IR filtration in lenses?

[Pinphot]

A bit of blue sky thinking here.
Assuming that the severe IR sensitity issues are going to remain structural to the M8 and Leica is commited to digital imaging long term, are there any technical reasons why a IR blocking layer could not be coated onto an internal lens element in new lenses? I'm not an optical scientist, but from what I have read about the science of IR coatings, it appears that a coating would have absolutely no adverse effect on lens speed or optical quality, and if anything would actually improve sharpness by cutting out IR which comes to focus at a slightly different focus point to visible image forming radiation. Also, because there is no new glass interface, the addition of flare/ghosting would not be an issue. Am i missing something obvious here?
Mark

 

[StuartR]

Well, nothing more obvious than the fact that it would prevent people from shooting IR photography with Leica lenses... Beyond that, existing lenses would have to be disassembled and recoated, which I think would be cost prohibitive, not to mention extremely labor intensive for Leica. Think how long it has taken them to just recode lenses...all that is taking a mount off and switching it out. To add the IR coating they would need to disassemble the whole lens, remove the element, coat it, replace it, put it back together and then test to make sure everything is still performing perfectly.

But anyway, if you are only talking about new lenses, I figure it is probably possible, but probably not optimal. I think the cyan vignetting would be a liability on film bodies where it would not be able to be fixed. If you are shooting slides with a 24mm lens with IR blocking, you would have slides that had permanent cyan fringing in the corners. I don't think Leica would like that to be a hallmark of their lenses.

 

[Pinphot]

I was only thinking of new lenses. If the coating was somewhere near the centre of the the optical axis, would the light path not be fairly straight thus minimizing or eliminating the cyan cast due to image forming light coming from overtly oblique angles which is where I assume the problem comes from? Again, I emphasize that I am not an optical scientist. Regarding those that wanted to do IR photography, perhaps integral IR filtering could be an option rather than an imposition.

 

[visiondr]

It seems to me that Pinphot is correct here wrt the angle of incidence of the light rays on the coating. If they are at or near parallel to the optical axis, there should be no cyan drift.

 

[Pinphot]

I look forward to Gore Vidal's opinion on this issue, and if Chomsky wished to express an opinion, I would be interested in hearing that too😀

 

[Gary B]

IR filter

IR filter

I was out at a local restaurant last night and took a few photos of my friend sitting across the table with my M8, v. 1.092, 35 Cron with the just recieved IR filter. She had on a very dark blue sweater. I was shooting at f2 under a large tung. light and got the terrible mangents. I guess that the problems are not solved as of yet.
Hope that something is done with the next firmware version.

Gary B

 

[jaapv]

I was out at a local restaurant last night and took a few photos of my friend sitting across the table with my M8, v. 1.092, 35 Cron with the just recieved IR filter. She had on a very dark blue sweater. I was shooting at f2 under a large tung. light and got the terrible mangents. I guess that the problems are not solved as of yet.
Hope that something is done with the next firmware version.

Gary B

If it is extreme, a bit of IR will leak through any filter, however, in that case it is not that hard to correct using appropriate profiles.What you describe is a situation where a DSLR with full IR filter on the sensor would have shifted towards magenta as well.

 

[StuartR]

Can you post the picture? Because I think people have been pretty much unanimous about the filters solving the IR problem. I know that I have not had problem with magenta blacks since I started using the filters...I have thousands of images since then. This is not to say that it did not happen, I am just suspicious that something else might be at play here.

 

[jaapv]

But anyway, if you are only talking about new lenses, I figure it is probably possible, but probably not optimal. I think the cyan vignetting would be a liability on film bodies where it would not be able to be fixed. If you are shooting slides with a 24mm lens with IR blocking, you would have slides that had permanent cyan fringing in the corners. I don't think Leica would like that to be a hallmark of their lenses.

This is incorrect, Stuart. I used a Tegea 9.8 mm lens with the IR filter between the lens elementsl and it exhibited no cyan vignetting at all. So it seems to me that this solution is indeed optimal.

 

[StuartR]

I am very glad to be wrong Jaap! But I do not want to spend millions to replace all my Leica lenses...hmmm.

 

[jaapv]

I am very glad to be wrong Jaap! But I do not want to spend millions to replace all my Leica lenses...hmmm.

I thought you were going to spend those millions on extending your lens collection, Stuart 😉 😛

 

[StuartR]

Right you are...how silly of me to forget.

 

[Ben Z]

I wonder putting an IR coating somewhere on one of the internal surfaces is a simple matter, or if they would need to recompute the optical formulae as a result. If the former, then it could be possible to have it done to existing lenses, if the latter then that'd be a lot of R&D and perhaps some retooling and only applicable to new lenses.

 

[jaapv]

I wonder putting an IR coating somewhere on one of the internal surfaces is a simple matter, or if they would need to recompute the optical formulae as a result. If the former, then it could be possible to have it done to existing lenses, if the latter then that'd be a lot of R&D and perhaps some retooling and only applicable to new lenses.

I doubt that to be the case Ben, the lens I mentioned had a slot taking the filter and having it in or out only makes a difference in the filtration. Otoh for instance the R 280/4.0 APO may not be used without a filter being present.

 

[Ben Z]

My 400/6.8 has a filter drawer in the rear tube also. A Series 7 (or a 49mm with the threads ground off) fits, which is probably how I would place an IR filter. But I recall reading how the optical designers at Leica figure the thickness and number of anti-reflective coatings into the optical computations of the lenses, so just wondered about the effect of adding an IR coating to one of the elements. Perhaps since it doesn't affect visible light it wouldn't be an issue. Way out of my knowledge base!

 

[Xmas]

If you use a dimmer on a tungsten filement light bulb then the relative intensity (%) of the near IR will increase dramatically... You need to over run bulbs to minimise the IR %, shots in dim singles bars may be at risk.
An internal IR filter should work ok.
Leicia could develop a inter element material like 'adsorban' but for IR, adsorban absorbs uV, but dont expect it to be popular with the IR photo crowd.
Noel

 

[visiondr]

Screw the IR photo crowd, I say!😀 🙄

 

[Gary B]

IR filter

IR filter

Trying to post attached picture for StuartR.

I did just download the new firmware v. 1.102 so I'll try it later today.

Gary B

 

[Xmas]

Some people like the M8's IR response... for B&W IR shots, stick a red filter on it...

Noel

 

[grduprey]

What happens to the photo if you correct the WB? Does it also correct the Magenta cast? I would guess it would effect it some and possibly reduce the effect. I have seen no magenta since using the filters, even under similar conditions as you show here.