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Measuring brightness in CIH setup

post #1 of 5
Thread Starter 
Want to make sure I'm not crazy and I'm thinking about this properly. I bought a light meter so I've been playing around with a bunch of different readings trying to get a general sense for the amount of brightness different options add or subtract from my setup.

I've got a constant lens setup and use the squeeze mode for 16:9 content.

If I measure my lux and covert to lumens, then fL with my projector *without* the lens in front of it, in 16:9, I get approximately 500 lumens, which translates in my case to roughly 11.1fL (1.0 gain screen, 44.6 sq ft). Not great but not the end of the world.

Then I put the lens in the light path. Multiplying the lux by the full 2.4 screen size, I get the same roughly 500 lumens, which makes sense I guess. But my fL goes down to 8.33fL. (1.0 gain, 60 sq ft). Now I'm starting to see why my setup feels pretty dim.

Then I realized that the measurement I was using for 16:9 content isn't right, because I'm calculating my fL using the 16:9 screen size (44.6 sq ft), when really I should be using the 2.4 screen size (60 sq ft), even though the content I'm watching is only 16:9. Does that sound right? Basically, since I leave the lens in place, the light from the projector is still spread out over the full 2.4 projection area, and when I put the projector in squeeze mode, it's not focusing all of the light down into the 16:9 area (like if the lens were moved out of the way), it's just masking off the sides of the image and I'm losing all that light.

In which case, my brightness for 16:9 material is also 8.33fL. This effectively means that by using an anamorphic lens, your brightness drops by about 25% on anamorphic content (8.33fL vs. 11.1fL in my case) simply based on the fact that you're making the image larger. Fair enough. But for 16:9 content, if you're in a fixed lens setup, you're still dropping your brightness by the same 25% compared to a non-anamorphic setup, even though the projected image is exactly the same size.

Does that all make sense, or am I doing something wrong?
post #2 of 5
Thread Starter 
A related question: When people talk about the zoom method losing 30% or so of the brightness compared to the lens, does that mean 30% less than the 8.33fL in my case? Because you're losing brightness simply by the image being so much larger AND you're losing additional brightness on top of that by not using the entire panel (the same way I'm losing brightness in 16:9 squeeze mode)?
post #3 of 5
Quote:
Originally Posted by popechild View Post

Want to make sure I'm not crazy and I'm thinking about this properly. I bought a light meter so I've been playing around with a bunch of different readings trying to get a general sense for the amount of brightness different options add or subtract from my setup.

I've got a constant lens setup and use the squeeze mode for 16:9 content.

If I measure my lux and covert to lumens, then fL with my projector *without* the lens in front of it, in 16:9, I get approximately 500 lumens, which translates in my case to roughly 11.1fL (1.0 gain screen, 44.6 sq ft). Not great but not the end of the world.

At this point are you projecting on the 16:9 area of the screen? Or "zoomed" so including overspill?
Quote:
Then I put the lens in the light path. Multiplying the lux by the full 2.4 screen size, I get the same roughly 500 lumens, which makes sense I guess. But my fL goes down to 8.33fL. (1.0 gain, 60 sq ft). Now I'm starting to see why my setup feels pretty dim.

That sounds about right, with an HE lens, if you're getting 11.1 ftL for the 16:9 area, when you put the lens in, since the area is 33% larger, the math says your ftL should be 8.325ftL, or exactly what you're getting.

Side note, this is why I don't know why people subtract for a lens, they don't "lose" any significant amount of light. You lens blocked no measurable light.
Quote:
Then I realized that the measurement I was using for 16:9 content isn't right, because I'm calculating my fL using the 16:9 screen size (44.6 sq ft), when really I should be using the 2.4 screen size (60 sq ft), even though the content I'm watching is only 16:9. Does that sound right? Basically, since I leave the lens in place, the light from the projector is still spread out over the full 2.4 projection area, and when I put the projector in squeeze mode, it's not focusing all of the light down into the 16:9 area (like if the lens were moved out of the way), it's just masking off the sides of the image and I'm losing all that light.

It doesn't matter what content you project, you're measuring just a portion of it. Well, if you mean the measurement wasn't accurate for the brightness you will see when you view 16:9 with the lens in place that's true. In that case it will be exactly the same brightness as scope.
Quote:
In which case, my brightness for 16:9 material is also 8.33fL. This effectively means that by using an anamorphic lens, your brightness drops by about 25% on anamorphic content (8.33fL vs. 11.1fL in my case) simply based on the fact that you're making the image larger. Fair enough. But for 16:9 content, if you're in a fixed lens setup, you're still dropping your brightness by the same 25% compared to a non-anamorphic setup, even though the projected image is exactly the same size.

Does that all make sense, or am I doing something wrong?

Nope, that all sounds about right. But remember it also means all content you watch is the same brightness.
Quote:
Originally Posted by popechild View Post

A related question: When people talk about the zoom method losing 30% or so of the brightness compared to the lens, does that mean 30% less than the 8.33fL in my case? Because you're losing brightness simply by the image being so much larger AND you're losing additional brightness on top of that by not using the entire panel (the same way I'm losing brightness in 16:9 squeeze mode)?

Yes, because when you zoom, the projected area is 77% larger than 16:9. Zooming, ignoring the variable f-stop of projection lenses, results in an image that's 44% or so dimmer than 16:9 vs an image that's only 25% dimmer. Of course most projectors gain a little brightness when they zoom so in reality the difference is less than the raw math (only based on area) would suggest.
post #4 of 5
A fixed A lens system basically turns a 16x9 projector into a 2.35 projector. So no matter what you scale, the brightness stays the same. There's no zooming only scaling. The 20-25 percent increase that an A lens gives you when compared to the zoom method is of course in reference to a 2.35 playback system.
post #5 of 5
Thread Starter 
Thanks folks. Makes since.
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