Flange focal length of Raynox DCR-250?

Ichthyophthirius · Post by **Ichthyophthirius** » Thu Feb 04, 2016 5:58 am

Hello,

Has anyone determined the "Flange focal length at infinity" of the Raynox DCR-250 adapter when used in normal orientation?

The focal length of the adapter is 125 mm, so I would expect the "flange focal length" to be around 110 mm. Does anyone know?

Kind regards,

Ichty

ChrisR · Post by **ChrisR** » Thu Feb 04, 2016 7:03 am

I haven't, but could measure it I guess.
Are you using it reversed, though?

Ichthyophthirius · Post by **Ichthyophthirius** » Thu Feb 04, 2016 8:30 am

Hi Chris,

I was planning to use it in forward orientation, with a Nikon AZ objective (infinity objective; large diameter exit pupil) in front. So I'm planning to use it as a tube lens. I then need the flange focal length in order to find out if I can mount the Raynox to a trinocular tube.

If the flange focal length of the Raynox was too short (less than 100 mm), I wouldn't go and buy one, so any measurements would be appreciated

From the information I found it wasn't clear which orientation of the Raynox, forward or reverse, would be the best for infinity objectives.

Regards,

Ichty

ChrisR · Post by **ChrisR** » Thu Feb 04, 2016 10:20 am

I read that they're better reversed - which makes sense, so I only ever used them that way.
This is what I measured - not wildly accurate:

rjlittlefield · Post by **rjlittlefield** » Thu Feb 04, 2016 12:14 pm

My numbers are similar, measured from a bellows setup in which I had carefully focused on infinity.

The labels on my DCR-250 say "Infinity focus at 95.5 mm from normal front of lens to flange of Canon camera." and "Infinity focus at 90.5 mm from male shoulder of reversed lens to flange of Canon camera."

The lens thickness is 15.5 mm, so I can match Chris's numbers to within 4-5 mm by taking each of his numbers to mean distance between the indicated point and the camera sensor.

--Rik

Ichthyophthirius · Post by **Ichthyophthirius** » Thu Feb 04, 2016 1:21 pm

Hi Rik, Hi Chris,

Thank you very much!

ChrisR · Post by **ChrisR** » Thu Feb 04, 2016 5:22 pm

I'm surprised they aren't closer than 4-5mm.
With hindsight though I was in a bit of a hurry - My "infinity object" was only about 20m away, which gives an error using thin lens formula, a little under 1mm too far from the sensor.
Rik interpreted my meaning correctly.