The "A Lens" - AVS Forum
View Poll Results: Have me buy a A lens or not for my situation
Buy the A lens and leave us alone. 8 80.00%
Spend money in different area of theater or blow it on Scotch. 2 20.00%
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post #1 of 11 Old 01-12-2013, 05:04 AM - Thread Starter
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This thread is similar to others but I want to give more specific information. I am really trying to decide what to do. I just bought a JVC RS66 (love it by the way). After spending couple of weeks with it now I started to consider a A lens. The JVC is pretty loud in high lamp and when using the zooming method to fill my 138" 1.5 screen it has to be in high lamp. At first I was think if I used a A lens I might not have to use high lamp, but I am watching my 16.9 image at 11.9 lumens and when I zoom that out to fill the 2.35 area it goes down to 7 lumens. The 20% or so that I will gain back from the A lens will not be enough to keep it from being in high lamp any ways.

So I guess by going to a A lens I would be gaining a little more contrast ratio and a little tighter image. The question is... is it worth it? I know that is a loaded question but what would you do? Spend the money on the A lens or continue zooming and spend the money in other parts of the theater?

Projector JVC RS66 - ceiling mounted
17' throw
Stewart 150 Ultramatte 138"
Bat Cave
Seating distance 16'
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post #2 of 11 Old 01-12-2013, 06:16 AM
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Just a quick question: What are you using to measure the fL as that seems pretty low compared to what I'm getting from an X35. My screen is narrower at 112" but the same gain at 1.5. I'm using minimum zoom and getting 14fL when using my Isco II lens and -9 on the iris in low power. I found that the fL readings using my i1 display Pro enhanced meter were low compared to my calculated fL based on a lux meter reading at the screen. This is due to the directional nature of the screen making it hard to get an accurate light level reading with the i1D3.

I found that there wasn't much difference between zooming brightness and using my lens either. This is likely because I'm at minimum zoom and there is a big change in brightness as the projector is zoomed at this end. There is likely a big change in contrast too, hence why I keep the lens. I wouldn't base getting a lens on expecting a huge difference in light output due to the effect of the projector's aperture altering the light output as it zooms. When I checked with my old HD350 it was something like 78 lux (lens) verses 74 lux (zoomed), so not even worth worrying about as you'd never see that difference even side by side IMHO.

The throw is going to be around 2.0 using these figures, so pincushion will be worse than I suffer (negligable) at 2.8 but well above the usual target of 1.5+

138" wide 2.35:1 is 103" width without the lens. 17' throw is 204" so I make that 204/103= 1.98 throw.

If it wasn't for the noise issue, I'd say just stick without the lens and buy a lamp when it dims too much, or do the same in low lamp and just accept a dimmer image. I meant to ask, but I'm presuming that you've opened up the iris (aperture) as well?

Zooming: Been there, done that, bought the lens...
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post #3 of 11 Old 01-12-2013, 09:15 AM - Thread Starter
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I am using a lux meter (this one) and using the calculation for lux reading * .0929 * screen gain (1.5) That is correct right?

My iris is set at -11 for 16.9 and when using 2.35 zoomed and high lamp -10. I know I can raise this but I hate to give up the blacks.

Thanks for all your info. I am going to spend the weekend watching movies (and enjoying the picture smile.gif and try and come up with a decision
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post #4 of 11 Old 01-12-2013, 09:48 AM
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Maybe not the best meter in the world as the range we need is it's lowest range and no decimal places, but I doubt it's that much different to mine (yes the calculation is the same as I use, down to the x1.5 for screen gain).

I've just open the iris more and stay in low lamp, the change in the contrast shouldn't be that bad verses wearing out the lamp quicker and the noise factor. I thought you were maybe at max iris already.

Another 'cheat' is to use HDMI standard and clip all above 235 which will gain you a little light over enhanced and allowing up to 255. A compromise that I use is Superwhite and I clip at 245 to allow for a small amount of headroom (especially if some processing in the Lumagen causes over 235 in some circumstances).

Zooming: Been there, done that, bought the lens...
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post #5 of 11 Old 01-12-2013, 09:31 PM
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ftL = fc * screen gain (e.g. 1.5 for UM 150). Your figures are not Lumens, or if they are you have something seriously wrong.

Lumens = fc * area of projected image (in square feet)
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post #6 of 11 Old 01-13-2013, 07:33 AM
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He has got something wrong...in the RS4810 owners thread he confirmed someone else with the same meter found that the meter wasn't accurate and his fL has been measured as being 28fL not 10fL. Cheap meters might be worse than no meter at all...

Zooming: Been there, done that, bought the lens...
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post #7 of 11 Old 01-13-2013, 07:41 AM - Thread Starter
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Kelvin I will check the post in the 4810 forum but I have never gotten 28fl??? EDIT - I went back and looked unless I am missing something I have not even posted in the 4810 forum. Can you show me the post you are talking about please. I want to fix it if I mistyped....

So I have been calculating wrong? I take the lux reading at the screen facing the projector. I then take that reading and multiply that by .0929 multiplied by 1.5 (gain). Do I have something wrong? I also have a Sekonic 358 but that reads in Ev. so I have not used it
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post #8 of 11 Old 01-13-2013, 09:56 AM
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Sorry I mixed you up with someone else who has been using that same meter. He found that the meter was completely inaccurate and measured 10fL when a calibrated meter confirmed he was really getting 28fL.

Apologies for the confusion. redface.gif

It looks like you might not have a problem with your projector, perhaps just with the meter.

Zooming: Been there, done that, bought the lens...
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post #9 of 11 Old 01-13-2013, 12:47 PM - Thread Starter
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I hate math se we will se how it goes smile.gif I am going to use my Sekonic meter and get the Ev reading and try and convert it to lumens and see what it says. I don't guess anyone has a simple equation on how to convert a Ev reading to lumens?
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post #10 of 11 Old 01-13-2013, 12:56 PM - Thread Starter
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Also I see the other confusion guys sorry. I said lumens and I meant to say ftL.
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post #11 of 11 Old 01-13-2013, 02:10 PM
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Quote:
Originally Posted by clrv View Post

Also I see the other confusion guys sorry. I said lumens and I meant to say ftL.
There you go. Conversions from fc to ftL and then to Lumens is easy. I'm not familiar with the Ev.
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