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VIS-IR Corrected Lenses
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PostPosted: Mon Feb 06, 2023 6:28 am    Post subject: VIS-IR Corrected Lenses Reply with quote
Hi all!

I'm chasing some information on lenses that are able to image in VIS-IR without any focus shift. Ideally I'd be looking at options from around 100mm focal length, and corrected for work around 400-1000nm. Ideally they'd cover a full frame sensor, but could get away with an APS-C image circle.

So far I've seen suggestions that the following may work, but nothing definitive!

- Zeiss Superachromats [250mm f/5.6, 300mm f/2.8]
- Leica APO R 280mm f/2.8
- Leica Telyt-R APO 280mm f/4
- Nikkor Ai ED 300mm f/4.5
- Leica APO-Telyt-R 180mm f/3.4
- Canon FL 300mm f/5.6 Fluorite
- Pentax Ultra Achromatic Takumar 300mm f/5.6

Cheers!
Marcus


PostPosted: Mon Feb 06, 2023 8:40 am    Post subject: Reply with quote
That should be correct Marcus. May I ask what you are trying to achieve?


PostPosted: Mon Feb 06, 2023 9:39 am    Post subject: Reply with quote
While not truly ultra-apochromatic I find many normal lenses are close enough to allow full spectrum shooting without noticeable focus issues.
I have heard everyday apochromatic lenses are often more affected here, the steps taken to get three wavelengths to focus together (and having no concern for wavelengths beyond the visible) causes a S-shaped wavelength-focus distance plot which can have very step tails.
Here are links to the 10 full spectrum shots from my flickr stream with assorted everyday lenses:
https://flic.kr/p/mptiuk
https://flic.kr/p/nJKBXN
https://flic.kr/p/rFuQaP
https://flic.kr/p/oUwRbU
https://flic.kr/p/r6davC
https://flic.kr/p/rBRRgn
https://flic.kr/p/tEwgp4
https://flic.kr/p/yMPhJY
https://flic.kr/p/yMPhn5
https://flic.kr/p/2kjhN19

When I've seen issues with softness it's generally been shooting with filters that restrict the image to IR & a small portion of the visible, but even with these focus is generally acceptable. It's possible the more restricted wavelengths encourage me to open the aperture compared to full spectrum shooting making focus shift more noticeable. Where EXIF is available these full spectrum shots have been between f4 & f7.1 probably mainly wide open on zooms so it seems subconscious stopping down doesn't seem to be the case!


PostPosted: Mon Feb 06, 2023 12:14 pm    Post subject: Reply with quote
kds315* wrote:
That should be correct Marcus. May I ask what you are trying to achieve?


Thanks for the reply, I'm looking at imaging in IR to add extra glow to hot objects (eg. car brakes in motorsport). I've got a modded full spectrum camera on the way which I'll be testing with my existing lenses to see how they go and how sensitive the sensor is to NIR light. Depending on that I was going to look for some better corrected optics for improved IQ!


DConvert wrote:
While not truly ultra-apochromatic I find many normal lenses are close enough to allow full spectrum shooting without noticeable focus issues.
I have heard everyday apochromatic lenses are often more affected here, the steps taken to get three wavelengths to focus together (and having no concern for wavelengths beyond the visible) causes a S-shaped wavelength-focus distance plot which can have very step tails.
Here are links to the 10 full spectrum shots from my flickr stream with assorted everyday lenses:
...
When I've seen issues with softness it's generally been shooting with filters that restrict the image to IR & a small portion of the visible, but even with these focus is generally acceptable. It's possible the more restricted wavelengths encourage me to open the aperture compared to full spectrum shooting making focus shift more noticeable. Where EXIF is available these full spectrum shots have been between f4 & f7.1 probably mainly wide open on zooms so it seems subconscious stopping down doesn't seem to be the case!


Thanks for those, nice shots! I'll have a bit of a play with the lenses I've already got and see how they go. My worry is that I'll mostly be shooting in darker environments where the NIR will be very bright, so any slight focus shift will be magnified significantly, hence my backup plan is some better corrected optics if needs be.


PostPosted: Mon Feb 06, 2023 1:48 pm    Post subject: Reply with quote
MG42 wrote:
kds315* wrote:
That should be correct Marcus. May I ask what you are trying to achieve?


Thanks for the reply, I'm looking at imaging in IR to add extra glow to hot objects (eg. car brakes in motorsport). I've got a modded full spectrum camera on the way which I'll be testing with my existing lenses to see how they go and how sensitive the sensor is to NIR light. Depending on that I was going to look for some better corrected optics for improved IQ!


DConvert wrote:
While not truly ultra-apochromatic I find many normal lenses are close enough to allow full spectrum shooting without noticeable focus issues.
I have heard everyday apochromatic lenses are often more affected here, the steps taken to get three wavelengths to focus together (and having no concern for wavelengths beyond the visible) causes a S-shaped wavelength-focus distance plot which can have very step tails.
Here are links to the 10 full spectrum shots from my flickr stream with assorted everyday lenses:
...
When I've seen issues with softness it's generally been shooting with filters that restrict the image to IR & a small portion of the visible, but even with these focus is generally acceptable. It's possible the more restricted wavelengths encourage me to open the aperture compared to full spectrum shooting making focus shift more noticeable. Where EXIF is available these full spectrum shots have been between f4 & f7.1 probably mainly wide open on zooms so it seems subconscious stopping down doesn't seem to be the case!


Thanks for those, nice shots! I'll have a bit of a play with the lenses I've already got and see how they go. My worry is that I'll mostly be shooting in darker environments where the NIR will be very bright, so any slight focus shift will be magnified significantly, hence my backup plan is some better corrected optics if needs be.


The ratio of IR to visible may indeed change how sharp the image looks, as might specific wavelengths of light present... Unfortunately the variables are too complex for me to investigate.
I do have one other shot that might be relevant if you're interested in hot objects in dim light https://flic.kr/p/qUd3F7 showing an electric hob element cooled blow the point of visibly glowing & having both visual only & full spectrum areas.

Working in motorsports I've taken quite a few panning images of cars going round a hairpin, some of these have been via a #25 (~590nm) filter. I don't think the brake discs here look much brighter than they do in pure visual shots in the same race, but perhaps on a more consistently twisty section of track this might differ.
https://flic.kr/p/mzKCSP
https://flic.kr/p/mDciGn

As to relative sensitivity to IR, I typically find a full spectrum shot outdoors in the UK has about half the shutter speed of a visual only one with the remaining conditions the same. The effect of shooting full spectrum is somewhat unpredictable but there are a few changes that are very common, foliage will be considerable brighter, and many manmade dyes will change. The team uniforms in the full spectrum grid shot I linked to earlier are black & orange to the eye, the black going brown to the camera.

Black fabrics seem to be particularly prone to changing in IR, I've had some stay black, some become fairly pale & others taking on a lilac hue.

I must admit I'd love to try out some of the ultra apochromats Klaus listed, but I've never seen one that isn't way more than 10x my available budget!


PostPosted: Mon Feb 06, 2023 3:06 pm    Post subject: Reply with quote
MG42 wrote:
kds315* wrote:
That should be correct Marcus. May I ask what you are trying to achieve?


Thanks for the reply, I'm looking at imaging in IR to add extra glow to hot objects (eg. car brakes in motorsport). I've got a modded full spectrum camera on the way which I'll be testing with my existing lenses to see how they go and how sensitive the sensor is to NIR light. Depending on that I was going to look for some better corrected optics for improved IQ!


You would most likely need a thermal camera which works at much longer wavelength than visibleIR if you are into recording such things as "hot brakes" - to record those they would need to be really hot ("weissglühend") to record them well....


PostPosted: Mon Feb 06, 2023 8:54 pm    Post subject: Reply with quote
I wouldn't be so sure about that...
Brakes in professional racing reach routinely 400-500C and with heavy braking get up to 700 or more. Steel starts to glow red at 700 C, but only a very tiny fraction of the emitted energy is in the visible spectrum! Let's pull out Planck's law and an online spectral calculator:

For a black body the amount of radiance between 400 nm and 700 nm at 1000k equals about the amount between 400 and 1000 at 750 k.
So assuming that everything else stays the same you would see the brakes glow a good 250 degrees earlier!

This is obviously a very rough calculation, brakes aren't perfect emitters, it ignores that all the surrounding things also emit in these wavelengths which would decrease the effect, who knows how your sensor handles it etc etc.
In reality the difference might not be visible that well. But a test would be quite cool if you have a furnace and some iron around!



Not a physicist tho, so please correct me if I'm horribly wrong Wink


PostPosted: Tue Feb 07, 2023 2:11 am    Post subject: Reply with quote
kds315* wrote:

You would most likely need a thermal camera which works at much longer wavelength than visibleIR if you are into recording such things as "hot brakes" - to record those they would need to be really hot ("weissglühend") to record them well....


Not necessarily!! I am not an IR expert at all, but just for fun I did convert a Sony A100 many years go.

A few examples:

* an iron cooking plate, turned on but not visually glowing (not even slightly dark red) gave a huge "glow" on the IR converted A100
* the large military radar installation on a mountain close to my home gave a strong "glow" when I took photos at night (again no visible light emitted)
* in a nearby cliff about 2000m above sea level there was a strong (IR) glow visible when I took images at night - again no visible light. I suspect a military installation radiating IR, but when looking at it during during daytime you see "nothing".

So "hot brakes" may actually work pretty well. Sure, we are not talking about 100°C here ... !!

S


PostPosted: Tue Feb 07, 2023 12:09 pm    Post subject: Reply with quote
kds315* wrote:
You would most likely need a thermal camera which works at much longer wavelength than visibleIR if you are into recording such things as "hot brakes" - to record those they would need to be really hot ("weissglühend") to record them well....


As mentioned by the others objects glowing in the visible spectrum will also glow in NIR. There's quite a good test detailed here: https://www.beneaththewaves.net/Photography/Thermal_versus_Near_Infrared.html

For evening/night motorsport you can get very significant glow from the discs at the ends of the straights, so there should be significant NIR emissions from them as well. That's the plan at least! Smile


PostPosted: Tue Feb 07, 2023 11:50 pm    Post subject: Reply with quote
MG42 wrote:

For evening/night motorsport you can get very significant glow from the discs at the ends of the straights, so there should be significant NIR emissions from them as well. That's the plan at least! Smile


So then ... why would you need a lens corrected for VIS and IR? From a photographic viewpoint it might even look better if the VIS image is sharp and the IR part is slightly out of focus (="glow"). Or do you plan to take images in the IR range only?

S


PostPosted: Wed Feb 08, 2023 7:59 am    Post subject: Reply with quote
stevemark wrote:
MG42 wrote:

For evening/night motorsport you can get very significant glow from the discs at the ends of the straights, so there should be significant NIR emissions from them as well. That's the plan at least! Smile


So then ... why would you need a lens corrected for VIS and IR? From a photographic viewpoint it might even look better if the VIS image is sharp and the IR part is slightly out of focus (="glow"). Or do you plan to take images in the IR range only?

S


That will probably depend on how much focus shift there is with my existing lenses. If it were just the brakes putting out IR then it might be okay, the problem from the few images I've found that are similar is that there is IR all through the image, and that means that everything becomes "soft" as the IR light blurs edges that should be sharp. With the brighter IR sections they also completely overpower the VIS part of the image, again hiding detail.


PostPosted: Fri Feb 10, 2023 1:40 am    Post subject: Reply with quote
For a Telephoto lens, 300mm and longer- A traditional Mirror lens may be worth looking at. I've used my 500mm Reflex-Nikkor with my DCS200ir.
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