Zoom vs. lens: Is my math correct? - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 17 Old 04-12-2020, 06:41 PM - Thread Starter
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Zoom vs. lens: Is my math correct?

I've been trying to do the math on whether my Panamorph lens is really helping much in my setup. I've been utilizing this calculator. My throw distance is 15 ft and my screen is about 106" wide.

Using the calculator, I'm getting the following values:
  • 34 fL zoom method
  • 57 fL for 16x9 anamorphic image at my screen height before lens expansion

Now, if I take that 57 fL number and multiply it by 61% (6% light loss through the lens, 33% light "loss" due to expansion), I get a result of 34.7 fL. This tells me that in my situation the lens is essentially pointless.

The calculator has a checkbox for if you're using a lens but it seems to just be doing a basic 1.33x multiple of the light output when using the zoom method. This gives me 45 fL, which obviously doesn't align with my math.

Am I doing this math correctly? Is the discrepancy due to the fact that the calculator isn't take the different zoom levels of the two options when checking the anamorphic lens box?

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post #2 of 17 Old 04-14-2020, 02:36 PM
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Quote:
Originally Posted by evanft View Post
I've been trying to do the math on whether my Panamorph lens is really helping much in my setup. I've been utilizing this calculator. My throw distance is 15 ft and my screen is about 106" wide.

Using the calculator, I'm getting the following values:
  • 34 fL zoom method
  • 57 fL for 16x9 anamorphic image at my screen height before lens expansion

Now, if I take that 57 fL number and multiply it by 61% (6% light loss through the lens, 33% light "loss" due to expansion), I get a result of 34.7 fL. This tells me that in my situation the lens is essentially pointless.

The calculator has a checkbox for if you're using a lens but it seems to just be doing a basic 1.33x multiple of the light output when using the zoom method. This gives me 45 fL, which obviously doesn't align with my math.

Am I doing this math correctly? Is the discrepancy due to the fact that the calculator isn't take the different zoom levels of the two options when checking the anamorphic lens box?
That calculator is worthless for brightness. it's only good for throw distance. You need a decent light meter and a test pattern test disc with a 100IRE test pattern to really know how many foot lamberts you are getting.
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post #3 of 17 Old 04-14-2020, 03:34 PM
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Your reference numbers must be off as the math seems valid. You can look at it in reverse. Whatever value you may actually measure at scope zoom will be brightened by something on the order of 27% brighter if that same projector and screen were going through an a-lens, less any additional loss by closing the zoom lens, but close to that.
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post #4 of 17 Old 04-17-2020, 10:13 AM - Thread Starter
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Originally Posted by ScottAvery View Post
Your reference numbers must be off as the math seems valid. You can look at it in reverse. Whatever value you may actually measure at scope zoom will be brightened by something on the order of 27% brighter if that same projector and screen were going through an a-lens, less any additional loss by closing the zoom lens, but close to that.
Yeah I figure the raw numbers are not right, but the relative values are more important. I've actually ordered a light meter to do a quick check to see if my thinking is in fact correct.

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post #5 of 17 Old 04-17-2020, 10:56 AM
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Originally Posted by evanft View Post
Yeah I figure the raw numbers are not right, but the relative values are more important. I've actually ordered a light meter to do a quick check to see if my thinking is in fact correct.
Again, that thinking is not correct. Whatever you might measure will be brighter for a comparable screen size unless you somehow have 33% loss from your optics, which won't happen. Even 20% brighter is something to be coveted..
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post #6 of 17 Old 04-17-2020, 10:58 AM - Thread Starter
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Originally Posted by ScottAvery View Post
Again, that thinking is not correct. Whatever you might measure will be brighter for a comparable screen size unless you somehow have 33% loss from your optics, which won't happen. Even 20% brighter is something to be coveted..
So you're thinking the light loss due to the zoom levels is negligible?

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post #7 of 17 Old 04-17-2020, 11:08 AM
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Originally Posted by evanft View Post
So you're thinking the light loss due to the zoom levels is negligible?
Well, I am including both primary lens and anamorphic lens in "optics." So, not negligble, it would be measurable, but still small, and could be counteracted by changing projector position. Depending on your lens choice you may be limited in positioning due to vignette or pincushion concerns, but it is not like you are changing zoom level from wide-open to closed, you are just closing it down to your regular 16x9 throw. If you did buy that light meter you could make the calculation, but I think 25-28% is what you will find.

If it did not make a difference there would not be a dedicated forum of people often paying as much or more for a lens than their projectors.
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post #8 of 17 Old 04-17-2020, 11:13 AM - Thread Starter
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Originally Posted by ScottAvery View Post
Well, I am including both primary lens and anamorphic lens in "optics." So, not negligble, it would be measurable, but still small, and could be counteracted by changing projector position. Depending on your lens choice you may be limited in positioning due to vignette or pincushion concerns, but it is not like you are changing zoom level from wide-open to closed, you are just closing it down to your regular 16x9 throw. If you did buy that light meter you could make the calculation, but I think 25-28% is what you will find.

If it did not make a difference there would not be a dedicated forum of people often paying as much or more for a lens than their projectors.
I think the difference may vary projector-to-projector. I know some lose basically nothing at different zoom levels, but supposedly JVC can vary more. With the zoom method, I'm at a zoom of 1.65, while with anamorphic it's at 1.22.

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post #9 of 17 Old 04-17-2020, 11:36 AM
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Originally Posted by evanft View Post
I think the difference may vary projector-to-projector. I know some lose basically nothing at different zoom levels, but supposedly JVC can vary more. With the zoom method, I'm at a zoom of 1.65, while with anamorphic it's at 1.22.
The loss from zooming simply as a factor of area lit by the same lamp is almost 44%, I think. My thought was the loss from changing zoom range won't be significant by comparison, even if it is measurable, but maybe I'm wrong.

Which Panamorph lens are you using by the way? One of the older Horizontal Expansion models I assume based on your math. On a sled?
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post #10 of 17 Old 04-17-2020, 11:43 AM - Thread Starter
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The loss from zooming simply as a factor of area lit by the same lamp is almost 44%, I think. My thought was the loss from changing zoom range won't be significant by comparison, even if it is measurable, but maybe I'm wrong.
Like I said, I think it can vary between different models and brands. Unfortunately I couldn't find a lot of info breaking down light level vs. zoom level on projectors to see how different projectors compare. I saw some writing on the JVC models not really benefiting from the lens at some throw ratios and zoom levels, but not a lot on others.

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Which Panamorph lens are you using by the way? One of the older Horizontal Expansion models I assume based on your math. On a sled?
It's an old UH380 HE lens on a custom sled, so it's actually somewhat easy to move it in and out of place. My throw ratio isn't quite high enough for a VC lens, otherwise I'd get one of those since the math for those work out quite well.

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post #11 of 17 Old 04-17-2020, 06:41 PM
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If you already have the lens, why bother with any calculation? Just try it with the lens and then without and see what you prefer. Done.

It really doesn't have to be that complicated. There's theoretical benefits of both approaches but what matters is what you think. You don't need a calculator to tell you that.


Plus, manufacturers specs are all useless or deliberately misleading. Even if brightness was the only issue, which it isn't, you'd have to take readings yourself at various stages of bulb life and from different parts of the screen under different levels of zoom. You'd also probably want to measure the effect on ansi contrast under regular lighting conditions (for your room). Not having excess light spilling out onto the walls and ceiling makes noticeable difference to contrast on my set-up.
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post #12 of 17 Old 04-18-2020, 09:41 AM
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I think your math is correct or at least the logic correct. I didn’t check the math.

The question is do you need the extra lumens.

If you think back to manual camera days and light meters and all that you set your exposure as a combination of shutter speed and aperture. Shutter speeds were setup to double with each click 1/30, 1/60, 1/120. etc. and aperture was in f-stops and each one doubled the area of the one before or doubled the light intensity. They went hand in hand if you wanted one click faster you got the same light by opening the lens one f-stop.

On a photo shoot outside I would setup and take a picture and one minute later go to take another photo and maybe a cloud covered the sun a little and I never noticed it. I would look in my camera and see my f-stop was off by 2. that’s doubled twice or a factor of 4. how could my eyes not know that? it was because my eyes also have an aperture and my iris adjusted 2 f-stops for the cloud and to my brain the light level never changed.

Eyes are the absolute worst light meters ever. Most humans can’t detect a half an f-stop or even a full f-stop that’s 50% to 100% as long as the brightness level is in the range where the eye can easily balance it.

Assuming you are fooling around with A-lens and zooming then I would assume your theater is not a well lit living room with tons of bias lighting, but rather a room where light is really well controlled. Then this gets back to theater levels of image brightness that used to be said 15 FL was about right for SDR and then HDR came along and projectors convert it to SDR with tone mapping and couldn’t ever come close to the TV brightness spec but I keep hearing for HDR projectors need about double or 30FL. I don’t pretend to understand HDR and projection but the extra brightness I’m told is for highlights and the main image is still around the 15FL mark. I’m also told there is no standard for it.

My point is I have been happy for years with SDR 15FL in the dark and about 25FL when I convert to media room and add in some bias task lights. When I do that the image doesn’t really seem brighter as the bias light is making my eyes adjust and I’m then compensating with the projector brightness. Everyone, well a lot of people say with 4K the resolution bump isn’t a big deal like it was with 1080p but they still do it for the brightness bump needed for HDR. I would guess if you had a real big screen and were on the edge of where your eyes wanted to compensate maybe you would notice. Otherwise A-lens / Zoom with what you are factoring in and how the eyes handle brightness I wonder if most could tell the difference.

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post #13 of 17 Old 04-25-2020, 01:05 PM - Thread Starter
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I HAVE ACTUAL MEASUREMENTS. These are with my calibrated SDR settings. Iris set to manual 0. I'm not accounting for screen gain in any of these.

Zoom method
21.83 ftL low lamp
32.79 ftL high lamp

Anamorphic Lens
26.01 ftL low lamp
38.92 ftL high lamp

So there is a 19% bump in ftL! Looks like the calculator was indeed off! Though the JVC's zooming does appear to knock out some of the advantage.

What's really interesting is that I measured my normal 16:9 screen (100") and came out with ~24 ftL in its calibrated SDR setting with the iris set to -5. ChadB measured it at 20 last year, but he was taking it off the screen which had a measured gain of about 0.8, which means the actual measurement today is around 20 as well.

My XY screen seems to have higher gain than my 16:9 screen, though I'm just eyeballing it. Based on this the zoom method appears to provide adequate brightness. Assuming the measurements are accurate of course.

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Just out of curiosity do you notice much of an image quality change to 1080p or 4k lens v zoom?
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post #15 of 17 Old 04-29-2020, 03:22 AM - Thread Starter
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Just out of curiosity do you notice much of an image quality change to 1080p or 4k lens v zoom?
I thought the image was slightly softer with the lens vs. without. I also noticed a color shift that could be corrected by altering the color temp to be a bit bluer.

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I agree with evanft but I'd add that a lot depends on the quality of the projector's lens and stretch processing. Even more depends on the quality of the anamorphic lens. I've seen some projector / lens pairings that look so clear that most would never notice any loss in sharpness. Some of those Schneider lenses are niiiiiiiiiiiice!

It's worth noting that the alternative (zooming) is also imperfect. I find an irritating loss in sharpness from not using all the projectors available lines. Projecting black bars causes a perceived (or maybe real) drop in contrast too. They always look a little gray / washed out on brighter scenes if you use a regular white or gray screen (regardless of which projector you use). It also causes excess light to leak onto the walls and ceiling, even if you coat the room in black velvet.

Basically, there's no perfect solution. The best I've seen is using custom resolutions on a CRT projector. As CRT can switch res, you can output 1920x810 and have it only project the image without black bars. The 2nd best (imo) is using one of Barco's 21:9 projectors with a native res of 5120x2160 with motorized curtains to mask the edges for 16:9 content.

I wouldn't bother buying an anamorphic lens just for more brightness. Most home theater projectors have enough brightness for a dark theater these days. I'd put the $3000-$10,000 into a better projector. I'd say it comes down to if you like the look an anamorphic lens adds to movies or not (I do). If money is no object and you can afford a good one, I'd say they look better overall than zooming.
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post #17 of 17 Old 04-30-2020, 08:25 AM - Thread Starter
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Originally Posted by Greyimporter View Post
I agree with evanft but I'd add that a lot depends on the quality of the projector's lens and stretch processing. Even more depends on the quality of the anamorphic lens. I've seen some projector / lens pairings that look so clear that most would never notice any loss in sharpness. Some of those Schneider lenses are niiiiiiiiiiiice!

It's worth noting that the alternative (zooming) is also imperfect. I find an irritating loss in sharpness from not using all the projectors available lines. Projecting black bars causes a perceived (or maybe real) drop in contrast too. They always look a little gray / washed out on brighter scenes if you use a regular white or gray screen (regardless of which projector you use). It also causes excess light to leak onto the walls and ceiling, even if you coat the room in black velvet.

Basically, there's no perfect solution. The best I've seen is using custom resolutions on a CRT projector. As CRT can switch res, you can output 1920x810 and have it only project the image without black bars. The 2nd best (imo) is using one of Barco's 21:9 projectors with a native res of 5120x2160 with motorized curtains to mask the edges for 16:9 content.

I wouldn't bother buying an anamorphic lens just for more brightness. Most home theater projectors have enough brightness for a dark theater these days. I'd put the $3000-$10,000 into a better projector. I'd say it comes down to if you like the look an anamorphic lens adds to movies or not (I do). If money is no object and you can afford a good one, I'd say they look better overall than zooming.
Good points all around, but I'd like to add some things:
  • I see no real increase in sharpness using the "whole chip". Maybe this was a thing back in the day, but now I find it pointless. The material going into my projector is either 4k native or 1080p upscaled to 4k, so there's really not anything that can be added by using more of the chip. If anything I would think it could make things worse as you're scaling an already scaled image, and only in one direction at that.
  • The black bars outside my screen are invisible. The black levels on my JVC are low enough that this isn't an issue.
  • Used anamorphic lenses are actually getting pretty cheap. I bought mine for about $450. Nice ISCO ones go for $1500 or less in some cases. I definitely think spending money on the projector is dramatically more important, but the cost is low enough on the secondary market that the additional expense wouldn't be enough to move you into the next projector level if you're already playing in the high end.

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