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Sometimes in lens reviews I read that a lens has a "cold" look, as opposed I suppose to a "warm" lens. So, for example, Voigtlanders are often described as "cold" lenses. What exactly does this mean? I kind of presumed it means the images are more blue than normal? How would that be? Does it mean more than that?

As long as we are on this subject, is there a technical term for the "character" of the lens, like whether it is "warm" or "cold"? Is that like the "mood" of the lens, or what would you call it?

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  • \$\begingroup\$ I'm not certain this is exactly what you mean, but Laowa have just released a series of anamorphic [cine] lenses they describe as blue, amber & silver, because of how they affect the lens flare. I actually have a 'fake' anamorphic which gives the flare a purple cast. \$\endgroup\$
    – Tetsujin
    Aug 20, 2022 at 7:08
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    \$\begingroup\$ It could be helpful if you provided a web link to an authoritative article that describes Voigtländer lenses as cold \$\endgroup\$
    – osullic
    Aug 20, 2022 at 11:07

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Lenses described as "warm" tend to allow longer (red, orange, and yellow) wavelengths through at slightly higher rates of transmission than shorter (violet, blue, and green) wavelengths. Lenses described as "cool" are the opposite, they let a little more of the shorter wavelengths (violet, blue, and green) through than longer wavelengths (yellow, orange and red) when equal intensities of the full visible spectrum are falling on the front of the lens.

This was more of a concern in the days of chemical film when a photographer was limited to the emulsions available with various films. Even with panchromatic B&W film, a 'warm' lens will allow red objects in the scene to be lighter shades of gray in a B&W print than equally bright blue objects in the scene. On cloudy days 'cool' lenses tended to give the sky a slightly purple/magenta tint with daylight balanced color film. On cloudless days a 'warm' lens would give the sky a slightly yellow-orange tint with the same daylight film emulsion.

With the ability to individually process raw digital image files this is much less of an issue than it was with chemical films. Digital photo processing allows the equivalent of designing a different custom film emulsion for each and every individual raw file we process and convert - and we get to do it after the fact! Not only can we control color temperature and white balance, which move pretty much all of the colors in a particular interpretation of the raw image data in one direction or the other (blue ←→ amber for color temperature, green ←→ magenta for WB correction), but HSL/HSV/HSB tools allow us to adjust the hue, saturation, and luminance/value/brightness of multiple different wedges of the color wheel independently of the others.

When Photophiles speak of the "character' of a lens, they're usually more concerned with how a lens renders a scene in terms of resolution, especially with regard to how sharp the in-focus areas of the scene are and how the out-of-focus areas are blurred. Things considered are the shape of blur as well as whether the blur has a uniform brightness, is brighter near the center or the edge, or even has sets of brighter and darker rings between the center and edge. A "smooth" lens renders the out of focus areas in a nice creamy way with the details of the background (or foreground) melted into a homogenous blob with smooth transitions from areas of one color and/or brightness to another, while a "busy" or "harsh" lens will render the details of out-of-focus areas in ways that can be distracting to the viewer. The direction and amount of field curvature and spherical aberration (two of the seven classic optical aberrations) significantly affect the "character" of a lens and the way out-of-focus areas of the scene are rendered as well as influence the lens' absolute resolution at the focus distance.

Even a lens made perfectly according to its "blueprint" has optical aberrations that are caused by the real thickness of refractive materials. The only theoretically "perfect" lens with no aberrations would be a theoretical lens with zero thickness. Most of the classic optical aberrations are due to the the fact that different wavelengths of light are refracted at slightly different angles at the air/lens boundary on the front and back of lens elements, or at the boundary between two lens elements glued together that have different refractive indexes.

How lens designers decide to deal with optical aberrations and how much correction they design into a lens by using various different elements made of various refractive materials affect the "character" of the lens.

What refractive materials are used in each of a compound lens' elements as well as what anti-reflective coatings lens designers choose to use to reduce lens flare and ghosting can affect the color transmission profile of the lens, which is what is often referred to as "cool" or "warm".

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  • \$\begingroup\$ How would a lens affect which frequencies of light would be "let through"? \$\endgroup\$ Aug 22, 2022 at 8:16
  • \$\begingroup\$ "What refractive materials are used in each of a compound lens' elements as well as what anti-reflective coatings lens designers choose to use to reduce lens flare and ghosting can affect the color transmission profile of the lens..." \$\endgroup\$
    – Michael C
    Aug 22, 2022 at 8:22
  • \$\begingroup\$ What we call "visible light" is just a term for the portion of the electromagnetic spectrum to which human retinas have a biological response. There's nothing unique about visible light compared to X-rays, radio waves, UV, etc. other than the sensitivity of the human retina. But different materials can be transparent or opaque to various wavelengths of EMR. Flesh, for instance, lets some portion of X-rays through but is more or less opaque to visible light. Bone, on the other hand, does not allow X-rays to pass through. \$\endgroup\$
    – Michael C
    Aug 22, 2022 at 8:27
  • \$\begingroup\$ Uncoated lenses reflect a significant portion of the light striking them. It should be fairly obvious that whatever is reflected does not pass through the lens. If the front of a lens is more reflective to longer wavelengths than shorter wavelengths, then more of the shorter wavelengths that fall on the lens will get through the lens, because more of the longer wavelengths are reflected than the shorter ones. \$\endgroup\$
    – Michael C
    Aug 22, 2022 at 8:33
  • \$\begingroup\$ All refractive lenses also absorb some of the energy of the light that passes through them. Depending on the material from which they are made, they can be more or less transmissive to different wavelengths. Color filters are made of materials which are even more selective about what wavelengths they allow to pass almost unabsorbed and what wavelengths are significantly absorbed as light passes through them. Narrow scientific filters can block almost all light except a very narrow range of wavelengths. Such filters are often used in astrophotography. \$\endgroup\$
    – Michael C
    Aug 22, 2022 at 8:37

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