Lens - screen size - throw calculator for digital systems?

[Frank Angel]

You all remember that very handy slideruler we use to carry around that let you setup every variable -- lens focal length, screen width & height and throw distance. It let you easily gave you any variable you needed if you input the others. You got the correct screen size and lenses for your particular setup with that nifty device. Schneider also had PC software that did the same. Is there an equiv that will calculate the same variables for digital lenses?

[Steve Guttag]

All of the projector manufacturers have one, online. I use my own spreadsheet to calculate them.

[Jim Cassedy]

<edited quote> You all remember that very handy slideruler we use to carry around that
let you setup every variable -- lens focal length, screen width & height and throw distance.
It let you easily gave you any variable you needed if you input the others. You got the
correct screen size and lenses for your particular setup with that nifty device.

I've still got one of those things. I think I got it free from one of the cinema supply houses when I
worked in NY years ago. I've seen then show up now on e-bay occasionally for as much as $50-$75

[Frank Angel]

Thanks Steve -- Now I recall actually using one a few years back, maybe Barco's and totally forgot about it. I was thinking about the old-school physical slide rule style that you could keep in the tool box. Of course with a cell phone and the internet...but then again, that slide ruler never needed to find a charger and a cable with the right freakin connector on it.

Jim, the older I get, the more I realize that we all live in Alice's Wonderland and we just try to convince ourselves we have control over anything that's going on, but really we are walking around n Fellini's SATYR ICON where very little makes any sense. I just saw a 1-sheet -- 1-sheets that most times came free or now for a few bucks -- for Universals DRACULA in an auction that sold for $500,000 plus!

I worked for a 90yr old theatre owner, Mr. Mattson in Caldwell TX, who owned the Mattsonian Theater and a Drive-In on the outskirts; both were dilapidated and he couldn't afford to run them simultaniously because they both shared equipment -- when it got colder in the winter, he'd closet the DI and run the hardtop but you needed the lenses and amplifiers and all the booth equipment, splicers, rewinds, etc., to be schlepped from the DI to the downtown theatre. He never even mentioned this to me and I only found out when one day I arrive a the DI to run the evening's double feature only to find a sign on the marquee that said,See movies at the Mattsonian Theatre Downtown. Drive In closed until the spring" made with varying size letters in at least three colors. So finding the place abandoned, I hitch-hiked back to town (quite amazingly, back then, hitch-hiking was quite a reliable mode of transportation) to the indoor theatre and after figuring out how to get to the booth, qoing across the hang ceiling and feeling crunchy things under my feet (rat and pigeon carcasses -- which is why the other projectionists called it the Rattsonian Theatre), I found half the contents of the DI booth strewn all about the room...reels of the double feature on house reels and not i cans, just thrown about, with lenses and such piled on a table. One of the titles was scope. When I yelled down to Mr. Mattson that there was no way i would be able to set up those anamorphics in time for the nights show, the ole man just yelled back that it'll be fine, just do it when you start the movie. ?? To say he was eccentric would be an understatement. So the audience witnessed the opening of SITTING BULL playing skewed across the screen and up on the side wall and out of focus. Same thing at the change-over to Projector 2. It was a disaster and a trauma that a projectionist never really forgets. But I digress...

This Rattsonion hard-top had a false wall between the exterior wall and the interior wall that ran the length of this 1000 seat theatre (typical of the time). He used the space for storage and on one wall had constructed this rickety shelving from floor to about 8 feet up with row upon row of cubby holes big enough to hold folded 1-sheets, Each of these cubbies were labeled with a year written in pencil or crayon, some so faded with age that they were hard to read. They starting with 1939....1940, 1941, etc up until the present, which at the time was about 1966. He had saved EVERY 1-sheet from every title he had ever run. I remember the first time I saw it, I ask why would he save all that worthless paper? He said in case some titles were ever re-released. He said quite frankly, he would probably just trash it all but that took time and effort and, like I said, he was 90yrs old. He asked me if I wanted to take any of them. Like a fool, I said, "Nah;" I was traveling light with not much more than a knapsack and where would I put any of this worthless stuff? Besides, I explained to the old man, "these are films before I was even born." Oh what fools, these mortals be.

I've have had a poster of the 2001: "Star Child" poster up in our apartment since I originally ran it. I think MGM charge me $2 for it. I just saw the same poster go for $3,000 plus, not that I would part with it, but I am just saying...how does any of it make sense? A buddy of mine has a house on a plot of land in Florida that was worth a million and a half in 2008 and a month later he is told it's worth 1/3rd that. Alice in Wonderland...nothing is real...we just think it is.

[Randy Stankey]

A pencil and a piece of paper is all I ever used to calculate lenses. Maybe a calculator if one was handy. It only takes a bit of high school algebra.

I find it perplexing that people seem unable to do such a simple thing without using some sort of app. Sure, if you're in the business of designing and installing projection systems, you would find it faster and easier to calculate which lenses are needed to outfit a 20-screen multiplex. There's nothing wrong with that but, as always, I believe that a person should know how to do it, themself, without need of an app. Even a person in the business who uses an app as part of their work should know how to do it by hand. Just the knowledge of how the math works is beneficial to doing one's job better. For example, it would be a way to do a quick "sanity check" on the results given by the app. But, alas, people just don't seem to know how to do things for themselves, anymore.

It's especially perplexing because I have never been especially good at math. In high school, my grade in Algebra I was 3%. For the whole semester! I had to take math in summer school because of it!

I didn't know it, back then, but I find it difficult to keep numbers straight in my head. I get the concepts, all right. If you asked me what "y=mx+b" means I can tell you, in a second, that it's the formula for a straight line. But, if you asked me to evaluate "y=3/7x+11" the numbers just seem to jump around on the page and I have a hard time making sense of things. Later in life, I found out that it's probably dyslexia. Regardless of whether that's actually true, I still feel like I have to work twice as hard to be considered half as good as others. That's why I find it so frustrating that people who don't have such problems seem to refuse to do "pencil and paper math." If a dumbass like me can do it, why can't others?

Even still, I find the concept of figuring out lenses and focal lengths to be a fun challenge. Like a puzzle. I grew up doing photography using manual cameras and tabletop projectors since I was about ten years old. Does anybody remember the "Sunny-16" rule? 125@ƒ-16 on a sunny day. (+1 stop for shade and -1 or -2 stops for direct sun.) You can push Tri-X by three stops but T-Max only gives you two stops. My God! Most people don't even know what the hell an ƒ-stop is, anymore! Anyhow, that sort of thing comes naturally to me because I've done it all my life. (Like the way Amish boys learn to calculate roof pitches in their heads by the time they are 15 y.o.) It confuses me when others don't seem to understand these kinds of things but it frustrates me (even pisses me off) when they refuse to learn! I understand that everybody's different. People have different interests. I get it but it bugs the crap out of me when people seem to refuse to learn something that should be considered a basic life skill!

That slide rule thing sounds interesting. I'd like to see one!

Perhaps, if somebody could get a good scan of one, others might be able to download the image, print it out, glue it down to a piece of poster board and make a copy. I've always liked playing around with things like that.

Somewhere, around here, I've still got an old "E6B" flight calculator. If you give me a little time to brush up on it, I betcha' I can still use it!

[Jim Cassedy]

<edited quote> I recall actually using one a few years back, maybe Barco's and totally forgot about it.
I was thinking about the old-school physical slide rule style that you could keep in the tool box.
Of course with a cell phone and the internet...but then again, that slide ruler never needed to find a
charger and a cable with the right freakin connector on it.:

That's the type I've got that's in storage right now with some of my other filmstuff. The picture below is of the
exact type I have. I got mine for free from some equipment supply house in NY back in the late 70's. The one
in the picture below is currently listed on e-bay for $50(us) + shipping. I was still using it on a regular basis
until I had to store some of my stuff during a move, and I liked it for all the same reasons Frank stated.
You didn't have to plug it in, or wait for it to boot up, and you could throw it in your toolkit and not have to
worry about it getting broken, etc, etc. Mine came in a thick cardboard protective sleeve, which protected
it from toolbox damage. As I said in another post on a different topic last week: "sometimes, less is more"

This Pic From The E-Bay Listing Shows The Front AND Back - One Side Was "US", & One Metric
LensCalculator.jpg

> It seems only the "professional" slide rule calculators that can do 35 & 70mm aspects and theatrical
screen sizes are priced high on e-bay. If you don't need 35/70 calculations, There are numerous ones
for computing 8 & 16mm lenses & throw distances for consumer type folding screens that can be had
for $10us or less.

[Frank Angel]

Yes, Jim -- thanks for posting those. I have the exact same one from MTE also for free. And all I really wanted to know was if when we went to digital, if anyone anyone made that same nifty tool as the ones Jim pictured above. Didn't realize Randy was going to get all passionate on me about it. I am not saying you shouldn't understand the the concept of ratios and math behind what the calculator is doing, that also doesn't mean if you USE the calculator, it doesn't mean your a dunce. Saving time by carrying this thin, brilliantly designed sliderule around to get quick and accurate numbers is maybe even easier than carrying a wad of paper around; besides, as good as anyone might be at doing the math, the slide rule will no doubt over the long term probably cause fewer mistakes...you know, those human errors humans make using pencil and paper. That's why when you see a NASA scientist carrying a slide-rule around in his pocket, you don't laugh and point and say, look he can't figure it out with paper and pencil. Such a handy tool for digital which I assume would be more complicated to design given that they would have to take into account the additional variable due to the zoom range of the lenses, which are not the single, fixed magnification that was the rule in digital projectors.

[Steve Guttag]

The biggest "trick" when doing lens calculations for digital is to realize you are always sizing for the full width of the imager, not particular ratio as we are now calculating throw ratio (length of throw divided by image width). So, since scope uses the full width, if the throw is 65 feet and the screen is 35-feet wide, the throw ratio is 1.86. If the screen is common height (scope screen), then the height is 14.64. Though the image width is 27.1-feet wide, the lens is always working with the width of the imager so, one multiplies 14.64 * 1.8963 = 27.8 so the throw ratio is 65/27.8 = 2.34. What the EF is doesn't matter as that is already "baked-in" to the lens' Throw Ratio. Now, if one is trying to use a lens that does not have a TR for the imager you are using (e.g. using a Rental/Staging lens that may have marking for a projector with a 1920x1200 imager), then one has to do the math to scale the throw ratios. But the vidiots went with throw ratio to keep the math easier, I guess.

Note, for common width screens, the throw ratios would be very similar between flat and scope as there is just a slight difference in width. In fact, I often will crop the scope image slightly to avoid moving the lens between formats (allows it to hold focus and alignment by not moving it). But, if one were to have the screen/throw as in the example above, using a 35-foot wide screen, the full-frame flat height would be 18.92' 18.92 x 1.8963 = 35.88 so the throw ratio is 65/35.88= 1.81

As you can see...a slide rule would be of little benefit. It is bulkier than the phone you already have since the math is nearly trivial. You can set up a spreadsheet with, again, a simple formula that would allow you to plug in the numbers, which is what I do so when it comes time to ordering lenses, I can see how close I am to the lens zoom range (there are normally overlapping lens ranges and I don't want to be any closer to the extreme end of the range than I have to.

[Gordon McLeod]

Pat Moore did a very good spreadsheet wen he was at strong that also did port size lumen requirements based on screen gain and even did a strong purchase order

[Jim Cassedy]

I had to dig way in the back of my file cabinet to find this, but I knew I still had another one of those screen size calculators stuffed away
somewhere. This is not the one I mentioned in my earlier post. This is a different one. I think I got it (free) at a trade show in the late 70's

ScreenCalc_Out.jpg
Age Has Turned The Little Slide Rule Window Yellow Since The 1970's ( "just like it's done to my teeth!"; he said with a sly drool )


One Side Has Lens Calculations For "Indoor" Screens Sizes & Throws, The Other Side is for "Outdoor" Applications

ScreenCalc_In.jpg

JC

[Joao Lopes]

Excel file for NEC projectors: Download here.

[Ioannis Syrogiannis]

Jim, it took me a few moments to figure out how 1% Crop makes 2% difference (.600 to .588 on "TYPICAL ASPECT RATIOS"), but then I put... one and one together.
That is 1% per side...

[Frank Angel]

NEC spreadsheet pretty comprehensive.