lens shift affects picture quality?

richard plumb

Distinguished Member
Hi

Does lens shift affect picture quality like keystone used to?

Newer projectors seem to project to the centre of the screen which isn't any good if you have a ceiling mount. So can I use full vertical lens shift to keep my mount on the ceiling without affecting the image?

I understand I'd probably lose any horizontal shift, but thats just a case of mounting it properly in the first place...
 
K

Kadders

Guest
Depends how good the lens is. What you are doing is projecting through a different part of the lens, where there could be more aberration. You need larger diameter lenses to allow shifting - part of the reason why it's not always available. This is also why you cannot fully shift in both horiz and virt at the same time.

Similar (or opposite!) principle to lenses for removing 'converging verticals' when photographing buildings - rather than tilt the camera, shift the lens:
http://www.zoerk.com/pages/p_psa.htm

Definitely better than keystone correction (which is an electronic non-linear stretch) in any event.
 

maj74

Active Member
The short answer is yes it does.

Certainly I discovered on my Sony HS-50 that if you use the lens shift, it causes the pixels to go very slightly out of alignment from the different panels. In the case of my projector which is mounted high up, I thought I had misaligned panels as I had a blue upper edge on any hard white lines e.g. credits, or the net cord in the film 'Wimbledon' caused by about 1.5 pixels of blue panel projecting above the others.

I thought I would try and get another projector and see if it had better alignment, so when UrbanT sold his HS-50 for a very attractive price, I thought 'What the hell' He assured me he could see no blue edge (or any other colour) that he could tell, so we conlcuded the deal and he shipped it to me.

I changed the projectors over, and hey presto, no blue edge.....

Until I used the lens shift to bring the picture down onto my screen just as my projector was set up, then the blue misalignment appeared just the same.

I then experimented with adjusting the lens shift of both machines and sure enough, the further the lens shift is applied, the further the blue pixels creep out of place.

I should point out that this is a very minor effect and many people possibly wouldn't notice it, but it is there and I do! :(

I wonder if this effect is caused by the different wavelengths of light (esp Blue) being diffracted by differeing amounts as the lens shift causes the light path to hit the lens at an increasingly oblique angle. If this is so then I would think that the same effect will be present to a greater or lesser extent on every projector that has a lens shift feature.

It's certainly worth mentioning that this is much less of an issue compared to using keystone correction which really does affect the picture.

Hope this long-winded reply helps!



So yes, the lens shift definately affects the picture quality, but by nothing like the degree
 

Rob100

Well-known Member
Can't say I've noticed and pixel alignment problems on my HS60, but using lots of vertical lens shift does cause the focus to differ slightly at top and bottom. I find if you look at the bottom of the image when focusing the top looks fine, but if you look at the top of the image when you focus the bottom is definately out.

However I couldn't live without lens shift...

Rob.
 

richard plumb

Distinguished Member
OK, so its minor compared to keystone, but still some effect.

Now answer me this - how come suddenly lots of projectors are projecting in the centre of the screen rather than the bottom/top edge?

Surely most PJ owners will have the PJ on the ceiling or low down on a table. By making them project to the centre many of us are forced to use shift.

Who can actually use a PJ without shift? if you have a shelf on the back wall? But then you have possible ventilation issues - not to mention projecting through the back of your head.

I just don't understand the sudden change of direction.

BTW I was looking at the panasonic AE900 (My AE100 projected to the top of the screen). Do the Hitachi TX200 or Sanyo Z4 project to the bottom?
 
K

Kadders

Guest
richard plumb said:
Now answer me this - how come suddenly lots of projectors are projecting in the centre of the screen rather than the bottom/top edge?
I think those that project toward centre-line are the ones which have lens shift. Like I said (or at least implied) above, it's possible to make a lens which will shift the picture without any aberration. Lens shift is not (or should not be) a bad thing. At the extremes, you might lose some light (but should not) and you might get the chromatic shift described earlier (but should not). All down to lens quality. The link in Quantum's post above shows this admirably (it's talking about camera lenses, but is the same principle).

Lens shift means the lens will be more expensive to produce - you need a larger diameter lens (bigger the lens, the more you can shift it) and it needs to be optically corrected to remove chromatic aberration. But shifting makes it much easier to position pj and screen, so it is a desirable feature. OK a slightly long-winded answer to your question!

Maybe you should get a demo to see if there is any problem.
 

chedmaster

Active Member
I notice absolutely no drop in PQ (although I am one blue pixel out of line I've never tried this non-shifted) using lens shift on my Sanyo Z1. Even at max up/down I can go pretty much max left/right as well.

The focus thing is a problem though, although there is a zone where the whole picture is in perfect sharp focus, as I try to defocus to reduce SDE it becomes uneven, the top defocusses first or vice versa.

Keystone correction is truly rubbish, you lose so many pixels and pixel mapping when using HTPC.
 

Mark_a

Well-known Member
My Sanyo Z3 is mounted more or less centrally top to bottom on a shelf behind me, but is off centre right to left. The lens shift brings it back into the centre of the screen and I can see not one single difference from where it's naturally centred. Then again, I don't shift it anything like to the extremes of where is could shift to. So I suspect smallish shifts have no effect, it's only at the outer limits it starts to suffer.

Regards

Mark
 

ELV

Active Member
I have a Z4 and use the lens shift verticaly quite a bit and hoizontally a little also,i noticed no problems with it, but iv'e never went out of my way to see any.
 

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