Film-Tech Forum ARCHIVE: Help with lenses

This is topic Help with lenses in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

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Posted by Joe Elliott (Member # 4006) on 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.

Posted by Dominic Espinosa (Member # 2122) on 10-14-2009, 10:22 PM:

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.

Posted by Joe Elliott (Member # 4006) on 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.

Posted by Karl Borowski (Member # 5424) on 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.

Posted by Richard Hamilton (Member # 321) on 10-15-2009, 09:04 AM:

quote: Karl Borowski
ubiquitous

quote: Karl Borowski
formulae

WTF

Posted by Mark Gulbrandsen (Member # 72) on 10-15-2009, 09:30 AM:

Rick,

If it's ancient Egyptian Cubits you should easily be able to help him out!

Mark

Posted by Richard Hamilton (Member # 321) on 10-15-2009, 11:44 AM:

Mark,
i made a few calls, and I think I heard my Egyptian friends flipping me off over the phone. Something about middle fingers [fu]

. Thats not aimed at you

Posted by Steve Guttag (Member # 268) on 10-15-2009, 08:36 PM:

Joe...this obviously goes to 1st generation DCinema projector.

The native aspect ratio of those projectors was 5:4 (SXGA, 1280x1024).

The lens markings are in "Throw Ratio"...this is how Videiots do things since they don't understand the math. The problem with throw ratio is that the lens must know what projector it is going into (its aspect ratio and, more specifically, the imager size).

Here is how throw ratio works. Take the throw and divide it by the WIDTH of the image...that is your throw ratio.

So, lets say you have a throw of 78-feet and a screen height of 15-feet [Wink]

The image without the anamorphics is going to be 15 x 1.25 (5:4 ratio) or 18.75-feet. divide 78/18.75 which gives you a throw ratio of 4.16. So your 3.2 to 5.7 fits into that range for the old house.

You multiply the 18.75 by 1.5 for the first anamorph and you get 28.125 and you will find that it comes to 1.875...which is wide enough for 1.85. Multiply 15 by 18.75 by 1.9 (the second anamorph) and you get 35.625 or a ratio of 2.375. Since the lenses are stamped to 2 sig. figs....really they are probably closer to the 1.85 and 2.39 ratios we know since in scientific terms, we can only calculate to 1.9:1 and 2.4:1

In the new house you repeat the exercise but we concerned about the width.

58/18.75 = 3.093 so in this house you will not be able to achieve full height for FLAT nor full width for flat...it will be close though. Scope choices are there. You can have a similarly undersized Scope with the 1.5 anamorph or you can use the 1.9 anamorph and zoom the lens out (with suitable refocus and lens shift. Or you could leave the prime lens alone and then reduce the number of pixels to form the scope image (bad idea as it reduced the light and resolution).

So if you take your throw and divide it by the shortest Throw Ratio you get: 58/3.2 = 18.125. Divide that by 1.25 and you get your max height of 14.5-feet.

SG

Posted by John Wilson (Member # 269) on 10-15-2009, 09:21 PM:

quote: Karl Borowski
Is that inches, millimeters, Ancient Egyptian cubits?

Geez Karl. Is it your intention to piss everyone off or are you just trying to hone that supposed talent you think you have there?

[Roll Eyes]

Posted by Karl Borowski (Member # 5424) on 10-16-2009, 01:56 AM:

No, John, I was trying to be funny and find out if they were in inches or mm.

I am pretty sure that you and Richard are being fucking dickheads and following me around from thread to thread.

Good thing you guys both have helpful comments though. I was too busy "making fun," of Joe, while the two of you both gave him helpful answers.

Posted by Joe Elliott (Member # 4006) on 10-16-2009, 03:58 AM:

Steve,

Yes, that makes sense, because when I was trying to calculate it as focal length, it mathamatically should not have fit in the old house either.

Although the math is going in one ear and out the other, I just spent 12 hours doing builds and commercials, and it is now 5am Friday, so I'm pretty tired. It should make sense to me by Saturday when I've had more sleep.

Posted by Richard Hamilton (Member # 321) on 10-16-2009, 08:56 AM:

quote: Karl Borowski
I am pretty

Posted by Steve Guttag (Member # 268) on 10-16-2009, 12:24 PM:

Karl, trust me...it isn't just Richard and John that think you are out of line...

-Steve

Posted by Richard Hamilton (Member # 321) on 10-16-2009, 01:31 PM:

Steve, please call me rick, "richard" and "fucking dickheads" are out of line [thumbsup]

Posted by Steve Guttag (Member # 268) on 10-16-2009, 02:00 PM:

Okay "Dick" [Wink]

Posted by Richard Hamilton (Member # 321) on 10-16-2009, 03:30 PM:

Hey...thats Mr. Dick to you, you ubiquitous kinda guy. [beer]

Posted by Mark Gulbrandsen (Member # 72) on 10-16-2009, 07:00 PM:

[Eek!]

Posted by John Wilson (Member # 269) on 10-16-2009, 07:29 PM:

quote: Karl Borowski
Is that inches, millimeters, Ancient Egyptian cubits?

quote: John Wilson
Geez Karl. Is it your intention to piss everyone off or are you just trying to hone that supposed talent you think you have there?

quote: Karl Borowski
No, John, I was trying to be funny and find out if they were in inches or mm.

And just how is that supposed to be read as you being funny? Do you see the graemlins down to the left when you post? Perhaps you should use them.

quote: Karl Borowski
I am pretty sure that you and Richard are being fucking dickheads and following me around from thread to thread.

Whatever you reckon. You're not helping yourself with your latest effort. Please read the rules you agreed to when you joined...especially Rule #1 which says:

quote:
RULE #1 Personal attacks These forums are moderated in the respect solely toward personal attacks. Everyone is welcome to participate in any capacity until the point comes at which other members are being singled out and directly insulted. This does not mean you cannot post a negative report on a product, service or customer support. All this refers to is the tone of the post if made toward a specific member of the forums. Disagreements are a daily occurence on the forums and will not be moderated, but personally attacking another member will not be tolerated. Please note "attacking" another member includes making discriminatory comments based upon skin color and/or sexual preference. Discriminatory comments are moderated at the exclusive decision of the Admin and Moderators, so make sure your post reads properly. If you are unsure if your post will "read" poorly toward another, please consider using smileys to assist in expressing the tone of your post or re-word it before posting, or refrain from posting it completely.
The bottom line is to please be polite, mature and professional towards the other members or you will be removed from the forums, regardless of your knowledge or stature in the industry. This includes sending harrasing emails to other members and comments made in the chat room. When in doubt, please do as your mother told you and keep your comments to yourself. Non-members, or members which we have not heard a peep from in ages, are of course fair game.

Now, is that clear enough for you Karl?

Posted by Sam D. Chavez (Member # 1841) on 10-16-2009, 11:37 PM:

Also, the same size screen at a shorter distance does not mean more light.

Posted by Steve Guttag (Member # 268) on 10-17-2009, 07:01 AM:

Actually...in the case of DCinema...it could, quite possibly mean a little more light but not for the incredibly silly reason that many people think (that somehow the air is SO thick that the distance the light has to travel somehow cuts into the light)

The reason it is probably going to be a little brighter is that the lenses used in DCinema, at the moment, are variable focal length (not really Zooms as a "Zoom lens" also denotes that the focus holds as you adjust the focal length and these lenses definitely don't do that). You will find that their f/stop tends to be faster when adjusted to the shorter focal length. You will find that variable focal length lenses also will print their f-stop in variable form too. The lower number (faster) will correspond to shorter focal length.

Though I have not measured the differences in DCinema lenses, I have in LCD projectors and you'd be amazed at the light through-put differences between the extremes of a variable focal length lens' range.

Steve

PS, thanks for correcting me, Mr. Dick sir.

Posted by Karl Borowski (Member # 5424) on 10-17-2009, 11:38 AM:

That's what I thought it was too Steve: a variable F/stop on a variable focal-length lens.

I've never heard of "throw" being used to mark a lens before. You learn something new every day.

Posted by Sam D. Chavez (Member # 1841) on 10-17-2009, 03:12 PM:

And indeed, you do have a lot to learn, film, tact, etc.

Posted by Mark Gulbrandsen (Member # 72) on 10-17-2009, 04:36 PM:

quote: Steve Guttag
but not for the incredibly silly reason that many people think (that somehow the air is SO thick that the distance the light has to travel somehow cuts into the light)

The air in some towns is pretty dam thick...

Posted by Gordon McLeod (Member # 33) on 10-18-2009, 12:23 PM:

sometimes the air is pretty thick here on film-tech [Big Grin]