Does lens transparency affect f-number?

[frantzmubarik]

Hi.
Since thermal lens can be made by different materials, and their transparency differs a lot, so I wonder if aperture value will take that into account?
Assuming two lens have exact same size, but made by different materials and having different transparency, will their f-number be same or different?
This is much les talked in visible light camera because glass all have rather high transparency, but I think thermal cameras have to consider that.
Thanks.

[Fraser]

The lens F number is a result of the physical characteristics f/D and not the lens element transmission figures.

The transmission figure of the lens and the required corrections are captured during thermal camera calibration. And yes, different materials used for thermal camera lenses do have different refractive indexes and transmission figures.

When a thermal camera lens is purchased, it will often have a Transmission figure included in its specification. The cameras designer takes the transmission figure into account in the design to ensure adequate thermal sensitivity. When you buy an optional lens for a thermal camera a lot of information is often missing as the OEM considers it to be of no interest or use to the end user. They have already done the design work and created calibration offset tables to account for the various lens nominal transmission figures. For greatest measurement accuracy an OEM will often recommend that a camera is calibrated with the optional lenses that it will be used with. Multiple lens specific calibration tables are then created and stored in the camera or host system. Each lens can vary slightly in terms of transmission, even if they look identical and have the same specification.

Thermal camera lenses are commonly quite “fast” in terms of F number and F1.0 is not uncommon, but the material from which the lens elements are made will influence the transmission figure as I have said.

Fraser

[Vipitis]

You have this for visible light as well, it's called T-Stop and can be found on high end video lenses for example.

The transmission is relevant for elements, material, coatings and also angle. You can lose about 1/3 by having bad coatings at the front. That is why T-stops are more useful to measure exposure, and F-stops just give you a guide to depth of field. However lens design can change how out of focus elements look and that impacts the whole image. But I doubt much thought is out into thermal lenses. Transmission figures, or spectrums are most useful.

[Ultrapurple]

Like Vipitis said.

I have just bought a visible light camera lens that has a f/2 aperture but transmits light like a f/3.2 (because of some optical processing going on in the lens). Most visible light lenses transmit with an efficiency of well over 90% and so we never bother about attenuation. But as has been noted, it can indeed be different with different materials.

[frantzmubarik]

Like Vipitis said.

I have just bought a visible light camera lens that has a f/2 aperture but transmits light like a f/3.2 (because of some optical processing going on in the lens). Most visible light lenses transmit with an efficiency of well over 90% and so we never bother about attenuation. But as has been noted, it can indeed be different with different materials.

How? Built in ND filter?

[Ultrapurple]

How? Built in ND filter?

Sort-of, yes. It has a circular darkening that forms an optical filter (an apodization filter) and gives creamy-soft bokeh. The manufacturer's page on the lens is here: Laowa 105mm f/2 Smooth Trans Focus (STF).

Even wide open the lens is pin-sharp in its focal plane - but backgrounds just melt away, with no distracting edges. Here's a sample image from the imaging-resource.com review:



(I believe this is 'fair use' of an image that I acknowledge belongs to imaging-resource.com)

We're a long way off the topic of thermal imaging here but lens physics works in similar ways irrespective of waveband.

[Fraser]

On the manufacturers page the image of the lady against the railings with a city landscape behind is very effective 
I never knew such dedicated bokeh lenses existed. I am used to people taking about Bokeh in connection with using C Mount CCTV lenses on M4/3 cameras using an adapter plate. Darned pity as that creates a lot of competition on eBay when I go after a nice quality C mount lens from a known brand ! There was a time when C mount lenses were relatively inexpensive on eBay. Not any more for the fast lenses that I need for night vision applications 

Fraser

[Mechatrommer]

I never knew such dedicated bokeh lenses existed.

played too much with thermal imaging?... https://www.diyphotography.net/9-lenses-special-bokeh/ the pursuit for super fast crazy bokeh TEA nuts communities has its place on its own. as usual i'm not easily get sucked by the madness, neither there nor here.

btw, i'm a canon guy so if i just want to get crazy and brag around, i will get one of this Canon EF 85mm f1.2L II USM but being a practical guy who usually shot candid/quick/fast paced/event photos, i just got bunch of zoom lenses (instead of prime) one of them is Canon EF 70-200mm f/2.8L IS USM since bokeh is also a function of focal length. i'm not sure the exact formula but i can get as much bokeh or even more at 200mm f2.8 as 100mm f1.4, playing with subject distance and angle also is another parameters, the farther away the subject (and background) from lens, the more bokeh and the shallower the depth of field is. i dont see it practical though in any thermal imaging business except if you have too much money and time to waste.

[nightfire]

This behaviour of having lenses with their material and coating and sometimes their sheer number of up to 20 pieces of glass in a big zoom lens is the reason why the video folks sort their stuff along with  that t-stops. Also some test sites like Dxo test also the t-stop.
In reality a zoom lens with constant f/2.8 aperture can have something like t/3.2 due to above discussed effects.

But talking about bokeh- there were earlier lenses that had specifically bokeh and out-of-focus control on their agenda: Have a look at the old Nikon Nikkor AF DC 135mm f/2- this had a dedicated defocus control ring to change the bokeh appearance.
(https://imaging.nikon.com/history/story/0032/index.htm)
It also had a sibling at 105mm. These were a bit different from the standard 105mm, that also had a very nice bokeh (got myself some copy on ebay earlier this year of that famous Ai 105mm/2.5)

[Ultrapurple]

Sometimes bokeh is a thing in thermal imaging.



OK, maybe it's not the most artistic example but it was the first that came to hand from my back catalogue.

[Ultrapurple]

@Vipitis - you're right about the various Nikon DC (Distortion Control) lenses, which have a kind of adjustable bokeh. There is some good info out there about these much-misunderstood lenses. One reviewer (who is equally loved and loathed; I make no comment) describes the Nikon 135mm DC as "the best 135mm lens I've ever used. If you want an ultra-sharp, zero distortion 135mm lens, this is it." I have yet to find a LWIR lens with similar accolades.



Lens configuration of Nikon Ai AF DC Nikkor 135mm f/2S, from Nikon History page.

The image below (from the same source) shows the subtle yet powerful changes wrought by the defocus control. Like many other 'tricks' in photography (and elsewhere) the untrained eye doesn't spot anything but subconsciously we do respond. Small details make the difference between a good image and a great one.

[Mechatrommer]

i guess the defocus control is just a hack or tweak to the existing lens construction, the original intention for the multi elements lenses is to avoid abberations and fringing...
https://www.bhphotovideo.com/explora/photography/tips-and-solutions/optical-anomalies-and-lens-corrections-explained

if there is no such thing as abberation, ie if single element glass can produce abberation free imageries, defocus control will be an expensive addition to the system. anyway, only imagephooleries can differentiate whether you are using front or back or whatever focus system, mortals wont be able to tell. thats why i guess they can buy a painting in 7 figures price..

here.. Rhine II... no bokeh at all... look at the price

[Ultrapurple]

$4.3M?

I'll take three 

[Ultrapurple]

Interesting to see that the bhphotovideo link included some technical info and a diagram of a lens with Fresnel elements, which were being discussed in terms of their use in LWIR lenses somewhere on this forum not so long ago.



By the way, front focus and rear focus are just ways of achieving the same thing (a clear, well-focused image) depending on the design of the lens and there is no way for anyone - directional cable enthusiast or otherwise - to tell what kind of lens arrangement was used to create an image.

(Oh, and I do have at least one optical filter that is directional - a Baader U).

[Fraser]

Thanks Ultrapurple  It was me asking about the visible rings seen in the ZnSE front lens element of a PM280 MWIR lens.

Now I know 

Fraser

[Bill W]

That is what they should look like.

I am told that any design with only 3 rings on it is caused by a designer not recognising a known bug in the Zemax lens design software !

[frantzmubarik]

Thank you everyone.
So does anyone know the approximate rate of transmission for lens from different materials?
I remember coated germanium lens can have more than 90% transmission efficiency, but I don’t know how about Znse or Chalcogenide glass.
Thanks.

[wilfred]

Hi.
Since thermal lens can be made by different materials, and their transparency differs a lot, so I wonder if aperture value will take that into account?
Assuming two lens have exact same size, but made by different materials and having different transparency, will their f-number be same or different?
This is much les talked in visible light camera because glass all have rather high transparency, but I think thermal cameras have to consider that.
Thanks.

The f-number is just the lens focal length divided by the aperture diameter. Which goes to explain why a one stop change either halves or doubles the light transmission when the f-number changes by the square root of 2 or approx 1.4 . ie 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and so on.

Lenses for movie cameras IIRC did have what was called T-stops which did have factored into them the light transmission of the lens glass. I think this was because they had to ensure negatives from multiple cameras would achieve a similar exposure on the film even if they had lenses with markedly different transmissivity (if that's a word). But I'll happily be corrected on that point.

[Bill W]

Thank you everyone.
So does anyone know the approximate rate of transmission for lens from different materials?
I remember coated germanium lens can have more than 90% transmission efficiency, but I don’t know how about Znse or Chalcogenide glass.
Thanks.

Transmission is dominated by the surface coatings, not the bulk materials themselves.
The materials are around 99% at a few mm (or they would not be used) but:

Ge 'high efficiency' is around 97% per surface.  Not robust, falls of when wet, so for sealed internal surfaces.
Ge 'high durability' is around 92% per surface and survives normal handling.
Ge 'hard carbon' is around 80%, pretty indestructible.

Chalcogenide is around 92%
Hard carbon for chalcogenide is pretty new.

ZnSe around 95%

All the data you need:
https://ispoptics.com/technical/coatings/

Bill