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04-07-2018, 04:24 AM   #1
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Reversing Hoya Filters...? (DA 21)

Apologies if this is in the wrong forum....

Following some recent sea silliness, I have just had to carefully clean a very salty front element on my DA 21 - my fault entirely. It seems to have survived and I am now looking at using a filter in the future to prevent potential damage in the future - the question:

Using a 43mm filter inside the stock hood on a DA 21 requires it to be screwed in, in reverse (i.e. the side that is normally facing the lens is now facing the outside world) - does this matter? Are filters coated on both sides?

This is the specific filter being used...

HOYA | The Difference is Clear

Thanks in advance!

04-07-2018, 06:23 AM   #2
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I have done exactly the same. I think that is decent filter and the rear mounting does require it to be screwed into the rear of the hood. To my knowledge the "outer" and "inner" coatings are the same. I have never seen any problems with the mounting you suggest. Some will argue that you shouldn't use any filter on the limiteds but for dusty, salty and wet conditions, I think it is prudent.
04-07-2018, 06:33 AM   #3
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Hmmm... it's not clear.

Test 1: If you hold the filter almost edge-on, does the front surface reflect light differently than the back surface?

Test 2: If you apply a drop of water to the filter surface, does it bead just as much on the back or front surface?

Or a solution: If the front and back surfaces are different, can you reverse the glass? Many filters are constructed either with a screw-in retaining ring or a stiff split-ring clip/spring. If so, you can take the filter apart, reverse the glass and make it so that the magic anti-static, scratch-resistant, water-repelling side is where it needs to be for your DA21.
04-07-2018, 08:34 AM   #4
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I use the Hoya Pro1D Protector inside the HD DA 21 hood. No issues. Works perfectly.

04-07-2018, 08:50 AM   #5
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Budget or bargain filters if coated at all may be coated on only one side. No way to tell except by careful examination.
04-07-2018, 09:08 AM   #6
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QuoteOriginally posted by photoptimist Quote
Hmmm... it's not clear.

Test 1: If you hold the filter almost edge-on, does the front surface reflect light differently than the back surface?

Test 2: If you apply a drop of water to the filter surface, does it bead just as much on the back or front surface?

Or a solution: If the front and back surfaces are different, can you reverse the glass? Many filters are constructed either with a screw-in retaining ring or a stiff split-ring clip/spring. If so, you can take the filter apart, reverse the glass and make it so that the magic anti-static, scratch-resistant, water-repelling side is where it needs to be for your DA21.
Thanks all for the responses, I hadn't thought about flipping the glass as a solution - good thinking! I will do some testing and then try this if there seems to be a difference.
04-07-2018, 10:25 AM   #7
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Most good filters (like Hoya) are coated on both sides, in most cases with the same type anti-reflection coating which consists of a number of thin layers and sometimes a hard topcoat. Even if coated on only one surface, flipping the filter will have little if any effect on the image since it just moves those surfaces to slightly different positions but doesn't change what they do.
04-07-2018, 11:29 AM   #8
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QuoteOriginally posted by Bob 256 Quote
Most good filters (like Hoya) are coated on both sides, in most cases with the same type anti-reflection coating which consists of a number of thin layers and sometimes a hard topcoat. Even if coated on only one surface, flipping the filter will have little if any effect on the image since it just moves those surfaces to slightly different positions but doesn't change what they do.
I don't think this is correct--as regards filters with only a single side coated. I believe it is reflections off the lens front element that cause the problem, and thus the filter side facing the lens is the important one as regards presence or absence of coating.

04-07-2018, 03:41 PM - 1 Like   #9
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QuoteOriginally posted by dms Quote
I don't think this is correct--as regards filters with only a single side coated. I believe it is reflections off the lens front element that cause the problem, and thus the filter side facing the lens is the important one as regards presence or absence of coating.
It really doesn't matter which way an uncoated side is facing - reflections from that uncoated surface are the same regardless of the direction the light comes. One way, the reflections have to go through a layer of glass and the other way they don't but the reflections are a function of the air-glass interface and don't care which is first or second.

If you take a piece of window glass (uncoated) and observe the reflections (actually two - one from each air-glass surface), then flip the glass around, nothing will change - same with a filter. The surface producing the noticeable reflections (uncoated side) will move a bit further away, but it will do the same thing reflection-wise. Reflections from front mounted filters would be a non-issue if it weren't for the reflection of the lens elements that follow which can bounce light back toward the filter. Otherwise, the only reflections from the filter would be away from the lens & camera, and wouldn't do anything. Inter-lens/filter reflections are what causes halos, ghosting, etc. (along with a few other things like reflections off iris blades etc,).

Another reason for coating lenses and filters is to enhance their transmission of light. Any light reflected cuts down on what goes through the lens - for regular window glass, that can be 10% or more. When reflection is reduced, transmission increases by a like amount. Again, however, there is no advantage here based on which side is coated; 90% one way - 90% flipped.
04-08-2018, 06:39 AM   #10
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1. The main issue with having to reverse a filter is that I don't believe a polarizer will work when reversed (at least not a CPL, not sure about an LPL) so reversing for that is not a workable solution - you would have to reverse the mounting.

2. Due to my recent experience with the MeFoto filters (in the Price Watch discussion), I've concluded that it's impossible (for me, at least) to determine if a filter is coated or multi-coated. I used to go by the various colored reflections that most anti-reflection coatings produce, but at least according to MeFoto, their polarizers are two-layer coated on both sides for improving transmission. They don't say anything about anti-reflection properties of the coating. Their UV filters on the other hand have the traditional multi-colored look I'm accustomed to from anti-reflection multi-coatings.
04-08-2018, 04:11 PM   #11
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QuoteOriginally posted by tibbitts Quote
1. The main issue with having to reverse a filter is that I don't believe a polarizer will work when reversed (at least not a CPL, not sure about an LPL) so reversing for that is not a workable solution - you would have to reverse the mounting.

2. Due to my recent experience with the MeFoto filters (in the Price Watch discussion), I've concluded that it's impossible (for me, at least) to determine if a filter is coated or multi-coated. I used to go by the various colored reflections that most anti-reflection coatings produce, but at least according to MeFoto, their polarizers are two-layer coated on both sides for improving transmission. They don't say anything about anti-reflection properties of the coating. Their UV filters on the other hand have the traditional multi-colored look I'm accustomed to from anti-reflection multi-coatings.
With my set of HD DA Limitedís - I only have one good 49mm polarizer and one good 49mm variable ND. Both filters work on The DA Limitedís plus the D FA 100 Macro WR. All of these lenses have a 49mm filter thread. On the 21 Limited once you remove the bayonet hood, the lens itself has 49mm thread. Put your polarizer on there and a 49mm pinch cap. Problem solved.
04-08-2018, 11:43 PM - 2 Likes   #12
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Hoya got back to me. Hopefully this helps to clarify things. Thanks again for all the advice.

Thank you for contacting us.

We would like to inform you that the way to reverse attach of the filter is not affect the filter's performance since the filters are coated equally on both sides.

Thank you for your continuous support.

Best regards,
Ken H.
Tokina/HOYA supporting unit, Kenko Tokina Co., Ltd

Last edited by tbelf; 04-08-2018 at 11:44 PM. Reason: Typo
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