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Old Dec 30, 2017, 1:08 PM   #1
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Default adding glass to change focus

Hi all,
I bought a very cheap, old, point and shoot (Ixus40), to experiment converting it to infra-red; but after removing the IR blocking filter, the camera would no longer focus. I wasn't surprised (I've read about others who have had similar problems after removing the IR blocker; the usual solution being to replace the removed filter with a piece of good glass of the same thickness.) So, that's what I did. The focus improved, but wasn't anywhere near sharp. After more experiments I finally achieved good focus with a piece of glass nearly twice as thick as the removed filter.
Now to the purpose of my post. I have a Finepix F70EXR which is not sharp wide open; but is sharp when zoomed to the sweet spot. Does adding a piece of glass, near to the sensor, to improve the wide open focus, make sense?
..... john
P.S. I have removed the IR blocker from a different camera (Kodak) which did not result in loss of focus.

Last edited by Shinnen; Dec 30, 2017 at 1:09 PM. Reason: diction
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Old Dec 30, 2017, 2:39 PM   #2
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G'day mate

Goodness me ... you're into a region I have no experience or even general knowledge with ~ but like others here, I will keep an eye on your results

Phil
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Old Dec 30, 2017, 7:09 PM   #3
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Originally Posted by Ozzie_Traveller View Post
G'day mate

Goodness me ... you're into a region I have no experience or even general knowledge with ~ but like others here, I will keep an eye on your results

Phil
Hi Phil,
Thanks for your reply.
I'm not going to take the F70 apart until I feel comfortable that I 'may' improve it's focus. Like you, I have little or no experience with this kind of thing, but I like the F70, if only it were sharper wide open.
.... john
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Old Dec 30, 2017, 9:05 PM   #4
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Originally Posted by Shinnen View Post
Now to the purpose of my post. I have a Finepix F70EXR which is not sharp wide open; but is sharp when zoomed to the sweet spot. Does adding a piece of glass, near to the sensor, to improve the wide open focus, make sense?
That sounds more like a problem with the lens that with the flange focal distance, so I'd say no. Adding a piece of glass will affect everything, not just some things.
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Old Dec 30, 2017, 9:23 PM   #5
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I have worked on a couple SLR lenses which had sharpness problems, and found the solution to be with the objective lens. In the case of a Tamron, it was mounted to allow small adjustments via cam slots in the holder. Some may need to be shimmed.
Older manual focus SLR lenses sometimes had markings for focusing with IR film. I suspect that the additional refraction of the glass in front of the sensor was enough to shift the focus for your IR conversions, but have doubts as to it working without the conversion.
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Old Jan 1, 2018, 9:26 PM   #6
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Thank you all for your help.
I think I'll let it go for now.
... john
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