Playing with some R25 filters

[ben_hutcherson]

Not too long ago, I bought an "Ebay special" R25 in 67mm since I couldn't find one in the used drawers at the local shop and figured for the $5 it cost it was worth a try.

I have ready access to a UV-Visible spectrophotometer, so thought I'd compare it side by side with a few others. I had a Tiffen and then an ancient Kowa branded bay 1 handy. Both showed a sharp cut-off at 600nm with effectively only some noise visible below that.

The Ebay special, on the other hand, had the sharp 600nm cut off but then had a wide band of translucency at around 350-450nm. So, it looks like I'm going to stick with a 67-72mm step-up.

Just for fun, I thought I'd check my Hoya R72 also, and just as expected it goes sharply at 720nm.

So, here are the spectra. These are absorbance spectra and not the more commonly published percent transmittance spectra. Absorbance is the log of the transmittance, which is why it's used(%T can not be quantitated directly). The higher the absorbance, the more it absorbs. Although not labeled, the R72 of course stands out, and the Ebay special is the one with the large dip in absorbance.

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[ben_hutcherson]

By the way, I have long argued that a strict UV filter is unnecessary because optical glass(depending on its exact composition) goes opaque somewhere in the 350-400nm range. In the lab, our UV-Visible spectrophotometers use quartz optics. When we want to measure a sample in the UV region of the spectrum, we have to use quartz cuvettes(sample holders) or plastic specified to be translucent over the desired wavelength range.

So, out of curiosity, I also dropped in a lens-this is nothing special, just a 50mm f/1.8 AF Nikkor. The signal attenuation over the visible region that you see is a result of the light being out of focus after passing through the lens. In any case, you can see that the cut off is nice at 400nm. It just goes to show me what I've been saying for years-a UV filter is nothing but a lens cap that you can shoot through(yes, I use them :) )

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[Bob Salomon]

By the way, I have long argued that a strict UV filter is unnecessary because optical glass(depending on its exact composition) goes opaque somewhere in the 350-400nm range. In the lab, our UV-Visible spectrophotometers use quartz optics. When we want to measure a sample in the UV region of the spectrum, we have to use quartz cuvettes(sample holders) or plastic specified to be translucent over the desired wavelength range.

So, out of curiosity, I also dropped in a lens-this is nothing special, just a 50mm f/1.8 AF Nikkor. The signal attenuation over the visible region that you see is a result of the light being out of focus after passing through the lens. In any case, you can see that the cut off is nice at 400nm. It just goes to show me what I've been saying for years-a UV filter is nothing but a lens cap that you can shoot through(yes, I use them :) )

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Except for that blue haze in color scenes.

[Martin Aislabie]

Isn't there a clue in the name - UV filter :D

It does what is says on the label ?

Martin

[ben_hutcherson]

I need to get my results together and published.

In any case, I gathered up a bunch of filters(specifically colored filters but also stuck in a few different "UV-class" filters today. These included UV, Haze, and Skylight filters. I found as much variance by manufacture as anything-for example an ancient 52mm Vivitar Haze had a beautiful sharp cut off at 400nm. The skylight, unsurprisingly, gave an interesting spectrum although it cut off gradually from around 410nm to 360nm. I checked a few different UV filters, and found most of their cut-offs to be in the range 360nm-in other words the same as a typical glass lens. I'll try to pilfer the card catalog at the camera store the next time I'm there and dig out a few name brand(Tiffen, Hoya, B&W) UV filters. It can be a bit of a daunting task when they have probably over a 1000 in the 52-58mm range with no rhyme or reason to organization other than their size(I need to dig up a few more colors also as well as some warming filters).

In any case, this is simple and easy enough for me to do that I'm going to start checking all my colored filters. It's easy to see if they truly are color filters or are "notch" filters like the most warming/cooling filters and the cheap 25A that prompted all of this. Also, it's a quick and dirty way to see if NDs and polarizers are truly neutral.

[Bob Salomon]

I need to get my results together and published.

In any case, I gathered up a bunch of filters(specifically colored filters but also stuck in a few different "UV-class" filters today. These included UV, Haze, and Skylight filters. I found as much variance by manufacture as anything-for example an ancient 52mm Vivitar Haze had a beautiful sharp cut off at 400nm. The skylight, unsurprisingly, gave an interesting spectrum although it cut off gradually from around 410nm to 360nm. I checked a few different UV filters, and found most of their cut-offs to be in the range 360nm-in other words the same as a typical glass lens. I'll try to pilfer the card catalog at the camera store the next time I'm there and dig out a few name brand(Tiffen, Hoya, B&W) UV filters. It can be a bit of a daunting task when they have probably over a 1000 in the 52-58mm range with no rhyme or reason to organization other than their size(I need to dig up a few more colors also as well as some warming filters).

In any case, this is simple and easy enough for me to do that I'm going to start checking all my colored filters. It's easy to see if they truly are color filters or are "notch" filters like the most warming/cooling filters and the cheap 25A that prompted all of this. Also, it's a quick and dirty way to see if NDs and polarizers are truly neutral.
You really have to get your terminology straight.
A UV filter is used to cut haze, it is slightly yellowish unless you use stronger ones like those meant for aerial work. The regular UV filter is used for both B&W and color.
A haze filter is slightly salmon colored and is a warming filter,or skylight filter for color work. It will cut some UV but not as much as a UV filter. Stronger colored haze filters are used for heavy overcast, snow scenes and shots in color at altitude to cut the blue in the shadows.

Most common filters sold for UV and skylight filters commonly sold in the USA are usually marked in Wratten numbers or with unique numbering. Heliopan, B+W, Rollei, Linhof, Leica are usually marked in the decamired system. Decamired filters are additive so if you put two KR 1.5 skylight filters together you make a KR 3 skylight filter. KR decamired filters are warming filters and KB series are warming filters.
If you put two 81A filters together you do not make an 81B.

There are charts published by various sources that gave the mired value of various light sources. By consulting the chart you could easily determine which value decamired filter would color correct for various Kelvin temperature films. Or you could simply consult the graphs that Heliopan and B+W used to publish in their brochures.

It is also interesting that virtually no camera or lens instruction books state that a UV filter is of no benefit. In fact, most camera and lens manufacturers sell UV filters and recommend their use. Although personally, for my digital work, with all lenses longer then a 25mm I use a Heliopan Digital filter which blocks all UV and IR and gives cleaner colors, better blacks and greater color separation between colors as well as sharper results compared to the same shot without that filter. But it will not work with extreme wide angle lenses. Nor does it do anything with film.

[Pere Casals]

Also, it's a quick and dirty way to see if NDs and polarizers are truly neutral.

This depends mostly on price.

Here you have a complete review about POL http://www.lenstip.com/139.25-article-Polarizing_filters_test_2015_Results_and_summary.html

Congratulations because having the push to use the spectrometer, this is the way to learn. I use an Ocean Optics USB 2000.

I'd add that an UV filter may block in part some of the violet light, having an effect in color film when because altitude there is also more visible Violet/blue than the film is equilibrated for.

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