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This topic comprises 2 pages: 1 2
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Author
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Topic: Help with lenses
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Joe Elliott
Master Film Handler
Posts: 497
From: Port Orange, Fl USA
Registered: Oct 2006
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posted 10-13-2009 01:29 PM
To start out with, I am posting this in the Film Handler Forum, as there maybe some projectionists out there that will have this information that never check the Digital Forum.
I am looking at moving an old 1.3k Christie projector into one of our small houses for alternative content use. It had been in a larger house and was replaced by a 2k Barco. I have tried using all of the lens calculators I could find, including the one on this site, and they all give you a specific lens model, which I don't need. I need to know if the zoom range will be compatable. I used to have the formula to figure that out, but it is long lost. Here are the specs:
Old house:
78ft throw, screen is 36X15 common height
New house:
58ft throw, screen is 28X15 flat, common width.
I know for scope I would have to redo the software masking, or possible need a different scope lens, but all of the cable and satellite stuff we have been doing is all flat.
My basic question is: Will the same flat lens work in the new house. I can post the lens sizes tomorrow, as I forgot to bring them home with me, but just working the math, someone should be able to tell me. Also if you could post the formulas to calculate the zoom it would be helpful. Most of the lens calculator software gave me the zoom range of their lens that would work, but did not give me the specific zoom needed on each screen.
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Joe Elliott
Master Film Handler
Posts: 497
From: Port Orange, Fl USA
Registered: Oct 2006
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posted 10-15-2009 02:05 AM
quote: Dominic Espinosa
I'm confused. It may just be that it's late and I'm tired so excuse me if I'm wrong.
The first screen is 36x15, I presume this is scope, yes? If that's the case then it's the same size as the other screen because flat on the first screen is the same as flat on the second screen.
So if both screens are the same size but the throw is 20' less you'll need a different lens, otherwise the image is going to be too small. You need to gain something like .10" per foot of throw to fill the second screen.
Again, I might be missing something. More info would be a big help though.
Actually yes, I believe you are right. Flat would be the same on both. The main lens is a variable zoom lens, so it may be able to zoom out to fill the screen. Let me give the specs:
The main lens is a Minolta 3.2-5.7:1 lens. It is paired with a ISCO 1.5 Widescreen lens.
The Scope lens is an ISCO 1.9 Superwidescreen lens.
These are special lenses made by ISCO, not main production runs like the ones on their website, so I cannot compare them up.
I finally found the lens calculator I needed, but I'm still not sure if I am reading it right.
Christie Lens Calculator
It is a DCP H/I. If I am reading it right, then you're right, I won't be able to use that lens, unless I mask it for scope, and then will still have black on the sides.
Well, back to the drawing board. I'll have to check the other screens, and see if any of the smaller theaters match up proportionally.
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Karl Borowski
Expert Film Handler
Posts: 161
From: Sulking in GameFAQ Forum
Registered: Sep 2009
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posted 10-15-2009 08:36 AM
I'm not familiar with your lens. Is that inches, millimeters, Ancient Egyptian cubits?
Looks like, when you talk about a ratio to one, that that is an F/number, which is irrelevant for what we are talking about here.
I will see if I can't dig up some formulae for you tonight. . .
But anyway, sizes are almost always marked in inches, millimeters, or both. Millimeters are almost ubiquitous with digital equipment.
Again I don't have formulae handy, but with even basic math, you can do a simple ratio where you convert millimeters/inches to feet of throw (I assume you'll want to worry about flat only, especially with a 1.3K projector). (Hint: 1 foot = 304.8mm / 1mm =~.003281 ft.)
Then compare your old ratio, approximating what length you're at if you can't determine that in a menu or are at one end or the other of the lens, to x over new feet of throw to see if you are in the ballpark or not with the new screen.
So: Lens length you were at on the old screen (converted to feet)/78 ft. = x/58ft.
x = [(lens length in feet)x58feet]/78feet
The length is going to have to be ~74.36% (about 3/4) the length of the old one (so shorter) to give you the same dimensions.
You don't have to worry about ft.L if it was bright enough on the old screen and the new one is closer.
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