IR leakage?

[ulf]

I agree that real world testing beats theorising, but then you have to have access to the suspected filter.

If the offending filter then is properly measured you will know the real reason for the suspected flaw.

That is much better than just rely on manufacturer's theoretical data.

 

Some years ago I found that in some situations, even my Baader U, with a rather high OD, showed some hints of leakage in combination with a good lens on my EOS 60D.

When comparing with identical images from a stack of U-360 and S8612, both 2mm, the leakage was eliminated.

That incident has made me suspicious about most dichroic filters.

 

When measuring and comparing the OD of the Baader U and the U-360 stack I could really confirm that there was a difference that I had seen in my images too.

 

---

I offered to measure Nemo Andrea's two Kolari filter, the badly flawed he got first and the replacement filter.

I would then naturally had paid for the insured return shipment.

 

Unfortunately he never came back letting me do that.

 

I am not prepared to buy a Kolari U just for measuring it.

[montanawildlives]

Thanks everyone for your responses. I think my example was too flawed in too many ways to nail anything down, starting with the essentially random white balance.

 

I've returned the Kolarivision filter (glad I bought it through Adorama). I am pretty sure it had significant IR leakage, primarily because the hotspot I was getting when I stopped down was very similar in shape and impact to what I got with the same lens and just an IR pass filter (720 nm) on the full spectrum camera. Not definitive, but consistent with the IR leakage hypothesis.

 

That said, I am so inexperienced I would not want this to be another nail in the coffin of Kolarivision's filters...it is VERY possible that I am just clueless.

 

I'm on to my next lens problem...see next thread ;-)

[Andrea B.]

Montana, I do not think you were clueless. There was something not right about your Kolari UV-pass filter. So I am happy you have returned it to get your money back and then to try again with something else.

 

*****

 

Also, seeing green in a white balanced UV image (unless you are below like 350 nm) is an alarm bell.

 

I need to mention here that seeing green in a white balanced UV image made under a typical broadband UV-pass filter is not at all unusual in a landscape photo at medium distance (20-40 feet). We tend not to notice green in a UV landscape photo because generally the false colors in a UV landscape photo are not very bright and not very saturated.

 

I'm going to go dig out an example or two. (Done.)

 

This is a link to some lens testing I did in 2014. Within this topic are some examples for which I super-saturated the UV photo to bring out certain problems. Some of those examples also show that there can be a lot of green in a UV photo.

https://www.ultravio...finity-visiruv/

 

I'm going to bring forward just one example from that topic here for all to see. Then if anyone wants to see more, they can hit the link. (Done.)

 

Pentax K5-full spectrum + Asahi 35mm f/4.0 Lens

 

Visible Reference

Sky, trees, grass in a park.

 

 

Ultraviolet with Baader UV-Pass Filter

The usual conversion shows the usual dull appearence.

 

 

The same UV photo with increased saturation.

And bam! There's the expected false yellow (sky) and false blue (grass). AND --- there is also some false green in the tree foliage. All this was lurking in the preceding photo.

So the point I want to make is that the false green can often appear in a UV photo without pushing the saturation depending on the scene being photographed and the amount of light and so forth. I usually do not regularly see false green in close-up flower work, but I have on occasion seen some dark false green in stems or foliage in close up floral work.

Finally we have to note that some of the green in the saturated photo is actually dark yellow. (Remember the recent discussions of the Abney Effect? Some colors when darkened also confuse the brain into thinking the hue has changed. Yellow is one of them. Dark yellow begins to look olive green.]

Side note: Pushing the saturation helps detect dichroic discoloration outside the main center portion of the photo. You can see some a lot of that here also. The Asahi 34/4.0 + BaaderU is not a good combo.

 

Now go look at that link and see how the false colours showed up in other lens tests. The lens also plays a big role in what and how much false colour you can get.

[nfoto]

I agree that pushing saturation might reveal other issues. The green tinged with yellow is something one can coax, or rather force, out of the shadows in many UV captures, given rough post processing. However, what initiated the present discussion was a photo plainly with "wrong" appearance, in all likelihood caused by using a lens with poor UV transmission, a filter leaking IR, and unsuitable "UV white" balance. As I said earlier, I obtained similar nearly 20 years ago for the same reasons. Occam's Razor prevails.

[Andrea B.]

Yes, definitely.

 

I was just responding to another comment in the topic and wanted to point out that false green is not at all unusual in a landscape photo. Been getting that for years.