Raynox DCR-250 - length of extention tube

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DoppelHans
Posts: 9
Joined: Fri Feb 07, 2014 10:27 am

Raynox DCR-250 - length of extention tube

Post by DoppelHans »

Hello,

I would like to construct a tube system for Mitutoyo-Objectives and want to include a Raynox DCR-250 lens as a tube lens.

I know that it is not the standard 200mm-lens. I read on a few posts, that the focal length is 125 mm or that the flange-sensor length is 118 mm. Both are not contradictory to each other, because the focal length starts at the lens principal plane which is located somewhere in the range of the lens or even outside. So if the principal plane would be 7 mm before the rear flange that 118 mm would be the distance from the rear flange to the sensor.

But the DCR-250 is not a symmetrical lens so there should be two different principal planes and also two different focal lengths, depending on which direction it is used.

My questions are:

How was the focal length of the DCR-250 calculated? Or was it measured?

Where could be the principal plane be located?

For which direction is that focal length of 125 mm calculated? Or in other words: In which direction is the lens used, ordinary like it is mounted on a camera objective or reversed.?

What about the other direction?

Thanx for help, Hans

hero
Posts: 72
Joined: Mon Jul 17, 2017 12:38 pm
Location: California

Post by hero »

I'm not an expert but the Raynox DCR-250 has a shorter focal length than the DCR-150 (which is what I use) when both are mounted in the normal orientation, and the DCR-150 is I think about 208 mm, so the 125 mm focal length that you cite is for the normal orientation.

I would imagine that reversing the lens would mean a change in focal length as you also state, but I have no data on this. I do think there are some forum posts about it.

Personally, my approach was to just empirically determine infinity focus for the tube lens setup. I used a variable extension tube to find the exact amount of extension I needed, then replaced the variable tube with a set of rings that would get me as close as I could. I didn't want to keep the variable tube behind the Raynox, because I had concerns about its rigidity, and I wanted to avoid inadvertently turning the ring when using the setup. In practice, small deviations from infinity focus aren't a problem for my use case.

A variable extension tube is a very useful thing, and comes in handy regardless of the choice of objective and tube lens.

rjlittlefield
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Post by rjlittlefield »

The DCR-250 is specified by Raynox to be "8-diopter". If exactly accurate, 8 diopter corresponds to 125 mm (=1/8 meter). I do not recall seeing any posting where anybody made an independent measurement of focal length.

The sensor-to-flange distance of 118 mm will have been determined empirically, for one orientation of the lens.

If the lens is reversed, then it will still have the same focal length (nominally 125 mm), but the sensor-to-flange distance will likely be different.

It is safest to test the lens in both orientations (normal and reversed), with your own camera and objective(s). Of course I recommend to search the forum for reports about which way gives best results, but I will not be surprised if you find posts listing both orientations. The major difference will be in the corners. Those will vary with sensor size and also with distance between sensor and objective. Unfortunately the distance between sensor and objective is seldom recorded, so if the corners are critical, you may need to do some further experimentation.

--Rik

DoppelHans
Posts: 9
Joined: Fri Feb 07, 2014 10:27 am

Post by DoppelHans »

thank you for your notes, it is a starting point. I know that there is always the option to get approximating values by testing. In this case, I just wanted to get some theoretical background.

Meanwhile I found an older post about the DRC-250 (by google, not by the forum search).

http://www.photomacrography.net/forum/v ... hp?t=29906

thanx, Hans

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