| Author |
Message |
Gerbrand
Joined: 13 May 2008
Posts: 199
|
| Posted: Fri Feb 20, 2009 2:40 am Post subject: Could my focus drift really be my house drifting? |
|
|
Hello,
Coming back from holiday I noticed that the center optical focus on my BG808 had drifted quite a bit. It does that sometimes and I had already noted a correlation with weather conditions.
Thus far I have always blamed the projector, but today I realised it could in fact be the mechanical distance between the screen and projector that is to blame. I did a rough calculation for the depth-of-view of a CRT projector projecting 1080p and came up with a number which is only of the order of 0.2-0.3 mm.
If this number is correct (please somebody correct me if I am wrong), this could easily explain the drift I am seeing. The projector is hanging from a wooden attic roof. So if the sun has been heating the roof, this could easily move by a couple of mms which is sort of in line with the number above.
Any thoughts? Do other people with their projectors hanging from wooden structures see similar changes e.g. related to the relative humidity?
Regards,
Gerbrand
|
|
| Back to top |
|
 |
Curt Palme
CRT Tech
Joined: 08 Mar 2006
Posts: 24396
Location: Langley, BC
TV/Projector: All of them!
|
| Posted: Fri Feb 20, 2009 3:14 am Post subject: |
|
|
|
Tighten up everything. It's amazing how much stuff shifts/loosens/drifts if it isn't screwed/clamped down properly.
|
|
| Back to top |
|
|
Mark_A_W
Joined: 15 Mar 2006
Posts: 3068
Location: Sunny Melbourne Australia
|
| Posted: Fri Feb 20, 2009 4:56 am Post subject: Re: Could my focus drift really be my house drifting? |
|
|
| Gerbrand wrote: | Hello,
Coming back from holiday I noticed that the center optical focus on my BG808 had drifted quite a bit. It does that sometimes and I had already noted a correlation with weather conditions.
Thus far I have always blamed the projector, but today I realised it could in fact be the mechanical distance between the screen and projector that is to blame. I did a rough calculation for the depth-of-view of a CRT projector projecting 1080p and came up with a number which is only of the order of 0.2-0.3 mm.
If this number is correct (please somebody correct me if I am wrong), this could easily explain the drift I am seeing. The projector is hanging from a wooden attic roof. So if the sun has been heating the roof, this could easily move by a couple of mms which is sort of in line with the number above.
Any thoughts? Do other people with their projectors hanging from wooden structures see similar changes e.g. related to the relative humidity?
Regards,
Gerbrand |
Ahh...play around with moving your screen.
I think you'll find you are 2 orders of magnitude out - you can move the screen 20-30mm with little difference in focus.
|
|
| Back to top |
|
|
Gerbrand
Joined: 13 May 2008
Posts: 199
|
| Posted: Fri Feb 20, 2009 9:18 am Post subject: Re: Could my focus drift really be my house drifting? |
|
|
| Mark_A_W wrote: |
I think you'll find you are 2 orders of magnitude out - you can move the screen 20-30mm with little difference in focus. |
I realised that myself this morning (that will teach me to do this kind of stuff in the middle of night  ).
The two orders of magnitude comes from the fact that I had divided by the magnification (that is the ratio of the screen size and the crt size, which is around 10) instead of multiplied by it.
The 20-30 mm makes more sense and also rules out the idea that drifting of the mount might be the cause.
So I'll follow Curt's advice and start tightening some screws. I suppose the screws holding the lenses and the screws holding the CRTs are the most important?
BR
Gerbrand
|
|
| Back to top |
|
|
|
|
