Confusion about focal length of dcr-5320

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Davids
Posts: 189
Joined: Sun Jan 31, 2016 2:20 pm

Confusion about focal length of dcr-5320

Post by Davids »

I'm confused as to the focal length do the raynox dcr-5320. One user found the best focus came from having the back of the lens approximately 192mm from the sensor ( http://www.photomacrography.net/forum/v ... f95a5f817a ). However, raynox ha posted the focal length to be 170mm. Does anyone know why there might be these discrepancies?

Thanks,
David

rjlittlefield
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Post by rjlittlefield »

I don't know anywhere that Raynox has posted the focal length of the DCR-5320 as 170 mm.

http://www.raynox.co.jp/english/dcr/dcr ... /index.htm describes the lens as "5 diopter", which would be 200 mm focal length (=1 meter / 5).

The same page lists the "focusing distance" as 17 cm, but the footnote explains that that number "is measured from top of the lens to the surface of object when the lens is set at infinity".

In this case the discrepancy would be explained by the physical thickness of the lens, more precisely the distance from the lens surface to the front principal plane. In non-technical terms, the focal length is measured from some special point inside the lens, while the 17 cm is measured from the surface of the lens.

It's also quite likely that the "5 diopter" specification is not exact. The idea that 2 diopter plus 3 diopter equals 5 diopter is an approximation that is true only in the limit of "thin lenses". For physically realizable "thick lenses" that necessarily have space between the principal planes, 2+3 is less than 5, following the 2nd formula at https://en.wikipedia.org/wiki/Lens_(optics)#Compound_lenses .

--Rik

Davids
Posts: 189
Joined: Sun Jan 31, 2016 2:20 pm

Post by Davids »

As always, thank you Rik! I knew you'd be able to clear up my confusion.

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