Focus

[Ryote]

We have just bought a Sanyo XP57L projector and have fitted an LNS-W31A Short Throw Zoom Lens. The main problem I'm having is that it looses focus about 2/3's down the projection. Initially I thought this was an issue with inserting spacers, but after spending an hour working at it we have noticed that when you move the lens up to a certain point this affect decreases. Anyone else experienced this issues and resolved it, if so how?

 

Many Thanks

 

Andy

[paulears]

Bit confused with the explanation?

 

If you mean that it's in focus at the top, or bottom, but you can't get both sharp, then I'm wondering if the projector is at a sharp angle to the screen. If you have the projector hitting the screen at 90 degrees, straight on, is that sharp? One thing with wide lenses is that they don't have a very big depth of field - so if the projector is low or high, and you have to use full keystone adjustment to get the image more square then it is very possible that focus sharp from top to bottom is impossible. The best you can get is sharp in the centre, and soft top and bottom equally. All it is in these cases is distance. With a short throw, and wide angle you can have big differences in the path length - maybe a 2 metre path to the closest bit of the screen, and over 3 to the furthest - taking a good few turns of the focus rack to sharpen up.

 

This is a kind of guess of a likely solution, as you need to explain how it's set up. Do the front on test first - if this is sharp all over, then the lens itself is fine. Wide angle lenses are always a compromise - often having to have a large aperture to get the light out, but this making depth of field quite poor.

[Ryote]

The first thing I tried was a front on test, The focus seems to be racked up to the limit and still we're not getting an all over sharpness, also as I said when the lens is moved up or down this also effects the sharpness. The only way to get anything resembling a sharp image is to have the projector sitting more or less on top of the screen and then the image size is minuscule. At the minute its being flown from a bar at around 7 metres from the screen and although it is usable for images it is useless for text. This focus problem seems to be present regardless of projector position or relative distance.

 

I realise this may all be unclear and for that I apologise

 

regards

 

andy

[gyro_gearloose]

I'm assuming your screen is flat? If it is, it might be worth trying to get hold of a curved screen instead. This will help because by being curved the distance from the screen to the projector remains constant (i.e. the edges are the same distance away as the centre). Of course the down side is that the shorter the projectors throw the greater the curve of the screen, so the further one sits from the projection axis the more distorted the screen appears.

[eviljohn2]

Should be fine with any XP projector set up right. Things to check are that the QR plate is attached properly and that the screws attaching the bracket to the lens don't poke out the other side, it's possible to fit the lens with the screws sticking out but you'll never get an even focus. You shouldn't need spacers to get it focused properly. :)

[duntokin]

Hi, we use the same kit to back project film in our hall.

Projector to screen approx 5 meters onto a 12'x 15' screen. We don't have any focusing problems as such though if you change the lens as we do you have to make sure its really well secured and seated using the right bolts.

 

I have found though, that the lens mounting quick release lever has to be firmly set or the lens can drop a bit, as its a really heavy load on the mounting plate.

 

What I do find irritating is that in some films where there is a lot of white/ v bright sets/scenery, like hospital corridors, snow, desert, etc there is a lot of burn-out and the contrast goes to hell in the centre of the image. Have tried loads of different settings on projector and DVD player but to no avail.

[Ryote]

thanks for the help guys I'm starting to think this problem is a unit based problem rather than a lens problem. Thanks Again

[FFM]

Should be fine with any XP projector set up right. Things to check are that the QR plate is attached properly and that the screws attaching the bracket to the lens don't poke out the other side, it's possible to fit the lens with the screws sticking out but you'll never get an even focus. You shouldn't need spacers to get it focused properly. :P

 

 

I'd suggest you follow eviljohn2 advice ref the screws. If the wrong screws have been used the lens housing could be bent, possible giving you the fault your experiencing. Don't assume because its new it will be in perfect condition, an inexperienced kid could have had his/her hands on the projector before it reached you, (ie a sales men!).

 

If you can get a true focus with a little force on the lens then send it back.

 

Best of luck