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Thinking about ultra-fast lenses (ƒ<1), it would seem useful to have a solid+liquid medium between lens and sensor. This wouldn't have been very practical (presumably) at the time of film, but with a digital sensor I don't think there's much issue with coating it with transparent glass. Removable lenses would be somewhat cumbersome, but not hopelessly so with the right index-matching fluid (as common in high-NA microscopy). Of course lens designs would have to be altered but I'm curious as to why this is not a thing (that I know of, beside a few esoteric mentions here and there)?

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4 Answers 4

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Since lenses work based on the difference of index of refraction across the interface as well as the curvature of the lens surface (the more the difference in I.R., the more the light rays are bent) putting a higher I.R material between the rear element and the sensor would reduce the effectiveness of the lens.

You could incorporate that into the design of the lens, but you'd need to use a substance with a very low coefficient of expansion across the temperature ranges the system would be used in. Can't have the fluid expanding and forcing its way into the lens proper, or contracting and either creating a vacuum bubble or suction on the lens mount/barrel. And a solid that tried to expand could conceivably crack the lens' rear element or the sensor.

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  • \$\begingroup\$ re the expansion, I imagine you could have an oil reservoir, which could be flexible. Not sure I follow the "reduce the effectiveness of the lens" argument. For one thing, an immersion microscope objective doesn't perform worse than a dry one – they're just different designs, and for different uses. I wonder if the same could be said here, for cases where the flange distance of standard (air) designs becomes impractically small (4 to 10 mm quoted in this video). \$\endgroup\$
    – baptiste
    Oct 25, 2020 at 5:57
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    \$\begingroup\$ @baptiste - An immersion microscope couples the slide cover light into the flat surface of the objective. Light is bent and focused by the curved surface of the lens as a function of the difference in refractive index of the surfaces. Any media in contact with the curved surface would reduce the refractive capabilities of the lens. If the media had the same index of refraction as the lens, the lens would no longer function as a lens at all. \$\endgroup\$
    – user10216038
    Oct 25, 2020 at 21:51
  • \$\begingroup\$ I'm not suggesting glueing any old existing lens to a camera with oil but designing a (multi-group, compound) lens that has 1.5 RI as output medium. \$\endgroup\$
    – baptiste
    Oct 25, 2020 at 22:19
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As you know, focal length defines the angle of view and the degree of magnification realized. Some of the lenses we use are “prime” having a fixed focal length. Some lenses are “zoom”, we choose and pre-set the focal length pre-exposure. Additionally, subject distance varies scene-to-scene. We or the camera’s automation must adjust focus based on conditions. In most cases, focusing is a mechanical action that changes the distance lens-to-sensor. However, in a few designs, focusing can be accomplished by repositioning the various lenses in the array keeping the back-focus distance constant. The latter places severe restraints as to subject distance. This is because the travel distance to focus on an object at infinity vs. magnification 1 (life-size) is equal to the focal length. As an example, suppose a 50mm focal length is in use. When focused at infinity, the rear nodal (measuring point) will fall 50mm forward of the sensor. When imaging at “unity” (magnification 1 sometimes states as 1:1) the rear nodal is positioned 100mm forward of the imaging sensor.

What I am trying to say, in normal use the distance, rear nodal to sensor alters considerably. Seems to me, any optical gain realized (if any) would over complicate camera/lens design.

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  • \$\begingroup\$ I don't know enough about the optics of multiple lenses, especially when the outgoing medium isn't air, to conclude that the design would necessarily be more complicated. Quite different, sure, but maybe not more difficult if starting from scratch? \$\endgroup\$
    – baptiste
    Oct 25, 2020 at 6:00
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It happens that until very recently there are two things that go between the lens and the sensor in high-end cameras: a mirror in DSLRs, and a curtain shutter. So until came designers figure out how they can completely avoid the two, the question is moot. And after that, it becomes a matter of practicality & cost v.s. added image quality.

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  • \$\begingroup\$ only to an extent – high-end leaf shutter and rangefinder cameras have existed for a long time. Digital sensor (vs film) might be the more relevant factor in my opinion, as it seems easier to have the digital sensor coated with glass or in contact with oil. \$\endgroup\$
    – baptiste
    Oct 25, 2020 at 21:35
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Mostly because the vast majority of lenses are not designed to do extremely high magnification as is the case with microscopy. Why increase complexity and cost to exorbitant levels when there is no added benefit for most applications?

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  • \$\begingroup\$ That could explain why it's not in widespread use – assuming the designs end up much more complicated than current optical trains, which isn't entirely clear to me – but with some gear already priced in the 10s of thousand, if not hundred thousand dollars, I'd expect to find some specialised options, if this was useful? \$\endgroup\$
    – baptiste
    Oct 25, 2020 at 19:53
  • \$\begingroup\$ How is it not entirely clear to you that what you propose would be much more complicated than current designs for non-macroscopy lenses? Beyond that, the fact that you see no such specialized options should tell you something about whether it is useful or not. If anyone could make a lens that does what, for instance, the Canon DigiSuper100 AF which costs around $200K and projects only an 11mm image circle can do for significantly less expense don't you think they would have? \$\endgroup\$
    – Michael C
    Oct 25, 2020 at 20:14
  • \$\begingroup\$ I find it hard to know a priori the complexity of a system that does not exist and requires deeper knowledge of multi-lens group design with refractive output medium than I currently have. Also, there are heaps of components in the history of cameras that would seem improbable at a given time – a flipping mirror? a dual lens? diffractive optics? nano-coating? The argument that if something doesn't exist it can't be useful doesn't often sway me (by that token no innovations can be found); lightfield photography is one recent counterexample (admittedly with limited market traction so far). \$\endgroup\$
    – baptiste
    Oct 25, 2020 at 20:31
  • \$\begingroup\$ ... I'll readily agree that it's very likely the reason this doesn't exist is because it's not been found of value by lens/camera designers – but I'd like to understand why from a physics point of view. Which I thought would be an interesting topic of discussion for this forum, but from the three answers I'm starting to feel it isn't. \$\endgroup\$
    – baptiste
    Oct 25, 2020 at 20:36
  • \$\begingroup\$ It has nothing to do with high magnification per se. Immersion microscope objectives are all about light coupling. See microscopyu.com/tutorials/immersion \$\endgroup\$
    – user10216038
    Oct 25, 2020 at 21:58

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