Varying magnification using a Raynox tube lens

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Stephane Savard
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Varying magnification using a Raynox tube lens

Post by Stephane Savard »

...and allows the overall magnification to be adjusted somewhat by changing camera-to-Raynox distance.
Here I'm quoting Rik from the http://www.photomacrography.net/forum/v ... hp?t=31810 thread.

This is the second time I come across this information on the forum, and I don't understand it. I'm doing some searching and a shopping list for photography using a Nikon objective, and have settled on using a Raynox tube since I may not have a suitable lens.

However, I don't understand the statement. I read that a DC-150 must be mounted with 208mm of extension, and the DC-250 with ~125mm of extension from the sensor (or is that from the camera mount?).

If someone was to increase the camera- to-Raynox distance, how does that not throw out the 125/208mm distance??

Also, what are the limitations of attempting the change magnification?

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Post by ChrisR »

Quick answers:
If someone was to increase the camera- to-Raynox distance, how does that not throw out the 125/208mm distance??
It does throw it out :)

It's not actually that amount of extension because they aren't "thin" lenses, but close - you have to focus at infinity to be "right".

Depending how your objective objects, or not, you get away with altering it, to some extent. Higher NA, less far. 10% should be ok, but you may find otherwise.
Chris R

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Re: Varying magnification using a Raynox tube lens

Post by rjlittlefield »

Stephane Savard wrote:I read that a DC-150 must be mounted with 208mm of extension, and the DC-250 with ~125mm of extension from the sensor
That word "must" is way too strong.

To explain...

When you move the tube lens away from its infinity focus position, you both 1) change the magnification and 2) introduce a certain amount of aberration, which may or may not visibly degrade the image quality.

The amount of added aberration depends strongly on the NA of the objective -- in fact it varies as something like NA raised to the fourth power. As a result, a change in extension that would be a serious problem at 20X NA 0.42 will be utterly insignificant at 5X NA 0.14, since the 3:1 ratio of NA's corresponds to an 81:1 ratio of aberrations.

With a low magnification objective, like that 5X NA 0.14 that I mentioned, you can change the extension almost however much you want, and added aberrations will not be a problem.
Also, what are the limitations of attempting the change magnification?
If you're looking for numbers, I suggest consulting the graph at http://www.science-info.net/docs/etc/Tube-Length-na.gif . Line A reflects the amount of change that could be made in one experimental setup without visibly affecting the image quality due to added aberrations. The left-most point on the graph is at NA 0.25 and about 220 mm, reflecting an increase in the tube length of a 10X NA 0.25 objective from 150 mm to about 370 mm, more than doubling the magnification. In contrast, the middle point is at NA 0.50 and about 14 mm, which would be less than 10% change in magnification.

Another limitation is field width coverage. On APS-C, you can push Mitutoyo objectives down to about half their rated magnification. Beyond that, the corners are liable to go soft.

--Rik

Stephane Savard
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Location: Canada

Post by Stephane Savard »

Thank you for the explanation!

Chris R: I shouldn't have used the word 'extension' in this case, I've been shopping so much for "extension tube sets" to go with the Raynox that it seared itself into my mind :)

Rik: I had seen this graph before, but couldn't understand how to read it, I get it now with your explanation.

This also throws yet another variable into my investigation, the fact that with the objectives I had in mind (MRP70040 and/or MRL00102) with Raynox tubes, I may have access to more of a range of magnification than previously thought.

A bit off topic, but has either of you seen any evidence whether the Micro-Nikkor 200mm AF F4D is suitable as a tube lens? This is the only lens I own in the 100-200mm range. I've having trouble finding any forum posts on this lens. Most are either specifically for the AI version of this lens, or one post where a user created a special adapter to recess the objective into the lens front (which I couldn't replicate). Would it be equal to or worse than the Raynox DC-150 plus the necessary adapters and extension tubes?

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Post by Chris S. »

Stephane, a different Chris, here.
Stephane Savard wrote:A bit off topic, but has either of you seen any evidence whether the Micro-Nikkor 200mm AF F4D is suitable as a tube lens?
I've used this lens for converging infinite microscope objectives; it works fine. Try stopping it down to see if this increases contrast with your specimen of this lens--it does with mine, apparently by knocking down internal reflection. But don't stop down too much, or you'll get vignetting. If memory serves, I settled on f/5.6-f/8. There is no need to get fancy in mounting the microscope objective to this lens--I just used a standard combination of two adapters--one to go from objective thread to 52mm, then a step ring to go to the 200mm micro-Nikkor's filter thread. Set the micro-Nikkor at infinity focus, of course.
Stephane Savard wrote:. . .Would it be equal to or worse than the Raynox DC-150 plus the necessary adapters and extension tubes?
I've never used a Raynox, but would venture to guess that any differences would show up only under pixel peeping. The Raynox might or might not show a bit more chromatic aberration. One optic might cover a given sensor size differently than the other.

If you are new to photomacrography with infinite microscope objectives, I suggest you use the 200mm micro-Nikkor you already have, and move ahead without worry. Later, once you are routinely producing solid photographs with infinite objectives, if you want to experiment to potentially obtain modest gains, feel free to try alternate converging optics.

--Chris S.

Stephane Savard
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Joined: Thu Jan 14, 2010 7:33 am
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Post by Stephane Savard »

Ah, I'll use the Micro-Nikkor - the shopping list of tubes, adapters and what not for the Raynox (not including the Raynox itself!) was starting to add up. I'd rather use what I have as much as possible!

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