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Advice on using a clear filter to protect scope objective lens please

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Old Thursday 9th June 2011, 14:10   #1
Malcolm Farrow
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Advice on using a clear filter to protect scope objective lens please

I recently bought a used Kowa 823, mainly to give digiscoping a try, and have been very impressed with its performance, particularly with the latest versions of the Kowa 21x and 32x eyepieces. It's a huge improvement over my previous Kowa 613 (which isn't bad in its own right!).

However, worried about getting muck or scratches on that expensive front element I bought a Hoya HMC UV filter to protect it. I put the filter on and did a few test shots in my garden and was suprised to see a weird fuzzy fringing on my bird images. At first I thought it was bad technique or that I had damaged the scope or an eyepiece somehow. But after a process of elimination I found it was the filter. Without the filter the images are fine.

I just wondered whether anyone else had experienced this problem. Have I just got a bad filter, is it something to do with the interaction of scope and filter or shouldn't I be using one at all?

Any advice would be much appreciated...

Cheers

Malc
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Old Thursday 9th June 2011, 14:37   #2
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I use a Hoya Pro1D UV to my Nikon ED50A. A couple of days ago I discovered a massive indent in the filter ring and I had no idea when that could have happened.
Anyway, the finding confirmed that using a filter was some good thinking.

But when I mentioned it here on BF, I was dissuaded from doing so, because the image quality would deteriorate at least when digiscoping (believe it was RJM who wrote that).

I'm a bit surprised to read that the filter use actually does impair the image quality, like he said. Perhaps you could borrow another top-notch filter and try it.
If the bad results remain, your scope simply cannot be used with a filter.

//L
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Old Thursday 9th June 2011, 15:07   #3
Malcolm Farrow
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Thanks for the reply and suggestion. It did cross my mind that perhaps the glass hadn't been fitted into the filter frame properly and had got slightly distorted, thus causing the distortion. If I look carefully I can even see the effect with the naked eye just by looking through the scope, but it's really obvious on the images.

I'd be interested in anyone else's experiences - do most folk use a filter to protect the scope's front element?
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Old Thursday 9th June 2011, 15:14   #4
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There’ve been 4 or 5 “protective filter” threads on BF over the years, the consensus being that even expensive filters can degrade the image a little & that cheap ones will degrade it a lot. IIRC, no objective testing data is cited in any of these threads, just anecdotes. I stopped using a filter some years ago when I dropped the scope & the filter shattered, scarring the objective lens which had to be replaced as a result (the scope was in a heavy padded case & if it hadn’t been for the flying sherds of filter glass everything would probably have been alright).
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Old Thursday 9th June 2011, 17:38   #5
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Quote:
Originally Posted by fugl View Post
I stopped using a filter some years ago when I dropped the scope & the filter shattered, scarring the objective lens which had to be replaced as a result (the scope was in a heavy padded case & if it hadn’t been for the flying sherds of filter glass everything would probably have been alright).
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Old Thursday 9th June 2011, 19:17   #6
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Quote:
Originally Posted by fugl View Post
There’ve been 4 or 5 “protective filter” threads on BF over the years, the consensus being that even expensive filters can degrade the image a little & that cheap ones will degrade it a lot. IIRC, no objective testing data is cited in any of these threads, just anecdotes. I stopped using a filter some years ago when I dropped the scope & the filter shattered, scarring the objective lens which had to be replaced as a result (the scope was in a heavy padded case & if it hadn’t been for the flying sherds of filter glass everything would probably have been alright).
I used UV filters on all my camera lenses and didn't notice any degradation in image quality at all. Is spotting scope more sensitive in this regard?
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Old Thursday 9th June 2011, 19:22   #7
fugl
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I don’t know, I was just reporting what I took to be the consensus so far. As I’ve said, I’m not aware of any objective data on this subject.
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Old Thursday 9th June 2011, 19:37   #8
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hi , many of the top scopes (leica...) have a clear lens build in front for protecting of the special coated lens.
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Old Thursday 9th June 2011, 20:02   #9
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I'd only use a filter when at the coast as sea spray can damage protective coatings as I've learnt to my cost :(
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Old Thursday 9th June 2011, 20:19   #10
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When I upgraded to a Leica APO77 from a cheaper scope I decided to get a filter for it as it was expensive and worth protecting. I got a good (well expensive) filter and was amazed at the way it degraded the image, it was soon removed and I've never bothered with one since.
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Old Thursday 9th June 2011, 21:38   #11
Malcolm Farrow
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Thanks for the replies - sorry I should have checked for previous posts.

Attached is a composite image that shows the difference in quality with and without the filter. The degree to which the image is degraded is pretty surprising, given that this is a top quality filter from a good maker.

The two images have been cropped from the central part of the image and re-sized but are otherwise unedited. The originals were successive raw files and the subject was a TV aerial a couple of houses away.
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Old Thursday 9th June 2011, 21:38   #12
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Quote:
Originally Posted by postcardcv View Post
When I upgraded to a Leica APO77 from a cheaper scope I decided to get a filter for it as it was expensive and worth protecting. I got a good (well expensive) filter and was amazed at the way it degraded the image, it was soon removed and I've never bothered with one since.
I use protective filters quite a lot on my scopes, and even in a side by side comparison (we've got three Nikon EDIIIs and one EDII in the family) I can't really see much difference. The differences between the scopes are bigger than the difference the filter makes - one is a true cherry, two are alright and one isn't really *that* good at all.

However, I also found some filters - even expensive ones from "good" manufacturers such as Hoya - degrade the optical quality. At present I use neutral protective filters from Canon and Nikon. No complaints at all, I prefer them over UV filters nowadays. With the Leica Apo 77 I used in the 90s I used an expensive Leica UV filter. That was also OK. It also saved me a lot of money, my Leica was one of the few that didn't suffer from a destroyed objective lens after extensive seawatching at the coast ...

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Old Thursday 9th June 2011, 22:08   #13
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Quote:
Originally Posted by Malcolm Farrow View Post
Thanks for the replies - sorry I should have checked for previous posts.

Attached is a composite image that shows the difference in quality with and without the filter. The degree to which the image is degraded is pretty surprising, given that this is a top quality filter from a good maker.

The two images have been cropped from the central part of the image and re-sized but are otherwise unedited. The originals were successive raw files and the subject was a TV aerial a couple of houses away.
That’s an amazing difference. The filter must be defective in some way. If I were you, I’d take it back to the retailer for money-back or replacement.
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Old Sunday 12th June 2011, 16:18   #14
etudiant
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Filters are touted as protecting the objective lens, but have not been worth the bother in my experience.
It is easy to get fogging between the filter and the objective lens, which might account for the softness in
Malcolm Farrows rather shocking images.
A soft rubber hood/sunshade is a better protection for the front of the scope imho, although that is difficult to reconcile with a stay on case without surgery.
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Old Sunday 12th June 2011, 20:40   #15
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Originally Posted by etudiant View Post
It is easy to get fogging between the filter and the objective lens, which might account for the softness in
Malcolm Farrows rather shocking images.
I've been using filters on a regular basis for close on 20 years now, and I never had any fogging between the filter and the objective lens. And I do go out birding in the rain quite regularly.

Quote:
Originally Posted by etudiant View Post
A soft rubber hood/sunshade is a better protection for the front of the scope imho, although that is difficult to reconcile with a stay on case without surgery.
Most scopes have some sort of sunshade nowaday, but in really bad conditions I tend to use one that's quite a bit longer than the one of the scope to keep the water or seaspray of the lens.

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Old Monday 13th June 2011, 16:37   #16
etudiant
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Interesting.
You must have better filters or better technique.
I had trouble with fogging/condensation on my ED50 scope using a Hoya HMC UV(0) filter, badly so in cool and wet conditions on Cape Clear, so it now has a screw on soft rubber sun shade instead.
That scope has no sunshade, sacrificed to save weight.
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Old Tuesday 14th June 2011, 15:15   #17
Malcolm Farrow
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Quote:
Originally Posted by etudiant View Post
Filters are touted as protecting the objective lens, but have not been worth the bother in my experience.
It is easy to get fogging between the filter and the objective lens, which might account for the softness in
Malcolm Farrows rather shocking images.
A soft rubber hood/sunshade is a better protection for the front of the scope imho, although that is difficult to reconcile with a stay on case without surgery.
It isn't caused by condensation or fogging. The filter is brand new and was only fitted briefly to scope in dry, sunny, warm weather. Whatever caused it, it's nothing to do with moisture or other external environmental factors.

I'll repeat the test just in case vibration or something similar caused the problem, it's the only other thing I can think of.

Thanks for the interesting replies.
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Old Tuesday 14th June 2011, 15:56   #18
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Optically the best solution would probably be Baader TurboFilm.
It would have to be replaced rather than cleaned but it is fairly inexpensive and one can make up one's own filter holders.
Here is a link:- http://www.alpineastro.com/optical_a...ccessories.htm (scroll down)
There are further instructions on how to construct your own filter cell.

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Old Wednesday 15th June 2011, 07:56   #19
Malcolm Farrow
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Just a quick postscript to add that I re-did the test with exactly the same result. Clearly the filter is causing the problem.

Hard to believe that a Hoya HMC filter could do this and I can only hope it's a rogue filter that got through QC. Otherwise you wonder what the implications could be for photography where the use of protection filters is even more common. Could be some people out there that have ditched a 'bad' lens when it was just a filter causing the problems. Makes you think...
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Old Wednesday 15th June 2011, 08:33   #20
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A standard camera lens is much less demading on filter glass quality since they tend to be used at much lower effective magnifications. Even the largest 600mm telephoto lens tops out at ~30x, which is on the low end of typical digiscoping magnifications.

Anyway, your experience is just another validation of what many others here have discovered when trying to digiscope with a protection filter on the scope. Too bad you never picked up on the old threads as you would have saved yourself some tuition.
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