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‘Scope Lens Query

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  • ‘Scope Lens Query

    So, looking for a bit of advice please as I’ve never been good with lens calculations and what is required.

    My 35mm projector has been up and running for a while in the home cinema, but now looking at showing some cinemascope films.

    Widescreen/flat features are projected fine but I have a small niggle when it comes to scope.

    Currently I have a Kowa anamorphic lens (photo attached).

    I also have a variety of lenses to use as the backing lens. These all came from the cinema the projector was originally installed in.

    The only one that seemed to project a decent image is the backing lens with the following markings:

    3.00 1:1.9 f=76mm

    Photo of the projected image is attached below too.

    Other ones I have produced a much smaller image.

    Whilst this is still watchable, I would love to fill the screen out a bit more. (Maybe I’m being picky!)

    So, I’m sort of guessing that the backing lens needs to be changed. Would love some pointers as to what size lens I need to look at.

    Here are some measurements:
    • Distance is 355cm screen to gate
    • Screen height is 120cm
    • Screen width is 220cm
    I have tried the lens I use for flat projection but obviously that fills out too much and the image is projected onto the walls too!

    Also, what does the adjustment do on the front of the anamorphic? It looks like it’s measured in feet. I’ve moved this around but it didn’t really do anything to the image.
    Help will be very much appreciated
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  • #2
    Hello Steve,

    There are several good threads in the old version of this forum that detail how to adjust the anamorphic lenses.

    In answer to the question on the calibrated ring, it is a rough adjustment that is dialed in for the distance from the projector to the screen.
    You will find detailed instructions in the old forum about exactly how to set these lenses.

    The focal length of the prime (backup) lens is one that fills the screen from top to bottom so the anamorphic can expand the image from side to side.

    Hope this helps. Delve into the archives for the really detained process.

    It is helpful to have the test film with details to the corners of the image.

    Side note: I adjusted the lenses at our theatre when I saw color fringing on the image from my seat on the organ bench.
    The test film was the flag in the opening scene of the movie "Patton"' where the stars of the flag were very helpful.

    From your picture of the screen it appears that you have a very small aperture plate and the image is not centered properly with the framing adjustment..

    Perhaps you could start without an aperture plate in the gate to see the complete image really being projected
    Last edited by Kenneth Wuepper; 11-25-2021, 01:17 PM.


    • #3
      Yes, frame the film and remove the aperture plate. Flat and scope use different apertures, even different flat ratios will use different plates although 1.85:1 was pretty much universal for flat. The scope image on the film fills the full frame height, so scope setup using a 1.85 aperture plate will be an exercise in futility.
      If you want to letterbox scope, the screen image should be 2.35x wider than the image height. So the image through just your backup, this would be the outside top/bottom lines of an RP40 frame, should be about 43% of the screen width. Then attach the scope adapter and the image should fill the screen.
      The focus adjustment on the scope adapter adjusts the relative focus of vertical and horizontal lines, there are plenty of both in your RP40. Adjust for the best balanced focus on these.

      Use the Schneider or Isco lens calculator to find the backup size you need. Both are in the "warehouse" here.


      • #4
        You may also find out that when all is said and done, your anamorphic lens will not allow perfect correction for astigmatism due to the shorter throw of an HT.

        Many years back, I had a pair of B&L anamorphics with that problem. I had a 23' throw.

        I was able to (carefully) remove the front correction element, and (carefully) shorten the the front of the lens barrel which allowed enough range to be able to fully correct for astigmatism.


        • #5
          If I'm understanding you correctly, your initial problem is just to fill the screen width during 'Scope projection, correct? You state that the current backing lens is "3.00 1:1.9 f=76mm". The "76mm" part is the critical number -- you need some lens with a shorter focal length, meaning a number less than 76mm. As a guess, without consulting any charts, but just looking at your supplied pic of your image on your screen, I'll guess that you'll need something around 70mm as your backup lens. You can use a lens with the EF marked in metric, or in inches, so 70mm would be about 2¾". (I just mention that because lenses can be a mixed bag and some may be found with inch-system increments.)

          One can calculate the precise focal length needed, using some of the available lens/image size calculators out there, but the increments of the available lenses will be what will be the final determining factor. (If you can find a 70mm lens as a loaner for a test, you can confirm that it's correct; if it's not correct, you can return it.) If 70mm isn't quite enough, you'd need something shorter, i.e. 65mm, but considering your dimensions, that seems like it would be too wide. (Or 67.5mm, but those lenses are not as common.) Note: by "70mm lens", I am referring to the focal length of the lens, of course, not a lens that is designed especially for use with 70mm film prints.

          The image we are seeing projected on your screen doesn't look exactly sharp either, so once you get the focal length settled, then you can work with the anamorphic attachment's field flattener adjustment setting, at the front of the lens to sharpen things up. You are using a very short throw distance, under 12', so I'm not sure how well that lens will behave at such a short projection throw...


          • #6
            Hello Steve, you would be more suited for a 65mm or 70mm base-lens to enlarge the scope image.Ideally a 67,5mm possibly.


            • #7
              There are lots of ways to calculate the right lens focal length. There used to be calculators on websites where you could plug in your info and they would tell you what lens to use. I know that there were some computer applications and I'm sure that there are some apps for your phone.

              I've used them before and I'm sure that some of them still exist but, to be honest, I just calculated by hand.

              It's easy: The focal length of your lens divided by the size of your image plane inside the projector is always equal to the size of the projector's throw divided by the size of the screen.

              (Focal Len. ÷ Film) = (Throw ÷ Screen)

              There are a few things to remember:

              1) Always keep your units the same. If the focal length of your lens is measured in millimeters and your throw is measured in feet, your answer will be messed up. Convert all your measurements to the same unit before starting. (e.g. Convert everything to meters.)

              2) If you have an anamorphic lens in the system, multiply your X-dimension by two. (Anamorphic lenses magnify the horizontal dimension by a factor of two.)

              3) Your throw is measured from the optical center (nodal point) of the lens to the exact center of the screen.

              4) Your calculations are probably going to give you an odd number for the lens size. As you know, lenses are made in certain increments. Pick the closest lens size that's shorter than your number.

              If you can just remember this, you'll be able to figure out what lens you need, any time. Heck! I've scribbled it on the lid of a pizza box.

              You asked what the adjustment on the anamorphic attachment does. It moves the focal point of the anamorphic portion of the lens so that it matches the focal point of the lens. If you don't do that correctly, your horizontal lines in your image won't be focused and your picture will look weirdly hazy.


              • #8
                That Cowa is a coke bottle. You could get some good use out of it blocking your car next time you do the brakes. ISCO is the preferred, the best is the ISCO Blue Star, .which is a 5 or 6 element lens. I can't remember. But the Blue Star is void of all anamorphic distortions and has excellent contrast. I used a pair on Dailies film projection on a bunch of movies. But they are very had to find... There are other ISCO's that will also work well. Schneider is ok, but not as good as ISCO. And I have seen Bausch & Lomb anamorphics that beat everything else out there. But those are also very few and far between. Most look like the COWA does.

                Update: There are two Blue Stars on Epay right now...
                Attached Files


                • #9
                  Hi everyone

                  thanks so much, some really useful tips and information here.

                  I removed the aperture plate and it slightly improved the size of the projected image. Also used the cylinder ring (which had been set to the same setting for nearly 50 years!) and managed to get a sharper-ish image. Attached a pic of this below. (Photo looks soft as I’d just cleaned the anamorphic with a Lens wipe!)

                  Going to test it again at the weekend with some scope trailers to see what it looks like with footage, rather than a test loop.

                  with regards to the prime lens, I’ve found this one on eBay:


                  It says 70mm - so presume that as this is slightly lower in scale than the 76mm one I have been using, this may fill the screen out a bit more? Am I right with that thinking? Plus, it’s UK based which helps for me.

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                  Last edited by Steve Pike; 11-26-2021, 09:57 AM.


                  • #10
                    A 70mm is a 70mm no matter who makes it. If it was a USA made lens it would be in inches. I believe some Japanese lenses were too. I see how hazy your picture is... That's because of the coke bottle.


                    • #11
                      Hello Steve,

                      You are making real progress now.
                      I do notice that the corners of your image are darkening. You might benefit from checking your light source.
                      Perhaps the distance from the reflector to the aperture is incorrect.
                      Be sure that the light source is properly focused as this can also degrade your image.


                      • #12
                        Yes, the Kowa is less than excellent. It's pretty much a copy of a Bausch & Lomb one from years ago. If the lens looks clean and clear all the way through, it's OK for most use although a better lens will give a better picture - these adapters were susceptible to internal cement damage if used on high powered lamp projectors that shows up as weird patterns visible looking through the lens.
                        Note if you're shopping that some adapters are "reverse", they shrink the image vertically rather than expand it horizontally so the backup is not the same size for a given image size. Also the threads differ on different lens makers with thread adapter rings probably NLA. If you're happy with the image as is then go with it, but maybe keep an eye on ebay etc for better optics.


                        • #13

                          What is the shred of material in the bottom of the aperture?

                          It shows up in focus at the top center of your image.


                          • #14
                            Hey all, thanks for the comments again really appreciate everyone’s help.

                            I’ve purchased the 70mm lens so that should help with filling out the screen a bit more. I’ll keep an eye online for a new anamorphic in the new year (when it’s not coming up to Christmas and money is being spent elsewhere!)

                            I have, today, tried a couple of scope trailers. Picture is acceptable, text is a tiny bit blurry but still legible - but for now, I’m happy and hopefully the 70mm lens will complete the setup for now until the new year

                            kenneth - the material at the top of the picture is actually some markings on the splice for the loop … must have taken the photo at that exact moment it went through!. I’ve adjusted my lamp so fills out a bit more too


                            • #15

                              Thanks for the positive feedback on your project.

                              Everyone here is very open and helpful especially with new entrants to the field of projection.

                              Let us know when you arrive at the next level.

                              Meantime, have a great Holiday Season.