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Actual Focal length
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PostPosted: Tue Aug 08, 2017 11:38 am    Post subject: Actual Focal length Reply with quote

I'm wondering if anyone has created a list of actual focal length of various lenses?

As we already know, manufacturers do not always specify the actual focal length of a lens on the lens itself, although some do make this information available in a leaflet with a lens.

What got me thinking about this is that I have a couple of 50mm f/1.4 lenses (Chinon and the late Miranda bayonet), and the optics looks suspiciously like the Tomioka 55/1.4. This has me wondering if the lens is different or just a sneaky rebranding. I don't currently have a Tomioka 55/1.4 to test them against.

PostPosted: Fri Aug 11, 2017 3:55 pm    Post subject: Reply with quote

The only time I've seen the actual focal length of a lens mentioned was when I read a review of it from a photo mag like Popular Photography or the old Modern Photography. There are online review sites that also do this, but they're all dedicated to reviewing the latest, newest gear -- not the old vintage stuff. So your best bet to find the actual focal length of an old lens will be to find a comprehensive review of it in an old photography-based print publication.

I don't know how they go about measuring it, otherwise I'd suggest doing it yourself. If you could get a precise measurement of the diameter of the main optic, then you can use the old astronomy formula: Focal length = (objective diameter) x (focal ratio). So let's say you have a lens with an objective diameter of 40mm and a focal ratio of f/1.4. Multiply the two and you'll get 56mm, which will be the lens's exact focal length.

PostPosted: Fri Aug 11, 2017 4:04 pm    Post subject: Reply with quote

If you have another 50mm lens made by another manufacturer, you could easily check the field of view. Put your camera on a tripod and see what it covers.

It would not tell you the actual focal length but you should see a difference if you are right.

PostPosted: Fri Aug 11, 2017 4:12 pm    Post subject: Reply with quote

Measure it! There are plenty websites with procedure...