How much leeway centralising Projector?

Idle Bull

Active Member
I've searched but couldn't find an answer, although I suspect the question has been asked before.

I hope to get the projector mounted tonight, and I'm confident I can get it central, but is there any leeway regarding the centre point (i.e. a few mm's either side?). It's a Benq W1070, so I don't think it has horizontal correction, but would, say, 5-10mm off centre really be noticeable?

thanks

Nick
 

Pecker

Distinguished Member
My current ceiling mount is dead centre to my room, but the middle of my projector lens is very slightly over to one side - I think a few cms.

Yes, it causes a very slight trapezoid effect, but this is very easily masked out at the edges, and/or top & bottom.

I've put up a test grid (horizontal and vertical lines) which really should up the problem more than anything, and the effect is invisible, to all intents and purposes. Even when I can see it, you need to stare at it to see it's there, but it doesn't actually look distorted.

Steve W
 

Idle Bull

Active Member
thanks for the reply Steve, it's put me at easy a little. I'm still going to endeavour to get the lens dead centre, but it's good to know if I'm off slightly it won't be a disaster.

Thanks

Nick
 

archie2000

Active Member
I know not all rooms are perfectly square, and your screen may not be exactly in the middle of the room width wise, but make sure you take the measurement for you centre point from the same wall that you measured to the centre point of your screen
 

Idle Bull

Active Member
I was actually thinking of setting the PJ up on a coffee table and getting the image perfectly squared up, and then dropping a plumb line from the ceiling to the centre of the lens. I can then line the projector up with the plumb line and mark the mounting points in the joist!

I also want to measure from the wall, and I've also ready about drawing 2 arcs from each corder of the screen at 2 different distances, and the straight line between the two should be dead centre.

My worry is that all three will yield different results (knowing my luck)!!:)
 

Idle Bull

Active Member
Didn't have much time last night, but I've now got a centre line. I used 2 methods, the first was to use some garden wire (which doesn't stretch) attached to each corner of the screen, to draw an arc on the ceiling, where the arcs intersect should be the centre, I then lengthened the wire and drew two more, the line through the two intersections should be the centre line.

I checked these with the centre point of the screen I marked on the screen casing using a laser level lined up with my centre line. It seemed spot on.

I then set a plumb line from the centre line and set the projector up on a coffee table with the lens centred on this and adjusted it until I had a squared image that filled the screen. Thankfully, the lens centre point was still in line with the plumb line from the ceiling centre point.

I should be able to work out the mount position from the lens centre, so feel reasonably confident I'll be central (+-5mm maybe?).
 

Pecker

Distinguished Member
At only 5mm, this is what I'd do - I'd project dead square so that the top and bottom of the image touch the to and bottom screen borders, with one side having a 5mm black bar, and the other side the image displayed overlaps the border by 5mm.

Then I'd zoom out a little so the side that's showing a 5mm bar expands to fill the screen. You'll then be overscanning by about 5mm top bottom and the other side now overscanning by 10mm, but that's well within tolerances.

I think safe projection area for 35mm film is something like 2.5%. A 100" screen is 221cm wide, 2.5% of that is about 55mm, so you're not even coming close to that.

Steve W
 

Idle Bull

Active Member
That's good to know Steve, although it could all still go wrong once I start drilling the holes for the mount! :) But I've spent quite a bit of time working out the position, so I'm pretty confident of getting it pretty well spot on.

Thanks

Nick
 

Idle Bull

Active Member
Things went pretty well last night, mounted the projector using A Vogels EPC6545 mount, which was the lowest profile one I could find (the ceiling in the cellar is only 6'5", which gives me 1" head room clearance).

The centre of the lens is around 5 inches from the ceiling and the top of the screen is around 6.5 inches from the ceiling, so, even with the lens shift at it's lowest, I couldn't avoid a slight keystone. I dialled in -1 on the digital keystone correction and that fixed it, it's a compromise, but the image quality didn't seem adversely affected by such a slight adjustment.

The good thing was that the top and bottom were parallel, so I must have got it central.

I will probably lower the screen a couple of inches, at the moment it is just brushing the top of the cabinets we have under it, but I think that it will drop in front once the install is finished and the cabinets are able to be pushed right back to the wall, so hopefully will be able to set the Keystone back to 0.

Image quality is pretty good. Couldn't find my one Blue-ray (TT3D closer to the edge), but I connected Sky HD and watched a bit of gold and some HD movies scenes which all looked pretty good, especially taking into account about 20metres of HDMI from lounge upstairs to HDMI matrix and then to projector.

One disappointment was the quality of unscaled DVD's played on my Panasonic DMP_BDT130 - I was using the THX optimiser on one of the Star wars discs,and then watched a bit of the movie and there were quite a lot of swirly artefacts! Looks like I might have to start replacing my DVD's with blu-rays!
 

Peter Parker

Distinguished Member
There shouldn't be too many artifacts on the Star Wars DVDs unless the image is very bright - they look pretty good even on my plasma. The worst I saw them was on a 3 chip DLP that was at over 50fL which is greater than plasma levels and over 4 times the ideal level for DVD. Were they visible in mostly darker areas or all over? How big is your screen?

Gary
 

Idle Bull

Active Member
Screen is a 92 inch Draper Baronet which I used with my old Barco 8 inch CRT. It is quite high gain I believe, 1.9 rings a bell, ideal for low output CRT, but maybe a bit much for a DLP?

I'm wondering if an ND filter might be a good addition? Just to tame the screen slightly?
 

Peter Parker

Distinguished Member
Hi,

Yes, it wouldn't hurt to try a ND filter (good quality treated glass like a Hoya HMC). I always used to set up my reflectance to 12fL or less - commercial theatres often run at less than 12, and I found that 12 or less was a really good level for me because it gave better black levels for digital and I tend to notice mosquito noise etc. For me, it also looked more 'cinematic' than brighter levels which looked more digital or tv like. With 12 or less, DVD looked very good. With HD material you can go brighter due to better compression algorithms, but then you may need two set ups - one for SD and one for HD, which isn't always easy to do (in your case, could be as simple as ND on, and ND off though. :) ). But that for me, would mean a move away from what I feel is the cinematic 'look'. But it's just a preference.

I take it HD material is fine?

Gary
 

Pecker

Distinguished Member
Sorry to go off topic a little here.

Gary, I'm looking at the Optoma HD91, which has been reviewed as being a little dim. Different reviewers have had different results, but the lowest lumens output I've seen rated is just over 550 lumens.

My sums:

By my calculations that should just get my 12fl on my CIA screen when using the full width.

My screen in 280cm wide (110"). That's the equivalent of a 126" 16:9 screen, which is 110" x 62", or 9.15' x 5.15'.

9.15' x 5.15' = 47.1225 square feet x 12 = 565 lumens.

Do I have those sums right?

Steve W
 

Peter Parker

Distinguished Member
Hi Steve,

With a unity gain screen, that's about right. If you have any gain, just multiply by the gain - is your perf screen around 0.8? If so, you'll be seeing around 9fL, and that's what I had with my last set up which looked fine to me in my room. Do you know what the measured lumens are of your current pj? It might give you an idea of what you're getting now and the chance to experiment with an ND filter to see if you could live with a dimmer image.

Unless you prefer a brighter image, you'll probably find it'll be OK, especially if the LEDs stays at that lumen output, though you will get some difference between formats as you zoom to each size, but I doubt it will be noticeable at all.

Gary
 

Peter Parker

Distinguished Member
Just had a quick look at the specs - around 1800 lumens when new, which could be 25 to 40% less out of the box and depending on the level of calibration, which could mean around double what you would have with the HD91. You could experiment with an ND2 on your current pj and try to live with that for a few weeks just to see what it's going to be like with the 91.

EDIT: forgot to add that the lumen specs are probably for high lamp, not eco, so you'll probably have to do the test in high lamp. You could try it in low lamp and see what you think (give it a week or two as a direct comparison will draw you to the brighter image), and if that's OK, you know that the 91 should be brighter.

Gary
 
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Idle Bull

Active Member
HD looks brilliant, I think you're right about DVD, I think the output is just a bit high, I haven't had time to play with settings as still working on room, so just a quick test.

How do I measure FL by the way, do I need a light Meyer? Or is there a way to calculate it?

Thanks

Nick
 

Peter Parker

Distinguished Member
Ideally you need a light meter because it's the only way to know for sure what you're getting - all lamps are different and will dim with age, and the specs usually don't take into account the settings for a decent video image (and different settings will give different outputs), so calculating it would only give a very rough answer. 1800 advertised lumens may be the max output with no adjustments. Input some settings like 'cinema' and the RGB adjustments used to get the colours looking more normal may drop the lumens by 25%. If you adjust to D65 as per an ISF calibration, you could drop the lumen output by as much as 40%, and that's on a new lamp. After a few hundred hours the number will have dropped further. If you use something like a 'dynamic' mode, you may get more lumens but at the expense of colour accuracy.

A photonic light meter will be more accurate (I would think) than a cheapie one, but the photonic ones start at around £60 IIRC. A cheapie may be OK for what you want to use it for, but I doubt it would be any good for on/off contrast readings due to low light accuracy.

To get foot lamberts (the reflectance level), you divide the projector lumens by the screen area in feet, so a 110 foot wide 16:9 screen like Steves will have a screen area of 47.26 square feet. If the lumens are 550 (divide the 47.26 into the 550) the fL would be around 11.6 on a unity gain screen. If it has a gain of around 1.1, multiply by that number and you will get 12.8fL. With normal UHP lamps, that number will fall as the lamp dims with age.

HTH

Gary
 

Idle Bull

Active Member
Any idea how accurate the light-meter apps on smartphones are? Are they good enough to get a close enough reading? Not sure I fancy spending another £60 to get a meter after the outlay I've already had.

Do I need to use a fully white image, is setting the background colour (when there is no signal) to white acceptable, or should I find something on-line to use?
 

Peter Parker

Distinguished Member
I've no idea about smartphone lightmeter apps, sorry. Might be worth a punt but I've no idea.

Its like a lot of things - how far do you go and how much do you spend? It can seem a bit much sometimes when all you want to do is watch a movie. An ND filter is around £30 IIRC (depending on the size you need), but if you can't live with the dimmer image, that's £30 wasted.

Although an ND2 filter will reduce the lumens by 50%, we don't perceive it as a 50% drop, To see a 50% drop you need to reduce the lumens by around 82%, so it's quite possible it will look OK.

Oops - sorry forgot to answer the last bit. You should use a 100% full white field generated from your source (BD player). There are a few test disks to choose from, but this one is free:

AVS HD 709 - Blu-ray & MP4 Calibration

Gary
 

Idle Bull

Active Member
I'll download that test disk tonight.

I set the projector up again last night (after removing it to decorate) and did a basic set up using the THX optimiser (with the light output on smart Eco and the cinema setting selected, Gamma 2.2( and the picture looked pretty great, even on DVD. I think it helped I was sat at a more optimal distance too.

I did think that it occasionally seemed a bit too bright, so think I'll try the ND2 filter, I thought I had a Hoya in my camera bag, but it turns out it's an ND10 which would be like watching through 3 pairs of sunglasses!
:)

Overall I'm pretty happy with the projector.
 

Idle Bull

Active Member
No time to download the test disk, but I managed to have a play with the projector and it definitely feels a bit bright. Even watching TT3D closer to the edge in 3D was a bright experience and I read the Benq 3D glasses are supposed to be quite dark.

Looks like an ND2 or even ND4 filter might be required.
 

Peter Parker

Distinguished Member
Do you have any other filters in your bag like an FL-Day? I think that was similar to an ND2 so that might give you an idea of how much dimmer it might look, albeit with a pink tint :)

If you can find one that gives you a good result )or a combination), you might be able to find the equivalent light reduction value and then know what ND you'll need.

You can get variable ND filters now, but I've a feeling some actually effect the colour as they become dimmer. I was going to get one to play with to get you in the ball park of what the nearest ND would be and then buy a good Hoya, but didn't bother in the end due to a lack of info. Canon do a 67mm one for £15 from Amazon for example, but I've no idea how good it is or even what it's made from (or coated or not):

67mm Variable ND Neutral Density Filter for Canon: Amazon.co.uk: Electronics

If you do get hold of a variable one that seems to work well, do let me know won't you? :)

Gary
 

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