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Getting some annoying Focus issues. Possible solutions?

post #1 of 5
12/24/12 at 11:04pm
Thread Starter 
First, Merry Christmas to you all. I'm up having a drink before bed and felt like venting a bit.

I have an Optoma HD3300 with an HTB anamorphic lens. I am at a 13' throw from my screen which is about 138". It is a curved screen that I built and I absolutely love it. The projector is bang on with the screen and it had to be since it has no lens shift and minimal zoom. I hate to be so close, but again, the zoom on this PJ sucks. The problem I'm having is softening at the right and left edges of the screen. Is this normal with all prism lenses, or do I just need to ante up and get a better lens? I realize I'm at a really short throw...perhaps I should sell my Optoma off and get a pj with a better zoom and have the projector further back from the screen. How much would that help?

I just spent a ton of money (relatively speaking) on new speakers, so I'm trying to see what I can do without spending a bunch of money. I love my cinemascope screen, but I know the picture can be way better and it bothers me. Help!?
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post #2 of 5
12/25/12 at 5:19am
Quote:
Is this normal with all prism lenses, or do I just need to ante up and get a better lens?

"Yes", and "Yes".

Prism lenses of the HTB caliber have no astigmatism correction. So you can get the image sort of focused in the center, but not at the edges and vice versa.

Some of what seem to be you "focus issues" will actually be "color aberration issues", which HTB lenses don't correct for, either. As the image progresses from center to edges, the colors separate out more. Fine detail is smeared over more pixels than intended, depending on the color of the channel. Combining them all together leaves you with an appearance of poor focus.

Both astigmatism and color aberration with an uncorrected prism lens will be worse at the low throw ratios you employ (less than 1.5).

So looks like a new lens, unfortunately, if you want your images to be crisp.
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post #3 of 5
12/25/12 at 9:06am
Thread Starter 
I figured as much. I did buy the astigmatism correction add-on, but I haven't found it has helped as much as I would have thought, especially at this throw. Thanks for the help. On a positive note, I used your spreadsheet to make my curved screen and it turned out fantastic. Any recommendations on a good short throw lens that won't break the bank?
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post #4 of 5
12/25/12 at 2:24pm
Quote:
I figured as much. I did buy the astigmatism correction add-on, but I haven't found it has helped as much as I would have thought, especially at this throw.

There are two main focal planes you need to consider when using prism lenses: vertical and horizontal. Prism systems don't change the vertical focus of the projector, bit they do change the horizontal focus. An astigmatism corrector adds optical power to the horizontal plane to "bump" it a little so that it's plane of focus co-incides with the vertical.

Fixed astigmatism correctors have a definite sweet spot at which they work best. After all they are "fixed", by definition!

At short throws, again you are correct, a fixed astigmatism corrector needs to be very close to ideal to give you best focus. At longer throws you have some more leeway.

To give you an idea of sweet spots: the original Panavision prism lenses, from the early 1950s, had a set of tables for determining the best astigmatism corrector to use. These tables started at a throw of 72 feet, and then up from there. Short throws like 13 feet weren't even considered.

The corrector lens won't fix color aberration at all.

A throw ratio of less than 1.5 is a challenge. You'll need a biggish lens, which isn't going to be cheap. Sorry, can't make any suggestions, but I'm sure others here might be able to help.

Glad you found the pincushion calculator useful. I wrote that a long time ago, but the numbers live on!
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post #5 of 5
12/25/12 at 3:53pm
Thread Starter 
I ended up messing around with the correction element again and double checked my throw ratio. at 13 feet and with an 89" 16:9 width, I'm at about 1.75, which isn't that bad. Again, I think it does come down to the lens which will have to wait until I can muster up some coin and convince my better half that it is, in fact, a worthy upgrade. During movie watching it isn't really noticeable most of the time because a person's eyes are focused towards the middle of the screen. It's the odd time when there's text on the extreme ends of the screen where it becomes a bother since I like things to be perfect. Thanks again for the help and I hope you're enjoying the holiday! Me, I have a rum and egg nog in hand and am watching my beautiful kids play with their new toys. Time to go engage. Cheers.
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