Testing my 340 nm and 315 nm bandpass filters. 50/50 success

[Stefano]

I bought a 340BP10 and a 300BP10 bandpass filters on eBay from here. The seller measured the 300 nm filter and it actually peaked at 315 nm. Usual equipment, full-spectrum Canon EOS M and Soligor 35 mm f/3.5.

 

The filters are tiny, so the only way to mount them was on the rear of the lens. I didn't use putty, I instead built a "holder" around them and they press fit my Soligor perfectly.

 

 

 

 

 

No residues, quick mounting/removal, perfect.

 

I first tried the 340 nm filter. I first put my usual UV-pass filter stack on the lens, ZWB2 (2 mm) + Chinese BG39 (2 mm), and quickly realized the "BG39" attenuates strongly at 340 nm, so I removed it. The ZWB2 filter was almost completely transparent, and I left it on the lens just in case. It looks like the filter has a good-enough blocking to be used alone, and that surprised me a bit.

 

I had a UV in-camera white balance, thus the strong green cast typical below 350 nm.

 

f/(?(either 4 or 8)), ISO 100, 60.1 s exposure

 

f/(?(either 4 or 8)), ISO 12800, 1/8 s exposure

 

Glass begins to darken.

 

I then just took monochrome images, as there isn't much color information anyway.

 

f/(4?), ISO 100, 30 s exposure. Brightness lifted in PN, and then image converted to B&W using the red channel only (raw). Resting on the glass is my ZWB2 filter, on the left is the dark Chinese BG39.

 

My hand. f/(4?), ISO 100, 30 s exposure

 

I then tried with my 315 nm bandpass filter, and I didn't get what I expected. I used it with the ZWB2 filter (it should transmit some there, as it should behave similarly to U-360/UG1, see graph below), and I also used it alone. My Soligor should transmit some there too: https://www.ultravio...ligor-35mm-f35/

 

UG1 and U-360 transmission (2 mm):

 

What I got was this:

 

f/(4?), ISO 3200, 1/2 s exposure.

 

Everything is soft. I can show other examples, but they are all like this.

Further testing showed a significant IR leak. In fact, the image seems to be mostly if not all IR. The next image was taken with a R72 on the lens (and the filter on the rear):

 

f/(4?), ISO 12800, 1/8 s exposure.

 

I don't know what causes this. Is it the filter? Or the lens? Or even the camera? If my camera allows it, I should see something at 315 nm with this filter and the Soligor. But even the infrared leak image is soft. I did clean both filters with a window cleaning spray and a "silicone glaze" as they were a bit dirty. Is it possible that some of it went inside?

[Andrea B.]

It is possible perhaps that you have a light leak around a port on the camera or around an LCD??? I've seen blotchy areas somewhat like that with my D610 upper LCD light leak. Put a lens cap on the lens and make some long exposures. (This is a test we all should do to check for light leaks.) If you find a blotchy area in the capped exposures, then try to cover the camera with a dark towel (or similar) and test again to see if it goes away.

 

If you have already tested for light leaks, then kindly ignore that suggestion.

 

Not sure what is your silicone glaze, but silicone stuff is very slippery. So I suppose it is possible something got inside. But I don't think it would look quite like that?

 

 

*****

 

The green photo -- what is the raw, no white balance color please?? Thanks.

 

 

*****

[Stefano]

The green is the color with a "normal" UV white balance. The raw color is orange. I may show the raw color later.

 

The second green photo has a leak at the bottom that I fixed. I will try the lens cap test.

[Andy Perrin]

Yeah, my money is on a camera light leak also. I have that with my old NEX-7 camera. It is very annoying, and they can be hard to track down. Foil around the camera to block parts of it is one way to narrow the search.

 

Stefano, for the indoor photos what were the light source(s)?

[Stefano]

The only indoor photos are the lens photos at the beginning taken with my phone and natural sunlight + a white torch. All UV photos were taken outside with sunlight.

[Fandyus]

How much did those cost you?

[Stefano]

I had a broken link, fixed. From the seller in this topic: https://www.ultravioletphotography.com/content/index.php/topic/4644-relatively-cheap-uv-bandpass-filters

 

They were 16.74 € + 5.85 € of shipping cost, each.

 

They are very small, a bigger filter would be much more useful, but they work very well (at least, the 340 nm one as it appears. But I still have to find the problem with the other one).

[Andy Perrin]

Stefano, aside from light leaks in the camera, your holders may be leaking.

[Stefano]

Which holder? The little filter?

[Andy Perrin]

Yes the handmade one.

[Stefano]

The leak in the 340 nm image I talked about in post #3 was due to that, a hole in the tape. I covered it and it disappeared

[Andy Perrin]

Remember you are forcing the 315nm much harder (lower sensitivity from camera and less transmission from filter) so it needs to be blocked better. It’s gonna find ALL the leaks. Even more than the 340nm did.

[Stefano]

If you multiply the ISO and exposure values from my 340 nm and 315 nm photos you will find that the latter had about half the "exposure" as the former (there is also the lens aperture to consider, but I think it was the same). Any leak that is not dependent by the filter itself should show up half as much.

[Stefano]

Another example at 340 nm, with the ZWB2 front mounted.

 

f/8, ISO 400, 600.2 s exposure (converted to B&W in Photo Ninja):

 

Raw colors (.CR2 file rendered as .jpg in Photo Ninja without any color correction):

 

If a typical UV white balance is applied, the color is a strong green as seen before.

 

In the background one can see some houses with a strong haze. To the naked eye, there was no haze. Rayleigh scattering is strong at 340 nm.

 

This photo was taken in the evening, and this partly explains the long exposure time (10 minutes). This filter is narrow and has a low peak transmission, plus it is in the deep UVA range, so long exposures are required.

[Stefano]

Big news! I "mounted" the 315 nm filter in front of a white LED torch, and it leaks significant amounts of blue (a pretty blue too):

 

Not only that, but I can see the LED of the torch through the filter this way, and it really resembles frosted glass. So this filter leaks blue, infrared and is frosted. The blue leak can be fixed with ZWB2 or ZWB1 at least, but the infrared leak and the softness are not that easy to fix.

[Fandyus]

I had a broken link, fixed. From the seller in this topic: https://www.ultravio...andpass-filters

 

They were 16.74 € + 5.85 € of shipping cost, each.

 

They are very small, a bigger filter would be much more useful, but they work very well (at least, the 340 nm one as it appears. But I still have to find the problem with the other one).

That's nice, I almost wish I could have gotten them but I'm sure they'll serve you better than me. I don't have a mirrorless and for that much money I should get an S8612 so that I have a non-leaky UV solution.

[Stefano]

Both the mirrored surface and the black surface are clean (not perfectly clean, but mostly, maybe even cleaner than the 340 nm filter), and so it seems that the "cloudiness" comes from inside. I don't know how these filters are built, but do they have two layers with a gap in between? is it a reasonable idea to try to open it and if so clean it?

[Stefano]

That's nice, I almost wish I could have gotten them but I'm sure they'll serve you better than me. I don't have a mirrorless and for that much money I should get an S8612 so that I have a non-leaky UV solution.

The seller has another 340 nm filter, like mine. The problem is that you need to rear mount it because it is tiny, and with a DSLR that's a problem.

 

I too should buy a S8612 one day.

[Fandyus]

The seller has another 340 nm filter, like mine. The problem is that you need to rear mount it because it is tiny, and with a DSLR that's a problem.

 

I too should buy a S8612 one day.

After examining my Industar 50-2, it seems like I might be able to rear-mount filters afterall, there's a good chunk of space between the furthermost part of the mount and the rear element at infinity, plus there's probably some more space inside the camera, as this is an APS-C DSLR made by canon and those have shorter flange distances due to the mirror being smaller I believe.

[Fandyus]

Also, I edited one of the pictures to illustrate how much darker the glass has become. Not to invade your work, I couldn't resist when I saw how much the blacks were lifted, so I normalized the image a bit.

[dabateman]

Stefano,

Do you have 4mm zwb1? You could try that to try and block the visible and IR leak.

I also would bet that your camera might not be sensitive at all below 335nm or 330nm. It seems rare to have deep uv sensitivity.

[Stefano]

Also, I edited one of the pictures to illustrate how much darker the glass has become. Not to invade your work, I couldn't resist when I saw how much the blacks were lifted, so I normalized the image a bit.

I think the blacks were lifted because the filter has some "glare", although not nearly as bad as the other.

[Fandyus]

I think the blacks were lifted because the filter has some "glare", although not nearly as bad as the other.

Optical problems are expected with solutions like this, but glare is not a big issue. I hope the filter serves well. You should probably look into UV trichromes now that you have this. Maybe use 395nm light, 365nm light and a fullspectrum flash + the 340nm bandpass, should work nicely.

Edit: to add, I've actually seen some sellers on eBay who sell wavelengths in-between 395nm and 365nm, I don't know how to hook up LEDs but someone here should really try a 395 - red, ~380 - green and 365 - blue trichrome. Probably wouldn't be as spicy as the ones Bernard did but it could still be interesting.

[Stefano]

Stefano,

Do you have 4mm zwb1? You could try that to try and block the visible and IR leak.

I also would bet that your camera might not be sensitive at all below 335nm or 330nm. It seems rare to have deep uv sensitivity.

I have a 3 mm thick ZWB1 filter. I tried looking at a halogen bulb with the camera, the 315 nm filter rear mounted and the ZWB1 held in front of the lens, it initially seems to suppress everything but the leak is still visible in photos with reasonably long exposure times (I think I tried 2 seconds at ISO 25600). The color of the leak was red with an in-camera "sunny" white balance.

 

Even if I fixed the leaks, I still have the softness issue. This filter produces images like it had mist on it. I'm not sure if this can be fixed.

[Stefano]

 

Optical problems are expected with solutions like this, but glare is not a big issue. I hope the filter serves well. You should probably look into UV trichromes now that you have this. Maybe use 395nm light, 365nm light and a fullspectrum flash + the 340nm bandpass, should work nicely.

Edit: to add, I've actually seen some sellers on eBay who sell wavelengths in-between 395nm and 365nm, I don't know how to hook up LEDs but someone here should really try a 395 - red, ~380 - green and 365 - blue trichrome. Probably wouldn't be as spicy as the ones Bernard did but it could still be interesting.

Trichromes/TriColours are definitely something I'm interested in. I already had the idea of using 385, 365 and 340 nm as I have LEDs there (I still have to mount the 385 nm LED, as well as the 340 nm one, and also I would like to have four of those 340 nm LEDs one day, mounted together, but that's future ideas). I like the wavelengths Bernard used (380, 345, 323 nm), I think I would push the long wavelengths at 385 nm.

 

Also, bigger filters would be better. Thorlabs sells a 1" (25 mm) diameter 340 nm bandpass filter, 10 nm FWHM (just like mine apparently) for 132.79 €: https://www.thorlabs.com/thorproduct.cfm?partnumber=FB340-10

 

This is not extremely expensive, but I would spend that money on something else. It's still not cheap for me.