Short throw 35mm projection: Reasonable focal length limits

[Jerry G. Axelsson]

Hello colleagues,

I am looking into 35mm projection in a small room were the throw is 3.75-4.00 meter only.
At this stage I have made tests with a 35mm focal length Schneider Premiere for FLAT which gives me an image which is 2.2 meters wide.
For scope I have used a 50mm Schneider Premiere intergrated anamorphic which gives a 3m wide image. The image is nice but somewhat distorted and has a bit of a Smile-box look.

My goal is to produce the largest possible image without apparent distortion for 1.85 and scope. Depending on future choice of projector, I may be facing limitations with what can be used in terms of using short focal lenghts without the image hitting the side of the machine. The projector is close to centered and has very little tilt (if any).

But at this point I would be interested to hear what focal lengths which you consider to be acceptable minimum with respect to image distortion when using red Iscos and modern Schneiders?

Any input would be appreciated.

Thank you kindly in advance.

[Vern Dias]

You're probably going to have to go with a curved screen to fix the pincushion. Also, many anamorphic lenses won't allow the level of astigmatism correction required at shorter throws. I had a pair of B&L anamorphics in my previous HT that I had to modify by physically shortening the front so that the correction element could be adjusted further inward to fully correct the astigmatism. That was with a 7 meter throw.

Honestly, if this is for a HT, you would be better off with video projection. I eliminated my 35mm film setup when I retired and downsized my residence about 12 years ago.

PXL_20221117_154009816.jpg
PXL_20221117_154442722.jpg
tic_20200813_084803c.jpg
You will have a far wider selection of titles with better image and sound quality, and no issues with vinegar syndrome, color fading, degraded prints, etc. You will gain the advantages of 4K HDR, lossless audio like Dolby Atmos, and DTS HD X, and the JVC RS3100 or RS4100, eshift X gives you an 8K equivalent resolution on the screen (no visible pixels and an amazingly film like image.

My HT is 12 feet wide, 13 feet deep, with a 9 foot ceiling. Screen is 52 inched high by 144 inches wide 2.76:1 with a 12 inch curve depth (required to correct the pincushion resulting from the use of an Isco 1.5x cinema DLP anamorphic lens). Projector is a JVC RS3100, Screen brightness is 15 fl (Low Laser) for SDR, 28 fl (Med Laser) for HDR, and 38fl (High Laser) for 3D (to compensate for the light loss caused by the shutter glasses).

One last screenshot of a scene on a blu ray release that was transferred from the original IB Tech elements shot in 1944:

PXL_20211118_011252879.jpg

[Vern Dias]

A couple of photos from my previous 35mm HT install:

IMG_0044.jpg
Back_Wall2.jpg

01010030.jpg

[Jerry G. Axelsson]

Hello Vern, thank you for your input. This arrangement we are looking into would be 35mm only at this point.

I have heard technicians suggest that the width of the image should be max 2/3 of the projection throw. Maybe there is something to this statement to give scope the space it needs to stretch out properly...

The intergrated 50mmm Schneider anamorph gives close to perfect focus even on this short throw. The problem is the distortion.
Even 35mm for flat looks great and is super sharp.

[Vern Dias]

Hello Vern, thank you for your input. This arrangement we are looking into would be 35mm only at this point.

I have heard technicians suggest that the width of the image should be max 2/3 of the projection throw. Maybe there is something to this statement to give scope the space it needs to stretch out properly...

The intergrated 50mmm Schneider anamorph gives close to perfect focus even on this short throw. The problem is the distortion.
Even 35mm for flat looks great and is super sharp.

The only fix for that will be to curve the screen. My suggestion would be to put up an image, measure the height at the center, mark it on a 1x2, then go to the extreme edge of the image and walk it toward the projector until the image height matches the height at the center and measure the distance to the screen or whatever you are using for a screen. This is the depth of curve you will require to correct the pincushion.

It's pretty easy to build a DIY curved screen. I built mine with poplar 1x2's and 1x3's and using white drapery blackout material.

Here's a photo showing how I constructed my floating screen:

PXL_20201123_211654734.NIGHT.jpg
The curve shouldn't affect the 1.85:1 flat image much. If it's to much for you, you should be able to file aperture plates to correct it.

The only way you will able to set the astigmatism (focus) on the anamorphic lens accurately is to project a cross hatch or similar test film loop and "rock" the projector focus back and forth. The vertical and the horizontal lines should come into sharp focus at the same time. If they don't, that the astigmatism is not optimal and this will affect overall focus to some degree.

1.85 will be sharp, no matter what the throw is.

Good luck.

[Mark Gulbrandsen]

You can easily do this with a Schneider or B&L scope lens. On the B&L you shorten the front edge of the silver barrel about 0.12", perhaps a bit more and also file off the backside of the guidance ears so the front element group can go all the way back in flush with the shortened front edge of the barrel. Keep track of the way the front group came out so it goes back in the same way. This should get your astigmatism down to 12 to 15 feet.

[Vern Dias]

A better explanation of the procedure I mentioned in my first reply to you. Thanks, Mark.

[Mark Gulbrandsen]

I made no other posts Vern, that was Jerry... Anyway, they will adjust well down to 15 feet. I machined mine in a lathe. The whole procedure takes under an hour. BTW, I like the chocolate Supers!! Had a customer in Wisconsin years ago that had painted his the same color...

[Sam Chavez]

Love the handy oil cans. Reminds me of a locomotive.

[Vern Dias]

I machined mine in a lathe. The whole procedure takes under an hour. BTW, I like the chocolate Supers!! Had a customer in Wisconsin years ago that had painted his the same color...

I did mine the old fashioned way: emery cloth on a flat surface.

[Mark Gulbrandsen]

We sometimes did that on garnet paper with lens element spacers when I was a tech at Canon. But we were only removing really tiny amounts of material when replacing lens elements... PIA, you'd be making trips into the darkroom to check.it on the lens projector then back to the bench to take them out again to take a teeny bit more off.?? Later TV lenses had matched groups.