Color shifts and other issues

[Cadmium]

I keep saying this, because I have come to believe this method is the best for testing for leaks:

First of all, don't use LED's for testing a leak.

Instead use some kind of usual subject or target, be it a flower, house, landscape... illuminated by some UV light source, or preferably sunshine, natural light.

Shoot the first photo with the UV filter, so that the exposure looks optimal.

Note all the settings and exposure for the first photo,

add a longpass filter to the front of the UV filter,

and shoot a second photo using all the same settings, same exposure time.

If you have no leak, the second photo should be black without even a faint ghostly image of anything.

If you have a leak, the image will look mostly black, with some faint to stronger image of some part of the first image.

A leak can then be more defined as to what the leak range might be by using various longpass filters with different cutoff points to isolate where the leak starts or ends even by further stacking various bandpass filters.

No big reason really to get too complicated with all that though, because if your second image is not totally black, then you will need to find a stack that doesn't leak.

A leak can sometimes be blocked by simply using stronger BG/S8612 or by using a different type of BG filter.

However, in your case, you already tried using 4mm(!) of BG suppressing and it didn't remove the leak.

So your stack has a leak problem that will not be mitigated by any amount of BG thickness.

Keep in mind that IF your Chinese BG39 is close to real Schott BG39 then is would have the same Red/IR OD suppression as S8612.

Of course, perhaps your Chinese BG39 isn't really like real Schott BG39/S8612, and so maybe it is not suppressing really at the correct suppression wavelength, but this is neither here nor there,

and neither is the idea that the Chinese U filter is not suppressing all of the visual range red that it should be responsible to suppress.

 

I say all this for the 'gallery', those reading, however many that might be, so they will know the best method for this kind of test, and not to use LED's.

I get so many pictures of LED's! Misleading.

If you do the test like I said, and if you don't have a completely black second image, then get filters that don't leak.

[Andy Perrin]

Yeah...I think I'm out of this discussion after this. Cadmium is right, use known long pass filters to test for leaks. LEDs are good (relatively) narrowband light sources but not helpful for testing lenses or filters. That's best practices right there. Eat green vegetables and use sunscreen. Be on time for appointments, and always, always brush your teeth.

[Stefano]

Today it was a beautiful sunny day, perfect for UV. It seems that I can get some nice colors with my camera/lens/filter.

 

f-stop: f/2.8, ISO 400, 1/8 s exposure.

 

I made this composition, it wasn't there... but still it is nice to see the two main UV false colors.

f-stop: f/2.8, ISO 800, 1/8 s exposure.

 

Using S8612 instead of chinese BG39 should improve results slightly I guess.

[Cadmium]

Stefano, darn good photos, however, see the center of those flowers where they are darker, you see how they are a bit warm, not darker black, not even as dark black as some other areas in the photo?

That is not because of your camera, and it is not because of your white balance, it IS because your filter (stack) leaks ~700nm~ range which warms up those mentioned areas which should look darker.

Also, your 'yellow' area, that should be more yellow, and that could be because your lens is not transmitting low enough.

But next time, if you don't already, stick a piece of PTFE (virgin white Teflon) in the pick to white balance from. Best to do a close up of the PTFE separately so you can optimally expose it separately,

it has to be exposed right, then use that for the white balance...

oh wait, you cant do RAW files... OK, see, you need a different camera to do that then.

Even I see some warmth in the background foliage, not sure? Any warmth you see is always a leak (except the yellow, and your yellow is washed out, meaning to me your lens is not transmitting UV well.).

This warmth is probably a result of the "BG39" you are using not blocking enough red/IR.