What Ultraviolet filter??

[Vincenzo101]

Hi,
on eBay there is a seller that sells UV only pass filters, one made of:

"optically bonded UG11 + BG40 glass"

and one other, made of

"UG11 + BG39 glass"

He says that "Combination with bg39 has lower IR transmission while combinarion with BG40 more of it along with more transmission in UV"

What UV ONLY PASS fiter should you choose? Thanks.

[Bjørn Rørslett]

It is very difficult to get a true UV bandpass filter without it having a "side lobe" transmitting in the IR range. So what one wishes for is sufficient IR suppression if the objective is pure UV photography. Preferable IR transmittance should be on the order of 10^-3 or lower. The best filters do 10^-4 or better. In practice one then can do, for example, studio UV photography using studio flash with uncoated Xenon tubes to give maximum UV output yet be ensured the IR coming through the filter is negligible (my newest flash unit is 1600 W/s and one really has to shield the eyes from its harmful UV delivery).

The above is a preamble to my recommendation of selecting the filter for which the UV/IR ratio is maximal. The transmittance itself of UV is not super critical as a lower value can be circumvented by increasing exposure time (or flash output) (within limits of course), as long as IR is suppressed to a sufficient degree.

If you are not familiar with www.ultravioletphotography.com do visit that site for loads of information on filters and all thing UV.

[charlie]

What UV ONLY PASS filter should you choose? Thanks.

The Baader U-Venus filter is an excellent UV pass filter.

http://www.baader-planetarium.com/en/filters/baader-u-filter-(venus-and-uv---350nm).html

[Bjørn Rørslett]

The Baader U-Venus filter is an excellent UV pass filter.

http://www.baader-planetarium.com/en/filters/baader-u-filter-(venus-and-uv---350nm).html

Of course. They have been my work horses for ages. But quite expensive if you need more than one. Plus they don't work well on wide-angle lenses due to the dichroic design. So having alternatives comes in handy ever so often. Or using rear filtration which can be pretty tricky to implement.

[Akira]

BG40 is all-to-easily deteriorated by the environmental moisture.  So, optically bonding UG11 and BG40 won't last long.

This should be the best available non-dichroic UV-pass filter so far:

http://www.uvroptics.com/index.php?AndreaU