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kal
Forum Administrator
Joined: 06 Mar 2006
Posts: 17860
Location: Ottawa, Canada
TV/Projector: JVC DLA-NZ7
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 Link Posted: Sun May 04, 2014 11:26 pm Post subject: |
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I suppose one thing is true for CRT projectors: There's no zoom on the lenses so there's only one "correct" throw distance for a given screen width. If you do install too far back you'll use less of the tube surface and therefore have less light output. So in that sort of situation moving the CRT closer would give you more light output which in turn would give you a higher contrast ratio (black level stays the same).
But that to me is an incorrect vs correct CRT installation.
Kal
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HogPilot
Joined: 21 Jan 2010
Posts: 2383
TV/Projector: Vizio P702ui-B3, Pioneer Elite Pro-151FD & 111FD
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| Link Posted: Mon May 05, 2014 4:12 pm Post subject: |
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| kal wrote: | I suppose one thing is true for CRT projectors: There's no zoom on the lenses so there's only one "correct" throw distance for a given screen width. If you do install too far back you'll use less of the tube surface and therefore have less light output. So in that sort of situation moving the CRT closer would give you more light output which in turn would give you a higher contrast ratio (black level stays the same).
But that to me is an incorrect vs correct CRT installation.
Kal |
Ah, that makes sense - thanks for the explanation!
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| ecrabb wrote: | | Curt Palme wrote: | | Interesting, Mac isn't returning my emails. Go figure. |
He's mad at us for making Hog a moderator. He took his ball and went home.
SC |
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ecrabb
Forum Moderator
Joined: 13 Mar 2006
Posts: 15909
Location: Utah
TV/Projector: JVC RS40, Epson 5010
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| Link Posted: Mon May 05, 2014 5:19 pm Post subject: |
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| HogPilot wrote: | | Kal's almost universally true statements about the relationship between throw and contrast ratio is due to the fact that the vast majority of LCoS, DLP, and LCD projector lenses have an iris in them that changes the f-stop of the lens as the image is zoomed in or out. There are a couple exceptions to this - Marantz's VP12Sx and 11Sx are all examples of projectors that have constant aperture lenses that have the same f-stop regardless of the zoom position. Of course, the lenses on those units are relatively expensive (not just because of their constant f-stop), so this is the exception rather than the rule. |
One point of clarification on this subject, Hog… Variable-aperture zoom lenses don't change aperture because they "have an iris that changes as the image is zoomed in or out". The aperture changes simply because most zoom lens designs simply use a much smaller portion of the lens at the longer focal lengths, therefore reducing the amount of light the lens passes. The net effect is similar to if there were an iris, but it's actually just a side effect of the design of the lens. In addition to increased contrast like has been mentioned, there is usually increased sharpness in the corners, and increased depth-of-field (which means a deeper focus plane or zone).
Constant (or fixed) aperture lenses, pass the same amount of light irrespective of the focal length setting. Downsides are that they're usually larger, heavier, and more complex than their variable aperture brethren of similar focal lengths.
Most zoom lenses have better or worse geometry at different focal lengths, too.
Cheers,
SC
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JayAllan
Joined: 03 Jun 2007
Posts: 175
Location: Los Angeles
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| Link Posted: Tue May 06, 2014 9:56 am Post subject: |
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| HogPilot wrote: | | kal wrote: | | HogPilot wrote: | | Is it opposite for CRTs? Shorter throw = higher contrast? |
I don't think so - not sure. I don't think it makes a difference (?)
Kal |
Oh okay, thanks. I only asked because JayAllan talked about mounting his RS1 closer to try to get more punch/contrast out of it, I didn't know if that was a CRT thing or not. |
My line of thinking was that by moving the projector much closer then with all things being equal if the brightness was set to where it was about to blow out in the highlights, by moving the projector should massively increase the amount of light hitting the screen (inverse square law) therefore raising the brightness level so that it would overexpose. Additionally as was mentioned I would be using a wider part of the lens which would further increase light output. Then I would turn down the brightness to get my highlights back which would in turn deepen the blacks. Of course I may be off since I am probably lightening the blacks at the same rate when I move the projector in. 
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HogPilot
Joined: 21 Jan 2010
Posts: 2383
TV/Projector: Vizio P702ui-B3, Pioneer Elite Pro-151FD & 111FD
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| Link Posted: Tue May 06, 2014 8:15 pm Post subject: |
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| ecrabb wrote: | | HogPilot wrote: | | Kal's almost universally true statements about the relationship between throw and contrast ratio is due to the fact that the vast majority of LCoS, DLP, and LCD projector lenses have an iris in them that changes the f-stop of the lens as the image is zoomed in or out. There are a couple exceptions to this - Marantz's VP12Sx and 11Sx are all examples of projectors that have constant aperture lenses that have the same f-stop regardless of the zoom position. Of course, the lenses on those units are relatively expensive (not just because of their constant f-stop), so this is the exception rather than the rule. |
One point of clarification on this subject, Hog… Variable-aperture zoom lenses don't change aperture because they "have an iris that changes as the image is zoomed in or out". The aperture changes simply because most zoom lens designs simply use a much smaller portion of the lens at the longer focal lengths, therefore reducing the amount of light the lens passes. The net effect is similar to if there were an iris, but it's actually just a side effect of the design of the lens. In addition to increased contrast like has been mentioned, there is usually increased sharpness in the corners, and increased depth-of-field (which means a deeper focus plane or zone).
Constant (or fixed) aperture lenses, pass the same amount of light irrespective of the focal length setting. Downsides are that they're usually larger, heavier, and more complex than their variable aperture brethren of similar focal lengths.
Most zoom lenses have better or worse geometry at different focal lengths, too.
Cheers,
SC |
Thanks for the clarification - I thought I'd read that in multiple places, but my knowledge of projector lens design isn't very deep so it's always good to learn something 
_________________
| ecrabb wrote: | | Curt Palme wrote: | | Interesting, Mac isn't returning my emails. Go figure. |
He's mad at us for making Hog a moderator. He took his ball and went home.
SC |
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ecrabb
Forum Moderator
Joined: 13 Mar 2006
Posts: 15909
Location: Utah
TV/Projector: JVC RS40, Epson 5010
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| Link Posted: Tue May 06, 2014 8:41 pm Post subject: |
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| HogPilot wrote: | | Thanks for the clarification - I thought I'd read that in multiple places, but my knowledge of projector lens design isn't very deep so it's always good to learn something |
NP. What relatively little I do know about lenses, I know mostly from being into photography years before digital projectors even existed. In fact, several of the more seemingly esoteric concepts in projection have direct parallels in photography. For instance, Schiempflug is a fundamental principle you need to understand to properly use large-format view cameras; it's the "tilt" in "tilt-shift" lenses or cameras. I remember first reading about higher-end sets with Scheimpflug, trying to get my head around what it was, and the light bulb coming on… "Oh, damn… It's a tilt lens!"
Cheers,
SC
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HogPilot
Joined: 21 Jan 2010
Posts: 2383
TV/Projector: Vizio P702ui-B3, Pioneer Elite Pro-151FD & 111FD
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| Link Posted: Wed May 07, 2014 5:40 am Post subject: |
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| ecrabb wrote: | | HogPilot wrote: | | Thanks for the clarification - I thought I'd read that in multiple places, but my knowledge of projector lens design isn't very deep so it's always good to learn something |
NP. What relatively little I do know about lenses, I know mostly from being into photography years before digital projectors even existed. In fact, several of the more seemingly esoteric concepts in projection have direct parallels in photography. For instance, Schiempflug is a fundamental principle you need to understand to properly use large-format view cameras; it's the "tilt" in "tilt-shift" lenses or cameras. I remember first reading about higher-end sets with Scheimpflug, trying to get my head around what it was, and the light bulb coming on… "Oh, damn… It's a tilt lens!"
Cheers,
SC |
Your revelations are much appreciated - I'm a very-amateur photographer myself with Canon G9 time, and a desire to step up to a Nikon D600 or D610 (I know the D600 has some shutter issues that should be fixed with a recall). I'll have a relatively steep learning curve with a full-frame DSLR but I'm looking forward to experimenting with it and shooting some nice 1080P video as well.
_________________
| ecrabb wrote: | | Curt Palme wrote: | | Interesting, Mac isn't returning my emails. Go figure. |
He's mad at us for making Hog a moderator. He took his ball and went home.
SC |
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