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01-01-2020, 05:56 PM   #1
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Using gel filter BEHIND lens?

Anyone ever use a gel filter on the rear element of a lens in a K-01? I'd be reluctant to try it on a lens without a rear holder on a Pentax with a mirror flopping around nearby, but the K-01 seems to have room (for those wondering why, it's a technique used with UWA lenses like fisheyes that can't use front-mounted filters).

01-01-2020, 07:45 PM   #2
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Interesting timing on this. I was just looking at gelatin filters at B+H trying to scope out what would work on the back of the Irix 11mm I just ordered, and wondering if it was worth the price difference from Lee to Kodak Wratten 2.
01-02-2020, 04:32 AM   #3
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How would you use it? How would it be 'mounted' or affixed? Intriguing, as I have a K-01 that could see more use...
01-02-2020, 07:06 AM   #4
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QuoteOriginally posted by clickclick Quote
wondering if it was worth the price difference from Lee to Kodak Wratten 2.
Personally I work with Lee filters because of their consistency, having said that Kodak are also an excellent choice as well.

QuoteOriginally posted by Oakland Rob Quote
nyone ever use a gel filter on the rear element of a lens in a K-01? I'd be reluctant to try it on a lens without a rear holder on a Pentax with a mirror flopping around nearby
I have worked with lenses that used rear gelatin filters, perhaps the biggest annoyance is the lack of GND* filters, secondly is how fragile the filters are. In the field it is easy to lose them and even easier to mishandle and inadvertently scratch them. But they work well, in some ways are superior to front mounted filters.


I wouldn't advise attaching a thick glass filter to the rear of any lens, not only can it damage the mirror mechanism/shutter should it come loose. But thick glass also has an effect on the optical properties of the lens, and attaching thick filter glass to the rear cell of a lens can have rather catastrophic impact on image quality.


*Graduated Neutral Density

01-02-2020, 07:40 AM   #5
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QuoteOriginally posted by Digitalis Quote
Personally I work with Lee filters because of their consistency, having said that Kodak are also an excellent choice as well.
Thanks! Appreciate the feedback. The Lee filters are about $23 versus $89 for the Kodak ones, so it left me wondering if there was a reason. I haven't handled this type of material, but just the name makes me concerned on durability. I was hoping to see someone making a graduated one, even if you couldn't adjust it and the horizon was dead center, it would be nice to have the option. I'm wondering if I could cut a graduated one down to fit. Have a hunch it's going to be about thickness not fitting the holder on the back of the lens versus the size of the square.
01-02-2020, 07:58 AM - 1 Like   #6
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QuoteOriginally posted by clickclick Quote
I was hoping to see someone making a graduated one
Not a chance, I have thought about the possibility of tediously half submerging a piece of ND gel in a concentrated solution of Sodium hydrosulfite or potassium permagnate, gradually withdrawing it as it bleaches out the dyes in the gel and thus creating a gradient.

Last edited by Digitalis; 01-02-2020 at 08:06 AM.
01-02-2020, 08:06 AM   #7
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QuoteOriginally posted by Digitalis Quote
Not a chance, I have thought about the possibility of tediously half submerging a piece of ND film in a concentrated solution of Sodium hydrosulfite or potassium permagnate, gradually withdrawing it and thus creating a gradient.
I like the way you think. If you do this, let me know....


01-02-2020, 08:48 AM   #8
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Here's the hack that got me thinking about using the gel with a lens that doesn't have a holder like the Irix (BTW, they sell some 30mm gels precut to fit I believe). Basically just cut and attach with a bit of gaffer's tape. Note the lens in this video has a bit of a recess. Other UWAs I've had, like my Pentax mount, seem to have a rounded rear element that protrudes a bit. So might not fit.



01-02-2020, 10:48 AM   #9
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QuoteOriginally posted by Oakland Rob Quote
(BTW, they sell some 30mm gels precut to fit I believe)
Found them on Amazon and Adorama, but not B+H - perhaps I overlooked them. Thanks

https://www.amazon.com/Irix-Gelatin-Filter-29x29mm-Lenses/dp/B073M4RY13?tag=pentaxforums-20&

https://www.adorama.com/irxifgnds.html?
01-02-2020, 06:11 PM   #10
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QuoteOriginally posted by Oakland Rob Quote
they sell some 30mm gels precut to fit I believe
You used to be able to buy 30mm punch filter cutters specifically designed to fit rear gel filter holders so you could buy bulk sheet filters from providers like Lee and Kodak. I'd imagine such a thing would be rather uncommon these days.
01-03-2020, 10:56 AM   #11
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QuoteOriginally posted by Digitalis Quote
I wouldn't advise attaching a thick glass filter to the rear of any lens
Some of the 6x7 lenses accept rear glass filters and I have used them a lot. What I have noticed is that the focus point changes and there is an increased chance of flare. Any glass rear filter should be multi-coated if possible. Image degradation from their use is actually quite small.
01-03-2020, 11:09 AM   #12
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I read some weeks ago about "Sensor filters" but can't find the link. I haven't seen them for pentax cameras... the benefit of this will be that you don't need different filters for different lenses... just "One filter to rule them all..."

01-03-2020, 07:21 PM   #13
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QuoteOriginally posted by desertscape Quote
Some of the 6x7 lenses accept rear glass filters and I have used them a lot.
I remember those. though It has been some years since I used those glass filters. Personally I use those rear slots for UV/IR and ND filters, GND and polariser filters are better employed on the front filter thread if there is one.
01-04-2020, 11:01 AM   #14
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Too bad they don't offer an STC filter for the K-01, but I can certainly understand why they wouldn't. They have them for a bunch of other Pentax models though.

The K-01 has so much room behind the lens I'm tempted to stuff a gel in there. I could store lunch in there, but the sides are kind of uneven, and there are some contacts and other things I wouldn't want to touch. And if I used gaffer's tape like the video, I'm afraid it could come loose and hit shutter or sensor, which would Be Bad.

There are lens adapters with drop in filters, like for PK to M43 adapters. And some lenses have drop in filters too.

I suppose one could try using a macro tube and put one in, but the tube itself won't work on the UWA lenses I'm interested in.

Maybe one could get one like the STC and attack it with a Dremel to get it to fit.

I'll keep looking; thanks.
01-08-2020, 06:58 PM   #15
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The STC is interesting. I wonder if you could adapt another Pentax one.
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