How does changing projector lenses affect brightness?

[guyburns]

I have some questions about projection lenses and screen brightness.

Background
I may be purchasing a BenQ PU9730 (http://www.benq.com.au/product/projector/pu9730/specifications/) for use in a large venue, projecting an image 3 x 5.3 metres. The projection booth is 18m from the screen (maximum distance), but the projector could be placed amongst the audience.

BenQ offers a choice of several lenses, and I will probably choose between the Standard and the Semi Long, with these considerations:

The Standard lens has a lower f-number (brighter?), but it would have to be placed amongst the audience. And because it has less zoom range, I would be more restricted in where I could place it, in this venue and others.

The Semi Long lens would allow projection from the projection booth (away from the audience), but I think it would project a less-bright image. For use in other venues, though, it has a much larger zoom range.​

Standard Lens

• focal length 26 ~ 34mm
• f-stop 1.64 ~ 1.86​

Semi Long Lens

• focal length 33 ~ 54mm
• f-stop 1.86 ~ 2.48​

Ques 1
Comparing the two lenses, and assuming the same size image on the screen (obtained by moving the projector while changing lenses) – is the Standard lens always going to throw a brighter image than the Semi Long lens?

Ques 2
For a single lens, and assuming the same size image on the screen (obtained by moving the projector while altering the zoom) will the brightness change as the zoom changes?

Ques 3
Assuming the brightness does change in Q1 and Q2, where does the energy in the wasted light go when the screen image is less bright?

• Does it heat up the lens? In particular, for the same size image, will the Semi Long lens runs hotter than the Standard lens?

• Is the wasted light reflected back into the projector, so the projector runs hotter?

Thanks in advance for any comments.

 

[scottdave]

Any decently designed projector should have sufficient fan/exhaust capability to remove heat, regardless of the mode of operation. This one says it has 2 lamps 350 watt each. I am guessing that is incandescent. Most of the power will be as heat, rather than visible light. Changing the F-stop will change the amount of light allowed to exit the lens. For a given amount of light, a larger screen will mean that it is "spread out" over a larger area (less light per square meter).

 

[sophiecentaur]

Any decently designed projector should have sufficient fan/exhaust capability to remove heat

This is very relevant. There are other places inside the projector that will be more affected by the waste heat energy. A slide projector used to have a fat i/r filter to save roasting the slide and the lcd element in a modern digital projector also needs to be protected. The fact that they have a long over-run for the fan says it all.

 

[Khashishi]

If the image size is the same, the brightness should be about the same in each case, since it's the same light power spread over the same area. Of course, the lenses could have different transmission, but I expect that most of the light (>70%) is transmitted through the lens. Even if one lens absorbs much more than the other (let's say 15% versus 30%), it's only a difference in transmission between 70% and 85% which probably isn't too noticeable.

 

[sophiecentaur]

If you use the long lens at its shortest focal length and the shorter lens at its longest focal length (i.e. more or less the same), the spec says they have the same f value.
But putting the projector further back (same image size) would involve using the long lens at a worse f stop and that would mean about 0.7 of a stop lower illuminance. Only noticeable in an A-B comparison but worth considering. Using the shorter lens as close as possible and the longer lens as far away as possible would be even more so.
You could investigate the possibility of a better long lens but that may not work - depends on compatibility, as with cameras.

 

[Khashishi]

Well, is the etendue limited by the lens or the illumination source? I assume that the lenses are sufficiently large that they aren't vignetting the light. If it's appropriately designed, you shouldn't have to worry about the f-number.

 

[sophiecentaur]

If it's appropriately designed, you shouldn't have to worry about the f-number.

I don't understand that.
f-number is relevant for photography so why not for projection? I have heard the point made before and didn't understand it then, either.

 

[Khashishi]

In photography, the CCD or CMOS sensor has a basically 180 degree field of view, so you are always limited by the lens as far as how large an angle of light hits the sensor. But with the projector, there's an internal lamp which shines through the LCD with some finite angle. The lens ought to be big enough for that angle, but need not be larger.

 

[sophiecentaur]

OK. I do know that there is an extra set of optics in a projector (mirror and condenser lens) that 'project' the image of the lamp onto the projector lens(?), maximising the amount of light getting to the picture. That would support your 'narrow angle range' idea. But you are implying that there is no point in having a wide projector lens and that still doesn't seem like the whole story.

 

[guyburns]

If the image size is the same, the brightness should be about the same in each case, since it's the same light power spread over the same area.

I don't think that's correct, because the explanation ignores the effect of the f-number. Sophiecentaur's comment seems to me to be more correct:

… But putting the projector further back (same image size) would involve using the long lens at a worse f stop and that would mean about 0.7 of a stop lower illuminance. Only noticeable in an A-B comparison but worth considering.

Some additional info required to fully answer my questions:

1. http://www.benq.com.au/product/projector/pu9730/downloads/. Page 17 of that PDF contains table for image size versus distance and lens used.

2. Throw Ratio, T = D/W (approx) where D is the distance between projector and screen, and W is the width of the image.

Standard Lens
• focal length 26 ~ 34mm
• f-stop 1.64 ~ 1.86
• Throw Ratio 1.73 - 2.27

Semi Long Lens
• focal length 33 ~ 54mm
• f-stop 1.86 ~ 2.48
• Throw Ratio 2.22 - 3.67​

To further the discussion, I'll answer my own questions to see what others think.

Ques 1
Comparing the two lenses, and assuming the same size image on the screen (obtained by moving the projector while changing lenses) – is the Standard lens always going to throw a brighter image than the Semi Long lens?

My answer: Yes – except for the case where the lens are both being used at f/1.86 (the only point of overlap).Ques 2
For a single lens, and assuming the same size image on the screen (obtained by moving the projector while altering the zoom) will the brightness change as the zoom changes?

My answer: Yes – because for the same size image, brightness depends only on the f-number – which changes as the zoom changes. I base that answer on what Wikipedia says about f-numbers:

Ignoring differences in light transmission efficiency, a lens with a greater f-number projects darker images.

A 100 mm focal length f/4 lens has an entrance pupil diameter of 25 mm. A 200 mm focal length f/4 lens has an entrance pupil diameter of 50 mm. The 200 mm lens's entrance pupil has four times the area of the 100 mm lens's entrance pupil, and thus collects four times as much light from each object in the lens's field of view. But compared to the 100 mm lens, the 200 mm lens projects an image of each object twice as high and twice as wide, covering four times the area, and so both lenses produce the same illuminance at the focal plane when imaging a scene of a given luminance.​

Ques 3
[I'll rephrase my original question]. For an image that stays the same size when projected by a certain projector (by changing lenses, distance, zoom, or any combination) I think brightness will become lower if the f-number is higher. Where does the lost light go?

An analogy: a projector always generates the same amount of light in the bulb, just as a microwave oven always generates the same amount of microwave power from the magnetron (assuming the power is set to maximum). So a similar question would be: if a microwave oven has nothing in it (neglecting whether or not that's desirable), where does the power go?

I'm not concerned about how the projector handles the heat, just interested to find out where the lost light goes.

 

[sophiecentaur]

just interested to find out where the lost light goes.

This link shows a picture of the two separate optics involved. A lot of the light from the bulb can escape off to the side of the condenser lens. Those lenses are not coated (as cheap as they can get away with, I expect) and there is the IR filter too (not included there) which will also be lossy. Off axis light in the lens tube will hit the black sides which may have rings to reduce scatter / increase contrast.
You should read post #8. He has probably got it right.

 

[pixel]

In photography, the CCD or CMOS sensor has a basically 180 degree field of view, so you are always limited by the lens as far as how large an angle of light hits the sensor.

The dimensions of the camera sensor determine the maximum field angle. The sensor is the field stop - the lens should not be the limiting factor as then not all of the sensor is used.

Also, if you look at the diagram that Sophiecentaur references, the projector lens is being used as an enlarger lens. The usual f-number calculations for image brightness need to be modified, as with a macro lens. Frankly, if I were in the OP's situation instead of trying to figure this out I would ask the vendor as they are most familiar with their product. Also, they might be willing to send a couple of lenses on a trial basis.

 

[sophiecentaur]

Also, they might be willing to send a couple of lenses on a trial basis.

A good idea. If they want the business then they should go along with that.

 

[guyburns]

The vendors might part with a few lens, but I doubt it. The Standard lens costs $1100, and all the others cost about $3000 (https://www.zylax.com.au/benq-5j-jam37-051-mid-throw-lens-for-p-series-large-venue-projector.html).

Thus my questions here. I have to be very confident before I buy, that the projector/lens combination is suitable.

Basically, do I want to pay extra for the Semi Long lens and the luxury of projecting from the booth (but with an image of lower brightness), or do I save $2000 and sit the projector amongst the audience for a brighter image? How much brighter? Is it worth it?

This is not a permanent installation. It will be me – a keen amateur – wanting to show about a dozen AV presentations over two or three years at the local auditorium.

 

[pixel]

The vendors might part with a few lens, but I doubt it. The Standard lens costs $1100, and all the others cost about $3000 (https://www.zylax.com.au/benq-5j-jam37-051-mid-throw-lens-for-p-series-large-venue-projector.html).

They could have several demo units sitting around that they would lend out for a couple of days. They may have a local rep who could come to your place to demo the unit with different lenses. They could answer your question about the lenses. You don't know until you ask.

 

[sophiecentaur]

This a difficult issue for you. Have you asked the suppliers about image brightness? They could advise better than us.
I can understand that you want the presentations to be as 'professional' as possible but $2k is a lot of dosh and it's pretty usual to have projectors near the screen, now that they don't clunk and whirr every time there's a picture change.
I should have realized that lenses with the sort of f values you quote would be pretty costly!