Measuring Light with A Lens and Zoom CIH - AVS Forum
Forum Jump: 
 
Thread Tools
post #1 of 21 Old 09-14-2011, 08:04 PM - Thread Starter
AVS Special Member
 
coolrda's Avatar
 
Join Date: Nov 2002
Location: Bakersfield, CA
Posts: 1,007
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
There’s confusion sometimes of how to properly measure light for comparison of the different CIH systems, projector lumens to size of screen needed for zooming or A lens use. Its important to calculate this correctly in order to get an understanding of what can be expected with both systems. Theres four ways in which to have a CIH, Retractable A lens, Fixed A lens, Expanded Zoom and Fixed Zoom(aka shrink method). I only want to discuss light, not the pros and cons of each.

When comparing to a full panel 16x9 image, all four options lose light. Its a fact. larger screen area lit reduces brightness of image. For example, lets take a THEORETICAL projector that puts out 500 lumens and produces 20 foot lamberts on a 15 square foot screen on a 16x9 screen, and, an equivalent 2.35 screen of 20 sq.ft. Using a A lens with 1.33 expansion will fill the whole screen using the whole panel, but light will drop when compared to the 16x9 image w/o the lens in, because of the greater area the light cover. If the A lens has less than 33 percent loss its somehow amplifying the light, if more than 33 percent loss, light is being lost. If an a lens is truly neutral the light should drop from 20 to 15 ftls.

The next method is fixed A lens. Light will be the same for both 16x9 and 2.35. The 16x9 is both 33 percent darker and has 25 percent less horizontal pixel density vs the above method.

Zoom Expansion is much like the retractible A lens in that the 16x9 image will have more light output. A 16x9 image of 15 sq. ft. is expanded to 26.25 sq. ft. When expanded for 2.35, this method now lights a screen area that is 1.75x more than the 16x9 screen. So light will drop 57 percent compared to unzoomed 16x9 image because the image expands 33 percent in all directions.

Zoom Fixed w/Shrink is similar to A lens fixed with the 16x9 image being darker and with less pixel density.

Now its common for a lot of confusion over what does what, A lens losses and gains, Zoom does this or that. The fact is the A lens doesn’t increase light over Zooming. Its justs allows less light to be lost. Were able to control the 1.33x expansion to the horizontal plane only. So the expansion, while taking place, doesn’t expand in both the vertical and horizontal plane, which is what decreases a Zoomed systems light when compared.

Here’s another way to look at it, as a constant. If that 15 sq.ft. is 16x9, then the 20 sq.ft. screen is 21.33x9 and the 26.25 screen is 21.33x12(16x9 in shape). So as far as light output goes, the A lens controls the expansion to eliminate wasted light, in essence turning a 16x9 panel chip into a 2.37 one. Another thing to remember is increasing isn’t the same as decreasing. You can say an A lens image has 33 percent more light than zoomed or a Zoomed image has 25 percent less light than an A lens method, both are correctly saying the same thing from a different perspective. So always use SCREEN AREA to calculate light at screen. So for A lens you use the 2.35 screen area, for zoom you use the width of the same 2.35 screen and 1.3333x to the sq. ft. for the off screen letterbox.

So why does that 33 percent gain only translate into a 10-20 percent light gain in the real world numbers. If all things were equal you would move your projector back to zoom to keep the image qualities equal. But thats not reality. The projector normally has a stationary or mounting point thats determined by the owners preference or the room. So the lens is zoomed and not projector moved back, to accommodate the wider screen. By the very nature of zooming, light is increased, therefore real world losses are less. It just depends on the projector how much.
coolrda is offline  
Sponsored Links
Advertisement
 
post #2 of 21 Old 09-15-2011, 10:56 AM
AVS Special Member
 
Gary Lightfoot's Avatar
 
Join Date: Jun 2000
Location: Surrey, UK
Posts: 4,436
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Liked: 35
With the zoom method, the light loss depends on where the pj zoom is within its range before it zooms larger - some will lose less light because they gain some back from the iris effect due to the change in F-stop value. In some cases the light loss may be negligible from 16:9 to 2.35.

Gary

Quote:
Originally Posted by elmalloc
Who says Cameron is "right" and why do we care about him so much - lol!

I trust Gary Lightfoot more than James Cameron.
Gary Lightfoot is online now  
post #3 of 21 Old 09-15-2011, 01:57 PM - Thread Starter
AVS Special Member
 
coolrda's Avatar
 
Join Date: Nov 2002
Location: Bakersfield, CA
Posts: 1,007
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Originally Posted by Gary Lightfoot View Post
With the zoom method, the light loss depends on where the pj zoom is within its range before it zooms larger - some will lose less light because they gain some back from the iris effect due to the change in F-stop value. In some cases the light loss may be negligible from 16:9 to 2.35.

Gary
Exactly and certainly there could be projectors that are more Zoom-friendly. My experience with the 5 projectors I've had is zooming all the way was comparable to going from low to high lamp. At the same time, lens users could move the projector much closer to the screen, as much as permitted by the lens aperture.
coolrda is offline  
post #4 of 21 Old 09-17-2011, 11:22 PM - Thread Starter
AVS Special Member
 
coolrda's Avatar
 
Join Date: Nov 2002
Location: Bakersfield, CA
Posts: 1,007
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Took readings with zooming and lending at different intervals and all readings with the JVC RS40 were consistent with other projectors I have. The light loss when I zoom is 15 percent compared to using the lens. If you are using the zoom method, the lens method will give you a 17 percent increase in light output.
coolrda is offline  
post #5 of 21 Old 02-14-2012, 05:54 AM
Member
 
kabrumbs's Avatar
 
Join Date: Apr 2006
Posts: 67
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
I'm trying to decide between the zoom method and anamorphic lens, and brightness will be the key factor. I will use a Sony VW95ES with, throw of 12 feet and 10 inches, screen of 42'' high x 98'' wide, StudioTek 130 (1.3 gain)

I'm using the http://www.eliteprojectorcalculator.com/ to calculate the foot lambert for a 16:9 screen with projector on Best Mode, Low Lamp.

Calculating for zoom method
I have to calculate the FL for a 16:9 screen of 98'' wide times 0.75, to acount the pixels that will not be using. So, it will be 16.5 fl * 0.75 = 12.375 fl...

Calculating for lens method
I will be using a Prismasoni HD6000. Some say that it has a 1% loss. So i have to calculate the FL for a 16:9 screen of 42'' high, times 0.75, times 0.99. So, it will be 23.9 fl times 0.75 times 0.99 = 17,74575

So I can expect an increase of 43% on brightness using lens against the zoom method.

Is this correct??
kabrumbs is offline  
post #6 of 21 Old 02-14-2012, 10:10 AM
AVS Club Gold
 
HogPilot's Avatar
 
Join Date: Jun 2006
Location: Good Ol' US of A
Posts: 2,869
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 18
Quote:
Originally Posted by kabrumbs View Post

I'm trying to decide between the zoom method and anamorphic lens, and brightness will be the key factor. I will use a Sony VW95ES with, throw of 12 feet and 10 inches, screen of 42'' high x 98'' wide, StudioTek 130 (1.3 gain)

I'm using the http://www.eliteprojectorcalculator.com/ to calculate the foot lambert for a 16:9 screen with projector on Best Mode, Low Lamp.

Calculating for zoom method
I have to calculate the FL for a 16:9 screen of 98'' wide times 0.75, to acount the pixels that will not be using. So, it will be 16.5 fl * 0.75 = 12.375 fl...

No need to multiply by 0.75 here since ftL are per unit area already. You'll be getting 16.5ftL in this configuration.

Quote:
Originally Posted by kabrumbs View Post

Calculating for lens method
I will be using a Prismasoni HD6000. Some say that it has a 1% loss. So i have to calculate the FL for a 16:9 screen of 42'' high, times 0.75, times 0.99. So, it will be 23.9 fl times 0.75 times 0.99 = 17,74575

Not quite. All you need to do is find the area of your 2.35:1 screen (42*98/144=28.5 sq.ft) and the number lumens your projector will be outputting in your listed configuration (42" high screen at mid throw*, best mode, low lamp - the calculator says 450 lumens). That gives you 450*1.3/28.5 = 20.5 ftL with a new bulb.

Quote:
Originally Posted by kabrumbs View Post

So I can expect an increase of 43% on brightness using lens against the zoom method.

Is this correct??

So if you assume 450 lumens, you're looking at 20% brighter with an anamorhpic lens. HOWEVER, keep in mind that the lumen numbers in the calculator you used are for mid-throw only, so they don't reflect the increase or decrease in lumens you'll get by using a shorter or longer throw, respectively. As was posted above, this will actually make your zoom method a little brighter and give you about a 17% difference in brightness.

There are 10 types of people: those who understand binary, and those who don't.

HogPilot is offline  
post #7 of 21 Old 02-14-2012, 10:18 AM
Member
 
kabrumbs's Avatar
 
Join Date: Apr 2006
Posts: 67
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Thanks Hog!!!

foot lambert Humberto, FOOT lambert!!! Now everything feets correctly!! Man, I hate this imperial metric system :P

[]s Humberto
kabrumbs is offline  
post #8 of 21 Old 02-14-2012, 10:23 PM
Senior Member
 
Gregory's Avatar
 
Join Date: Aug 2001
Location: Illinois
Posts: 328
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Originally Posted by HogPilot View Post

No need to multiply by 0.75 here since ftL are per unit area already. You'll be getting 16.5ftL in this configuration.

So, now I'm confused. Assume constant width: if you are projecting a 1.78 image on a 1.78 screen you would use the full panel, however if you are projecting a 2.35 image on a 1.78 screen you are only using about 76% of the panel, so wouldn't you multiply by 0.76?

Let's say I have the following three scenarios with the associated calculations. Are these correct?

Assume the following:
Screen: 1.3 gain
Lumen output: 356 L (for these examples, assume that the lumens will not change due to zooming - in reality they will increase when zooming from 1.78 to 2.35.

Scenario#1: 1.78 image on 1.78 screen (constant width)
- image size: 49"x87.22" (100.04" diag.)
- image area 29.68 sqft
- uses 100% of panel on the projector
(note: the "1" in the equation is for the full panel in the projector being illuminated)

FL = 356L * 1 / 29.68 sqft * 1.3 gain = 15.59

Scenario#2: 2.35 image on 1.78 screen (constant width)
- image size: 37.11"x87.22" (94.79" diag.)
- image area 22.48 sqft
- uses ~76% of panel on the projector
(note: the "0.757" in the equation is for ~76% of the panel in the projector being illuminated - light reduction due to black bars being projected)

FL = 356L * 0.757 / 22.48 sqft * 1.3 gain = 15.59

FL's remain the same as Scenario 1 even though the lumens on the screen are less because the surface area decreased.

Scenario#3: 2.35 image on 2.35 screen with zooming (constant height)
- image size: 49"x115.15" (125.14" diag.)
- image area 39.18 sqft
- uses ~76% of panel on the projector
(note: the "0.757" in the equation is for ~76% of the panel in the projector being illuminated - light reduction due to black bars being projected, but are now thrown off of the screen due to zooming)

FL = 356L * 0.757 / 39.18 sqft * 1.3 gain = 8.95

FL's are now decreased because the surface area is larger.

Scenario#4: 2.35 image on 2.35 screen with A-Lens (constant height)
- image size: 49"x115.15" (125.14" diag.)
- image area 39.18 sqft
- uses 100% of panel on the projector
- assumes no light reduction due to the A-Lens
(note: the "1" in the equation is for the full panel in the projector being illuminated)

FL = 356L * 1 / 39.18 sqft * 1.3 gain = 11.81

FL's are greater than Scenario 3 because the A-lens allows the full panel to be used.

Thanks
Gregory is offline  
post #9 of 21 Old 02-16-2012, 10:43 PM
Senior Member
 
Gregory's Avatar
 
Join Date: Aug 2001
Location: Illinois
Posts: 328
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Can anyone confirm if I am calculating correctly in my post, above?

Thanks
Gregory is offline  
post #10 of 21 Old 02-16-2012, 10:50 PM
AVS Special Member
 
MovieSwede's Avatar
 
Join Date: Feb 2007
Location: Gothenburg
Posts: 6,758
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 10 Post(s)
Liked: 36
I dont think you calculations are correct since there are alot of variables. A lens will absorb some light and both zoom and lens depends on how wide the iris is as you noted.

Good movies are as rare as an on topic discussion.
MovieSwede is offline  
post #11 of 21 Old 02-17-2012, 06:17 AM
Advanced Member
 
taffman's Avatar
 
Join Date: Nov 2007
Posts: 651
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 12
Quote:
Originally Posted by MovieSwede View Post

I dont think you calculations are correct since there are alot of variables. A lens will absorb some light and both zoom and lens depends on how wide the iris is as you noted.

I agree. The A-lens will probably absorb about 2 - 5% of the light, depending on the quality of the lens. And, with the zoom method, if you are zooming from a position of maximum telephoto the f number of the lens will progressively decrease as you zoom out to 2.35, so you will be losing much less light than the calculations indicate. In fact, for some scenarios, zooming out may not dim the picture very much at all. The accepted number seems to be that an A-lens buys you about 10-20% more light than the zooming method. That is good of course, but hardly a compelling reason, by itself, to buy an A- lens, since you can easily gain that much light from the projector itself by changing menu settings, or going from eco to normal lamp mode.
taffman is offline  
post #12 of 21 Old 02-17-2012, 10:30 PM
Senior Member
 
Gregory's Avatar
 
Join Date: Aug 2001
Location: Illinois
Posts: 328
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Originally Posted by taffman View Post

I agree. The A-lens will probably absorb about 2 - 5% of the light, depending on the quality of the lens. And, with the zoom method, if you are zooming from a position of maximum telephoto the f number of the lens will progressively decrease as you zoom out to 2.35, so you will be losing much less light than the calculations indicate. In fact, for some scenarios, zooming out may not dim the picture very much at all. The accepted number seems to be that an A-lens buys you about 10-20% more light than the zooming method. That is good of course, but hardly a compelling reason, by itself, to buy an A- lens, since you can easily gain that much light from the projector itself by changing menu settings, or going from eco to normal lamp mode.

I understand what you are saying, but I just want someone to check the basic calculations that I'm using.

I understand that an A-Lens will loose some light just from the glass (for conservative numbers I always assume 5%). I also understand that zooming out will will get you more light with a decrease in contrast.........this is projector dependent, as for example, JVC's have a smaller lumen difference between the extremes than Panasonic. I was trying to keep it simple for comparisons sake by just assuming that the same lumens will exit the light source for all scenarios.

Given the lumen output at the projector lens (or at the A-lens) do the calculations make sense? My basic concern is that I'm incorrectly reducing the light output by ~ 24% when projecting a 2.35 image on a 1.78 screen, as in a constant width method (image height is reduced). With a 2.35 image the projector panel is basically masked, because the movie has black bars encoded and the pixels are told to turn off for those areas. This should result in less light (~ 24% less) exiting the projector's lens or A-lens (assuming an ideal lens). This is how I've always looked at, but based on earlier comment by HogPilot I'm questioning my logic.

I'm still thinking my logic is correct, but just looking for some confirmation.

Can anyone confirm?
Gregory is offline  
post #13 of 21 Old 02-18-2012, 02:31 AM
AVS Special Member
 
Gary Lightfoot's Avatar
 
Join Date: Jun 2000
Location: Surrey, UK
Posts: 4,436
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Liked: 35
When you project a full white field onto a 16:9 screen, the projector is putting out X amount of lumens. If you measure the centre of the screen you will measure the number.

If you then blank the pixels above and below a 2/35 portion of the screen, and measure in the same place as before, you will get the same number, so the same image brightness remains as before. You're not dimming the entire image, so you don't need to reduce the number by 25%.

As for A lens light reduction, I measured a Prismasonic in pass-through (16:9) mode so the light was passing through the glass onto the screen, and with the glass in place the lux at the screen dropped from 101 to 99, so approx 2% drop. With an ISCO you can't do that, but I would think the light loss would be very similar.

It's difficult to calculate actual lumens because as you've already found, zoom mode outputs vary with the amount of zoom and from projector to projector. Plus, you don't really know what the light output of the lamp is, especially with all the different modes and settings pjs have, and that's before you've calibrated it.

The only way to know for sure is to use a light meter and measure.

HTH

Gary

Quote:
Originally Posted by elmalloc
Who says Cameron is "right" and why do we care about him so much - lol!

I trust Gary Lightfoot more than James Cameron.
Gary Lightfoot is online now  
post #14 of 21 Old 02-18-2012, 04:59 AM
AVS Club Gold
 
HogPilot's Avatar
 
Join Date: Jun 2006
Location: Good Ol' US of A
Posts: 2,869
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 18
Quote:
Originally Posted by Gregory View Post

This is how I've always looked at, but based on earlier comment by HogPilot I'm questioning my logic.

I'm still thinking my logic is correct, but just looking for some confirmation.

Can anyone confirm?

Foot-Lamberts (ftL) are calculated very simply: lumens * gain / screen area in feet = ftL.

For example, let's take a 100" x 56.25" 16:9 1.0 gain screen, and let's say you have a projector putting out 500 lumens. Your screen area is 100 * 56.25 / 144 = 39.0625 sq ft, which gives you 500 * 1 / 39.0625 = 12.8 ftL.

Now you want to watch a 2.39:1 movie, so you're projecting letterbox bars on the top and bottom of the screen, and your new active area is 29.05625 sq ft (25.6% less than your 16:9 image). However, you've also lost the lumens that were being projected into the letterbox bars, so your lumens have proportionally dropped by 25.6% to 371.9. So now you have 371.9 *1 / 29.05625 = 12.8 ftL. Image brightness remains unchanged because your projector's light output is dropping by the exact same amount as your image area.

Regarding the use of an anamorphic lens vs zooming, as you've already stated that the amount of lumens you recover by changing f when you zoom is really projector dependent, so you won't really know how much you get back by zooming without a direct measurement. I have never owned a projector that had an iris that was aggressive enough that I recovered all the ftL I lost by zooming and increasing the image size. If one chooses to use high bulb mode to recover some lumens, you'd need a separate calibration as the spectral output of a bulb on normal vs high can vary quite significantly, and there's no guarantee that the increase in output will compensate for whatever you lost by expanding the image size.

There are 10 types of people: those who understand binary, and those who don't.

HogPilot is offline  
post #15 of 21 Old 02-18-2012, 08:58 AM
Senior Member
 
Gregory's Avatar
 
Join Date: Aug 2001
Location: Illinois
Posts: 328
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Originally Posted by Gary Lightfoot View Post

If you then blank the pixels above and below a 2/35 portion of the screen, and measure in the same place as before, you will get the same number, so the same image brightness remains as before. You're not dimming the entire image, so you don't need to reduce the number by 25%.Gary

Quote:
Originally Posted by HogPilot View Post

Foot-Lamberts (ftL) are calculated very simply: lumens * gain / screen area in feet = ftL.

For example, let's take a 100" x 56.25" 16:9 1.0 gain screen, and let's say you have a projector putting out 500 lumens. Your screen area is 100 * 56.25 / 144 = 39.0625 sq ft, which gives you 500 * 1 / 39.0625 = 12.8 ftL.

Now you want to watch a 2.35:1 movie, so you're projecting letterbox bars on the top and bottom of the screen, and your new active area is 29.05625 sq ft (25.6% less than your 16:9 image). However, you've also lost the lumens that were being projected into the letterbox bars, so your lumens have proportionally dropped by 25.6% to 371.9. So now you have 371.9 *1 / 29.05625 = 12.8 ftL. Image brightness remains unchanged because your projector's light output is dropping by the exact same amount as your image area.

So, in this case, a 1.78 image and a 2.35 image shown a 1.78 screen work out to be the same screen brightness, as my calculations have shown for Scenarios 1 and 2 and as both of you have stated. Although, both of you appear to arrive at the answer differently. Gary states that you do not have to account for the reduction of light from the black bars on a 2.35 image (and in this specific case makes sense) and HogPilot, like myself, are accounting for it.

I think the reduction in light by projecting the black bars is the more correct way to calculate screen brightness, as it works for other scenarios, as well.

Just trying to understand the fundamental calculations.......do I still not understand something?

Thanks,
Greg
Gregory is offline  
post #16 of 21 Old 02-19-2012, 02:57 AM
AVS Special Member
 
Gary Lightfoot's Avatar
 
Join Date: Jun 2000
Location: Surrey, UK
Posts: 4,436
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 4 Post(s)
Liked: 35
Rather than use calculations that have already been posted, I thought I'd try and word it in a way that might be easier to visualise.

If you can get a light meter, you can then experiment with things and see how it works with the calculations. That's kinda how I started, and found it to be a lot of fun. Plus you find out exactly what's happening in real terms.

Gary

Quote:
Originally Posted by elmalloc
Who says Cameron is "right" and why do we care about him so much - lol!

I trust Gary Lightfoot more than James Cameron.
Gary Lightfoot is online now  
post #17 of 21 Old 02-19-2012, 04:33 AM
AVS Club Gold
 
HogPilot's Avatar
 
Join Date: Jun 2006
Location: Good Ol' US of A
Posts: 2,869
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 18
Quote:
Originally Posted by Gregory View Post

So, in this case, a 1.78 image and a 2.35 image shown a 1.78 screen work out to be the same screen brightness, as my calculations have shown for Scenarios 1 and 2 and as both of you have stated. Although, both of you appear to arrive at the answer differently. Gary states that you do not have to account for the reduction of light from the black bars on a 2.35 image (and in this specific case makes sense) and HogPilot, like myself, are accounting for it.

I think the reduction in light by projecting the black bars is the more correct way to calculate screen brightness, as it works for other scenarios, as well.

Just trying to understand the fundamental calculations.......do I still not understand something?

Thanks,
Greg

As Gary said, he and I are actually saying the exact same thing - it doesn't matter whether you change the image to 4:3, 2.66:1, 1.85:1, or keep it at 1.78:1. The loss of image area due to the black bars is always exactly proportional to the loss of light, so your ftL - your brightness off of the screen - remains exactly the same. What I showed mathematically was really redundant, and doesn't need to be performed as long as the image isn't being zoomed at all. I was just doing it for illustrative purposes to help you understand what's going on.

If you do zoom the image, you'll want to take a new light measurement and re-calculate the ftL you're getting because both light output and screen size have changed.

There are 10 types of people: those who understand binary, and those who don't.

HogPilot is offline  
post #18 of 21 Old 02-19-2012, 07:14 AM
Advanced Member
 
taffman's Avatar
 
Join Date: Nov 2007
Posts: 651
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 12
As an example of how zooming to 2.35 produces far less light loss than you would expect:
My projector is a Panasonic AE4000 projecting on to a matt white (1.0 gain) screen.
The screen width for 16:9 is 87 inches, and the screen width for 2.35 is 110 ins. Screen area at 16:9 is 29.6sq ft, and screen area at 2.35 (INCLUDING the black bars) is 47.2 sq ft. You have to include the black bar area for this calculation because as far as the projector is concerned you are just zooming out to a larger 16:9 image - it knows nothing about the black bars). So based soleley on the area increase of the projected picture, the screen brightness when zoomed to 2.35 should be 62% of the screen brightness for 16:9.

Now we go to the Projector Central caculator for the AE4000, which takes into account the effect of relative aperture change with zooming.
Based on my projection throw of 18 feet, the chart shows that the brightness for an 87 ins wide screen is 10FL, and the brightness for a 110 ins wide screen is 8FL. So the 2.35 brightness is only 20% less than the 16:9 brighness!! So when you zoom out to 2.35 from a position close to the maximum telephoto (throw) of the lens, you lose far less brightness than is predicted from the screen area increase.
taffman is offline  
post #19 of 21 Old 02-19-2012, 11:54 AM
Senior Member
 
Gregory's Avatar
 
Join Date: Aug 2001
Location: Illinois
Posts: 328
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 10
Quote:
Originally Posted by Gary Lightfoot View Post

Rather than use calculations that have already been posted, I thought I'd try and word it in a way that might be easier to visualise.

If you can get a light meter, you can then experiment with things and see how it works with the calculations. That's kinda how I started, and found it to be a lot of fun. Plus you find out exactly what's happening in real terms.Gary

Gary,

Totally agree with taking actual measurements, but I do not have a projector yet. I want to perform the calculations to insure that whatever projector/screen that I get will be bright enough. I do factor in the actual projector and throw distances for the projectors that I'm researching, as well as using the lumen data from reviews.........these all go into the calculations.

Quote:
Originally Posted by HogPilot View Post

As Gary said, he and I are actually saying the exact same thing - it doesn't matter whether you change the image to 4:3, 2.66:1, 1.85:1, or keep it at 1.78:1. The loss of image area due to the black bars is always exactly proportional to the loss of light, so your ftL - your brightness off of the screen - remains exactly the same. What I showed mathematically was really redundant, and doesn't need to be performed as long as the image isn't being zoomed at all. I was just doing it for illustrative purposes to help you understand what's going on.

If you do zoom the image, you'll want to take a new light measurement and re-calculate the ftL you're getting because both light output and screen size have changed.

HogPilot,

Agreed...........the calculations that I have been using show the same thing when not zooming. It's all proportional............light reduction due to the black bars is exactly balanced by the reduction in image area.

So, when zooming, as I probably will end up doing, all that is required is to account for the increased image area, which will reduce screen brightness.
Of course it's more complicated than that, as stated before, as the projector is zoomed out some light will be gained back.

Quote:
Originally Posted by taffman View Post

As an example of how zooming to 2.35 produces far less light loss than you would expect:
My projector is a Panasonic AE4000 projecting on to a matt white (1.0 gain) screen.
The screen width for 16:9 is 87 inches, and the screen width for 2.35 is 110 ins. Screen area at 16:9 is 29.6sq ft, and screen area at 2.35 (INCLUDING the black bars) is 47.2 sq ft. You have to include the black bar area for this calculation because as far as the projector is concerned you are just zooming out to a larger 16:9 image - it knows nothing about the black bars). So based soleley on the area increase of the projected picture, the screen brightness when zoomed to 2.35 should be 62% of the screen brightness for 16:9.

Now we go to the Projector Central caculator for the AE4000, which takes into account the effect of relative aperture change with zooming.
Based on my projection throw of 18 feet, the chart shows that the brightness for an 87 ins wide screen is 10FL, and the brightness for a 110 ins wide screen is 8FL. So the 2.35 brightness is only 20% less than the 16:9 brighness!! So when you zoom out to 2.35 from a position close to the maximum telephoto (throw) of the lens, you lose far less brightness than is predicted from the screen area increase.

I've been looking at the Panny AE-7000 and the JVC X30.

One thing to be careful about the Projector Central calculator is not knowing the actual lumen output that they use if using the calculator to give you an actual fL........I think they use the max spec., but derate for some bulb aging. It would be nice if they would allow you to enter your own lumens. If you are using the calculator to give you relative information, such as the zoom effect or screen gain changes, then it is good. I mainly use their calculator to give me the throw ranges and then read reviews to get the lumen output at whatever zoom and picture modes that they use. Protector Central generally tests at full-zoom (max light output), while Projector reviews generally tests at mid-zoom. I use this data to estimate the lumen output to expect at my desired throw and then use that data in my calculations.

Question on your screen size. You state that your screen width is 87" wide for 16:9 and that your 2.35 screen width is 110". Shouldn't the 2.35 width be 115", unless you aren't zooming to the full height of the 16:9 screen height (49")?

Gary, HogPilot and taffman,

Thank you very much for taking the time to help me understand this better. I think we have all been probably saying the same thing all along........I'm just a little slow!

Oh, BTW, how good are these iPhone light meter apps?

Thanks,
Greg
Gregory is offline  
post #20 of 21 Old 02-19-2012, 02:15 PM
Advanced Member
 
taffman's Avatar
 
Join Date: Nov 2007
Posts: 651
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Liked: 12
Quote:
Originally Posted by Gregory View Post

Gary,


Question on your screen size. You state that your screen width is 87" wide for 16:9 and that your 2.35 screen width is 110". Shouldn't the 2.35 width be 115", unless you aren't zooming to the full height of the 16:9 screen height (49")?

Thanks,
Greg


Quite right Greg! - I have an ALMOST CIH system! My 2.35 screen height is 47 inches and my 16:9 screen height is 49 inches. I prefer a slightly bigger 16:9/4:3 picture than a 100% CIH system. I can do this because I have a totally flexible 3-way motorized masking system.

Incidentally, if you re-visit my calculations, this time using an A-lens on the 110 ins wide scope screen, you get an illuminated screen area of 35.9 sq ft, and that gives a brightness of 8.3FL, compared with 8FL with zooming. So, for my particular set up, an A-lens would buy me nothing in screen brightness!
Interesting hey!
taffman is offline  
post #21 of 21 Old 02-19-2012, 02:25 PM
AVS Special Member
 
CAVX's Avatar
 
Join Date: Aug 2005
Location: Brisbane, Australia
Posts: 8,354
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 8 Post(s)
Liked: 31
Just don't discount those extra half million pixels on screen though. They DO make a difference.

Mark Techer

I love my Constant Image Height system!
CAVX is offline  
Reply 2.35:1 Constant Image Height Chat

User Tag List

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off