Do all 50mm lenses have a focal length of 50mm

[Pixl8tor]

I believe that lens focal lengths are normally measured when the lens is focused to infinity.
I would imagine that the actual focal length of every 50mm lens is not exactly 50mm but may be something close to 50mm, say within a range of 48 to 52mm.
So a 50mm lens from Canon may not have the exact field of view as a 50mm from Nikon or Sony or Sigma etc. Likewise for every other focal length of lens.
Can anyone direct me to a reliable web site that would confirm this?
I'm just curious, it's not that I'm that concerned about the field of view of the lenses that I have (or want).
Bill

[William W]

I believe that lens focal lengths are normally measured when the lens is focused to infinity.
I would imagine that the actual focal length of every 50mm lens is not exactly 50mm but may be something close to 50mm, say within a range of 48 to 52mm.
Can anyone direct me to a reliable web site that would confirm this?

I can't direct you to a website which would confirm that there is no lens measuring exactly 50mm Focal Length (within stated machine tolerances), but I do know of published Canon and Nikon documents which state that the nomenclature focal length on their lenses, is not exact.

For example it is stated (with explanation) in several of the Canon White Papers on Lenses and also (from memory) in the several editions of their book, "Lens Work"

WW

[Pixl8tor]

Thanks WW.

[William W]

I have been doing thinking on your proposal of the range - i.e. 48 to 52... I think that would be very close (for a "50mm lens").

I base this opinion of the fact that there are, and have been for many years, lenses labelled 45mm and 55mm.

WW

[Manfred M]

I have also read that they can be a bit liberal when they state the maximum aperture; a lens marked at f/2 might really be f/2.1.

[Tronhard]

I recall that DXOmark have charted some of that characteristic in their lens metrics, from open through the range.

[xpatUSA]

I have also read that they can be a bit liberal when they state the maximum aperture; a lens marked at f/2 might really be f/2.1.

Yes, and I have also read that effective apertures can get rounded to the nearest "standard" value.

Then there is the matter of "focus breathing" when focusing at less than infinity. When focusing at less than infinity, the lens still has a focal length but it can vary considerably from the marked value, e.g. Nikons. I had one such, long ago - the 60mm micro-Nikkor f/2.8 'G'.

See an article by the well-known Nasim Mansurov:

https://photographylife.com/focus-breathing