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UltravioletPhotography

Filters available in larger sizes?


OlDoinyo

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With the recent interest in wider-angle lenses for UV work comes the question of what filters to use with them. Simple Woods-glass filters such as the 403 are available in many sizes and work reasonably well with film, but are not really up to the demands of digital shooting. Dichroically-augmented WG filters such as the Baader U2 are better for digital work, but this filter, at 49mm, is too small for most mid-to-short wide-angle lenses; even with a step-down ring, there will be problems with obstructive image-circle truncation, and the dichroic coatings do not function optimally at angles far from the perpendicular. I would be interested to know what else is known to be out there; even non-dichroic filter stacks might be worth considering.
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I don't currently know of any larger diameter UV-pass filters which would not require an added IR-blocking type filter. Uviroptics on Ebay offers larger diameter IR-blockers, so stacking is probably the only feasible way to go currently if you want larger diameter front-mounted filtration.

 

How about just rigging up a rear mounted filtration system for the wide angles? Then you could use your existing filters. I have never done this, so I do not know if rear mounted filters would interfere with the flange focal distance needed for the lens?

 

You can get larger square UV-pass filters from some of the filter manufacturers (check Omega, Asahi, Edmund and others). Squares could perhaps be used on wide angle lenses with a Cokin or Lee type filter holder. I've forgotten how big those square filters are typically. Still might not be big enough?

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Rear mounted filters most definitively change the register distance. Thus one would mainly be able to use this approach for mirrorless cameras.

 

I have used rear mounted filters for my Panasonic GH-2 cameras for years. It is a feasible solution for using Baader and similar filters on very wide lenses.

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Rear mounting is quite the can of worms--it would seem to require cutting up some very expensive filters so that their fragments can be affixed to the rear of the lens (which is most likely not designed with this in mind.) Plus, there are the focus shift issues mentioned, as well as mirror clearance.

 

I do not yet own a UV-capable wide angle; my stock Minolta 25/2.8 has a lot of CA and hotspots badly to boot when UV is attempted. My 14/2.8 has very thick glass and will not even take a front filter, so never mind that. I have, however, been intrigued by some lenses I have heard discussed elsewhere on this forum.

 

Is there such a thing as a non-dichroic IR-blocker which does not block UV (I already have a collection of dichroic hot mirrors, but those may not work well when there is incoming light more than 45 degrees off the lens axis.)

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It might be cheaper to get an old NEX and convert it yourself for "rear-mounting", then to pursue filters in 82mm size.

 

By the way, some older ultra-wide angle lenses were designed with built-in UV-cut rear mounted filters.

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There is no problem mounting say a Baader filter inside a lens adapter for a mirrorless system. One might have to remove the filter itself from its retaining ring though: for m43, the lens adapters hold internally mounted filters up to around 43 mm. Any bigger and the ring has to come off, or one needs a rear-mounting filter box into which filters can slide sideways.
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Sorry this is dark, but it is cold and dreary here now, with a tiny bit of snow.

This is using the Nikkor 18mm f/4 which has 86mm x 1mm pitch (coarse) filter threads, and the Nikon D7000.

This comparison has both filters mounted on the front of the lens.

 

This is not a dichroic filter, it is a simple stack, Hoya U-360 2mm thick + Schott BG40 2mm thick, compared to the Baader U.

No adjustments via Photoshop other than re-sized. I left these a little bigger than I normally would so you can compare more detail.

 

The side-by-side image shows the same white balance used for both shots, straight out of the camera, no adjustments.

post-87-0-93575900-1451870351.jpg

 

The second image is the Baader U, CNX2 marquee white balance.

post-87-0-05074100-1451870409.jpg

 

The third shot shows the Hoya U-360 2mm + Schott BG40 2mm stack, CNX2 marquee white balance.

post-87-0-57930700-1451870472.jpg

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Vegetation seems to be paler in the stacked-filter shot, especially the presumably dead vegetation in the foreground. It is difficult to tell if this is due to exposure difference or to IR leakage. I notice that the side of the barn is paler, too. Nevertheless, this is an encouraging first step. I wonder what 403 plus S8612 (0.75mm) would yield.
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Hi Clark, Well maybe on a sunny day next time. ;-)

If B+W 403 is Schott UG1, which I believe it is, then using the same 2mm thickness of UG1 (403) with 0.75mm thick of S8612 you would be raising a 720nm-740nm IR leak into the danger zone, at or above 1E-03.

S8612 1mm thick has the same Red/IR suppression as BG40 2mm thick (compare blue and red lines on left hand graphs, respectively), so those can be interchanged, but dropping the thickness of the S812 is like dropping the thickness of the BG40.

UG1 2mm + BG40 2mm, vs U-360 2mm + BG40 2mm looks the same 'on paper' (calculated in a graph), however the UG1 stack version will have a longer exposure time, but the U-360 stack version has the same exposure time as the Baader U.

This has been consistent with any lens or condition that I have compared the UG1 and U-360 stacks with the Baader U.

This is why I recommend U-360 for UV stacking instead of UG1 (however, I recommend the Schott version when choosing a UG11 or U-340 stack, because the Schott version (of those two 'equivalent' glass types) has superior visual range blocking per thickness, that is not suppressed by any form of stacking).

 

post-87-0-59916300-1451885964.jpg

 

Here is another example of U-360 2mm + BG40 2mm, laminated, using the Kuri 35mm in better weather.

http://www.nikonians.org/forums/dcboard.php?the-white-horse&forum=214&topic_id=6171&mesg_id=6171&page=2

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Of what I have seen so far, it looks like the LUV U/U2 prestacks show the most promise for digital UV at wide angles; the single mount would be expected to minimize ring thickness problems and they are available in a number of sizes. Like most digital UV filters, they are expensive, of course. For film, simpler filters will do, which are more modestly priced.
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That is a UG11 + S8612 stack, and has a slightly longer exposure time than the U-360 + BG40 stack which is consistently the same exposure time as my Baader U shots with my D7000/Kuri 35mm.

The color seems closer to the Baader U also, even before individual WB. So I personally prefer that stack. Any stack is available in any size of course, separate or laminated, as apposed to the Baader, which is why I mentioned it in this topic.

Exposure times example:

Baader U 2s

U-360 2mm + BG40 2mm 2s

UG11 1mm + S8612 1.75mm 2.5s.

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