retrofocus wrote:
Keep in mind that the longer wavelength IR light reduces the needed f-stop of about one stop for getting the same DoF as in regular light. I normally never use a smaller aperture than f/8 or a max. of f/11 to avoid hot spot creation - f/16 often will cause hot spots in IR light due to earlier IR light diffraction setting in which only happens in regular light at around f/22. Meaning testing for hot spots at f/16 is kind of an extreme condition since you shouldn't use it in IR photography. I consider a lens not suitable in IR light if it shows already a hot spot at f/5.6 for example.
Do the lenses you tested have an IR focus mark, or is the focus aligned with regular light that pretty much no noticeable shift occurs? Most Leica M rangefinder lenses have no IR focus mark since it's not needed, but I hve the mark on some older LTM and also Voigtlander lenses.
I am aware I am doing an extreme test. Nevertheless, the CV Apo and the Biogon pass this harsh test with flying colors. In addition, the CV Apo has a circular aperture at f/2.8, f/5.6 and f/11, giving it a slight advantage over the Biogon in my book.
The Distagon and Biogon have an IR mark, the CVs do not.
What I would like to know is what makes the Biogon more immune to IR hot spots than the Distagon. Obviously it is not as simple as the T* coating as both lenses have it.
Nov 02, 2025 at 08:28 PM
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