is 261mm while the BFL is 211.1mm.

Both parameters are usually given on lens datasheets. They're both given for the lens focused at infinity.

- Leigh

[Dan Fromm]

Raghav, this forum is concerned with large format photography. We don't use zoom lenses. We use lenses that cover formats much larger than do most lenses offered in c- and cs-mount. Your 13 x 13 chip is somewhat smaller than the 4"x5" and larger film most of us use.

C- and cs-mount flange focal distances are, respectively, 17.526 mm and 12.526 mm. Both mounts are threaded 1" x 32 tpi. C-mount was invented for some 16 mm cine cameras, later used on a few Super 8 cine cameras and some video cameras. CS-mount is, as far as I know, used only on some small chip video cameras. Back focus depends on the lens, not on the mount. C-mount lenses usually have to clear a shutter or beam-splitter. Long lenses in c-mount have back focus much longer than flange focal distance. My unlamented 500/5.6 Tele-Athenar II had roughly 20" back focus.

Very long lenses have been used on c-mount cine cameras.

If you want more information you'll have to tell us what you're trying to accomplish. Putting it in our context (film, no smaller than 4"x5") would be helpful.

[raghavsol]

hi,
Thanks to Leigh for clearing about BFL and FFL difference and mentioning that both of the paramaters are mentioned on spec sheet. Probably I will ask next time for these parameters. I think BFL is more relevant for me. One more thing as you have suggested "For example, for the Nikkor T 360mm/f8 the FFL is 261mm while the BFL is 211.1mm". It means if my Zoom lens is somewhat of 100mm length then the total size of the system from lens 1st surface to sensor will be around 100 + 211 = 311mm. which will be a huge system. But in contrary i have seen many zoom lens + sensor systems of 250mm focal length with total size appreciably less than 311mm.

Moreover, recently i came to know the following two thumbrules in this case, which i would like to share with you all and look for your comments:
1) the total size of the system can not be more than the EFL of the lens. Because mostly the optics design is such that the principle plane(the reference point for EFL) lies beyond the optical system to the object the side. So mostly BFL = EFL - ZoomLensSize instead of BFL = EFL+ZoomLensSize what i understood earlier.
2) The more the number of element the more you can shift the principle plane to object side and hence can reduce the BFL. That is why the telephoto lenses (200mm focal length) normally have C or CS mounts which are for 12.5 or 17.5mm BFL.

raghav

[raghavsol]

Hi Dan,
thanks for your reply and valuable information. As you said "If you want more information you'll have to tell us what you're trying to accomplish. Putting it in our context (film, no smaller than 4"x5") would be helpful."

Actually my purpose is very simple, It is not specific to medium or large photography it is about basic optics. I can put it in this way- what will be the BFL for a lens of some EFL. Obviousally, in case of single lens design the relation will be BFL = EFL. But what will happen in case of multiple lens designs like coplex zoom lenses? I have to design an imaging system with 2 degree FOV(for which FL nearly equal to 400 mm on 13mmX13mm sensor) but not more than 200mm in length is it possible or not? That is my purpose/problem.

raghav

[GPS]

...
That is my purpose/problem.

raghav

Guys, just read his 16 posts and understand.;)

[Dan Fromm]

Raghav, you don't understand English. I gave you the C- and CS- mount standards. All C-mount lenses have the same flange-to-film (or chip) distance, all CS-mount lenses have the same flange-to-film (or chip) distance. Similarly for 35 mm/digital/medium format SLR systems. Every lens in one of those systems has the same flange-to-film distance.

A zoom lens -- there are many C-mount zooms -- has a fixed flange-to-film (or chip) distance that is defined by its mount.

Shut up and buy a 400 mm lens in your camera's mount.