Film-Tech Forum ARCHIVE: Lens Help

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Posted by Steve McAndrew (Member # 8865) on 06-28-2017, 03:14 PM:

I have two 50mm lenses, generally they look identical however one has a much smaller rear lens (facing the film) than the other. Why would this be?

Thanks

Posted by Stephan Shelley (Member # 8587) on 06-28-2017, 07:53 PM:

Usually the ones with the larger rear element are for 70mm.

Posted by Sam D. Chavez (Member # 1841) on 06-28-2017, 08:13 PM:

Stephan is right but in short lenses like 50mm, I doubt anyone made a 50mm lens to play 70mm film. Different brands and models have all sorts of differences including aperture size. I would stick with pairs that look identical if they are to be used on the same screen.

Posted by Steve Guttag (Member # 268) on 06-28-2017, 09:28 PM:

Where to begin here.

Later lens designs like the ISCO Ultra-Star Plus (red barrel) or the Schneider Cinelux-Premiere are designed to capture more light by better matching the typical reflectors in use. These lens have a larger inlet pupil than prior designs. They have significantly more light throughput than prior deisgns despite having similar f-stops (the standard Ultra Star Plus is an f/2.1 but one can remove the standard ring for an f/1.7 or replace the ring for an f/2.4 or f/2.7. Even at f/2.4 they are brighter than prior designs. The Cinelux Premiere has a variable iris that also allows as fast as f/1.7 and can be stopped way down though anything much more than f/2.7 will result in a "black hole" of light in the middle.

As to 70mm format lenses, sorry Sam, your memory is getting you on this one. Short EF lenses for 70mm go about as far back as Todd AO (ISCO's T-Kiptikon series went pretty short) and via their .56X magnifier (used in the T-Kiptikon line originally) can get you down into the 40s. However, with the 70mm surgance in late 80s, both ISCO and Schneider made an entire line of under 100mm EF lenses for 70mm film that would get you down to 50mm and due to 8/70 being a popular alternative to IMAX for a brief period while studios like Disney were supporting it, there were also a selection of lenses with short EFs that could handle 8/70 as well as 5/70 too.

Posted by Sam D. Chavez (Member # 1841) on 06-29-2017, 01:04 AM:

Not a memory issue on this topic. One simply does not see these lenses out there in cinemas. Point out one example of a 70mm film based cinema running today other than the odd IMAX and I'll concede the point. I've only ever seen the lenses you refer to in a catalog, or a specialty situation. I have lenses as short as 45mm in int. scope lens but nothing that can play 70mm that short for sale or everyday use. Should not have stated they were never made. That was too attractive to prove wrong.

Posted by Pete Naples (Member # 761) on 06-29-2017, 04:27 AM:

On a total aside, I can't bear to see Isco & Schneider lenses thrown away. Not simply on what they cost when new but that they are superb examples of optical engineering. I've a fair few boxed in storage, one day I'll dream up some way of displaying them. Occasionally even now, a customer has a use for one, which is great! Most recently was 'Hateful 8' in Ultra Panavision 70mm. It just so happened that I had a pair of 70mm lenses in the focal length required by two venues who were running it over here.

Posted by Steve Guttag (Member # 268) on 06-29-2017, 05:07 AM:

Sam,

I've used them for 5/70 presentations. Heck, the Uptown theatre in DC used 62mm EF lenses for 70mm film. I used shorter than 100mm EF lenses for 5/70 at the Colonial Williamsburg Visitor's Center "Patriot Theatres" too. And yes, in IMAX venues were we augmented them with 5/70 systems, we used VERY short lenses as well for 5/70 (I want to say it is a 42.7mm lens but would need to look it up to confirm).

Thy were not ficticious lenses.

Posted by Sam D. Chavez (Member # 1841) on 06-29-2017, 09:18 AM:

90mm is a long way from 50mm cited in the original message.

Every example you cite is in the past.

The chances of a random 50mm lens capable of 70mm playback showing up alongside a run of the mill 50 is mighty high and I was answering the question the most direct way I could. Your explanation of the difference in pupil size was excellent but somewhat common knowledge. The "speed" of the lens is just a calculation involving the size of the specific set of optics vs an actual measurement of the light that is passed. I found this out in my early days, many years ago.

Posted by Steve Guttag (Member # 268) on 06-29-2017, 09:32 AM:

The speed of the lens does not tell the whole story as the f-number is strictly based on exit diameter versus focal length. The Ultra-Star Plus was notably more light efficient than its predecessors and the Ultra-Star before it, even at f/2.4 was as bright or brighter than the f/2.0 Ultras that came before them.

The Schneider 70mm lenses designated for 5/70 or 8/70 film (below 105mm) would not look out of the ordinary as compared to many equivalent lenses of similar focal lengths of the day. Furthermore, towards the end, Schneider would supply the 70mm format lens in lieu of a 35mm format lens if they were out of the 35mm format lens so it is very possible for a late-model theatre to end up with a 70mm format lens below 100mm.

That said, checking things over, it would appear that once one got to 50mm EF, yes, the lens would be notably different since it would be made for 101.6mm lens mounts. At 60mm is where the transition seemed to occur from standard 70.6mm barrels to 101.6mm