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Old Nov 4, 2013, 9:54 AM   #1
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Default distance of filter from lens

Hi,
I'm wondering if the distance of a filter from the lens is important. I've been fiddling around with making a 'homegrown' variable neutral density filter, using a cpl and linear pl (circular next to the lens); but the camera seems to be focusing on one of the filters, because I'm seeing what appears to be dust, or something. (There are other issues too ..... namely a blue cast when I get to the point of blocking out most of the light.) But I'm mainly interested in whether the filter should be mounted right next to the lens, or can a spacer be used in between the lens and the filter. Sorry about the long winded explanation.
Thanks,
....... john
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Last edited by Shinnen; Nov 4, 2013 at 9:57 AM. Reason: spelling
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Old Nov 4, 2013, 1:58 PM   #2
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If (for whatever reason) your filter is a small distance away from the lens,any dust,hair,mark or whatever on the filter might be picked up by the AF system- and of course,vignetting would be an issue also...
Given the issues you're experiencing- I'd consider a fairly inexpensive solution...
http://www.parkcameras.com/22432/Ham...FU_HtAodrm4Aig
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Old Nov 4, 2013, 2:19 PM   #3
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G'day mate

From experience, when you start to polarise already polarised light, the colours go haywire. I get violent violet images doing this

I have also got a [german made] B+W ND400 & a ND1000 [can't remember the brand] and both of them cause miserable streaking & splotches across the image

I don't know whether any of these filters truly work

ps- I can post images if you wish
Phil
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Old Nov 4, 2013, 3:27 PM   #4
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Hi Simon/Phil,
Thanks for the help. So, it's best to put filters right onto the end of the barrel ..... no spacers ..... OK.
It sounds like the Hama filter is the solution alright. Now I just need to find one here.
I got the idea of using a cpl and a lpl from a webpage, but haven't been able to make it work. Thought I might do some motion blur pictures.
...... john
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Old Nov 4, 2013, 8:38 PM   #5
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The blue or purple cast is common with some crossed polarizers due to the shorter wavelengths passing more easily through the filter.
When you reduce the amount of light using filters this way, autofocus systems have difficulty. The best way to do what you are doing is to manually focus, or lock focus with the polarizer set to pass the most light, then adjust for the exposure you need.

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Old Nov 5, 2013, 2:52 AM   #6
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Quote:
Originally Posted by Ozzie_Traveller View Post
I have also got a [german made] B+W ND400 & a ND1000 [can't remember the brand]
The brand is B+W.

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... and both of them cause miserable streaking & splotches across the image
I'm very surprised to hear that. B+W makes some of the finest filters available. I suspect that yours are either counterfeit or were grossly mishandled somewhere in the supply chain.
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Old Nov 5, 2013, 8:47 AM   #7
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Quote:
Originally Posted by Shinnen View Post
I'm wondering if the distance of a filter from the lens is important.

There are two considerations.
  1. If the filter is too far from the lens, the filter barrel(s) could cause vignetting, especially at short focal lengths/wide angles of view.
  2. If the filter is too close to the lens, the filter's surface could come in contact with the objective lens, and both of them could be damaged as you mount, dismount, and operate the filter.
Quote:
Originally Posted by Shinnen View Post
I've been fiddling around with making a 'homegrown' variable neutral density filter, using a cpl and linear pl (circular next to the lens); but the camera seems to be focusing on one of the filters, because I'm seeing what appears to be dust, or something.
Are you using a macro lens? If not, it's unlikely that you would be able to focus on something so close to the objective lens. I once was using a bellows on my Minolta SRT-101 and Rokkor MC 50mm f/1.4 lens, when a housefly landed on the objective lens. Only the six legs were in focus. (The body was outside the depth of field.) Before I realized what I was looking at and could press the shutter release, the fly flew away. That was the only time I was ever able to focus on the objective lens.
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Old Nov 5, 2013, 8:49 AM   #8
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Possibly they were cleaned with a solvent which wasn't compatible with the material.
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Old Nov 8, 2013, 9:09 AM   #9
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Thank you all for your thoughts.
TCav ...... no ..... no macro.
Brian ..... solvent ..... maybe. I do use Windex occassionally, but I've never noticed a problem before. I will try the manual focus idea.
........ john
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Old Nov 8, 2013, 9:49 AM   #10
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Quote:
Originally Posted by Shinnen View Post
... but the camera seems to be focusing on one of the filters, because I'm seeing what appears to be dust, or something.

Quote:
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Are you using a macro lens?
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TCav ...... no ..... no macro.
Then I suspect that it's unlikely that you're focusing on the filter. Could it be dust on the sensor instead? Can you see it in the viewfinder? ... in 'Live View'? ... while distant objects are in focus?
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