50x lessons part 2

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Macro Photog
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Joined: Sat Dec 13, 2014 11:45 am

50x lessons part 2

Post by Macro Photog »

I called this out as a new topic because I want to understand more. When I was purchasing parts to adapt my objective to the camera I was advised to purchase some extension rings so the tube lens could focus at infinity. Several people suggested distances in the 60 to 70mm would do the trick. I tried distances in the suggested length and did get some good images at 68 +/- but had a lot of trouble getting subjects in focus. Out of frustration I tried shorter and longer lengths. I seem to be getting my best images with no extension tubes at all. The objective is in its adapter, and that is attached directly to the raynox. Since I'm not technical why is this working vs. the initial advice?

I want to be clear this is not about the purchase of the rings. They are a trivial expense and I will use them later for some other project. It is also not about the initial advice because I'm sure it was correct and did not work as well for me due to my inexperience. I just want to understand as much as I can to ensure I am maximizing the potential of my objective.

Thanks in advance.

ChrisR
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Post by ChrisR »

Just to clear a couple of things:
1) do you have the Raynox reversed - it may make a difference?
2) do you have the Raynox focusing exactly at infinity?
3) have you found the same with lower mag objectives?
Chris R

Macro Photog
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Joined: Sat Dec 13, 2014 11:45 am

Post by Macro Photog »

Hi Chris,

1) do you have the Raynox reversed - it may make a difference?
No. I will try that.

2) do you have the Raynox focusing exactly at infinity?
I'm not sure how I would know.

3) have you found the same with lower mag objectives?
My other objectives are finite. This is my first exposure to an infinity objective.

ChrisR
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Post by ChrisR »

Ah, OK, it would be good to normalise thing to what most people do :).

I find it easier to screw the objective on the front of a 200mm camera lens, so though I've played around with the Raynoxes as tube lenses, this is mostly second-hand info..

1) Reversing the Raynox is reported by a few as giving improvements to edge definition, (though possibly at the expense of central definition). You'll need a bunch of step rings and something to use as a female-female threaded part at some diameter like 52mm, I used a 55-52 step-down ring which had 52mm tapped right through.

2) Having the tube lens at infinity means the objective is properly working at its designed subject ans object distances. Higher NA (than say a 10x) means you get less tolerance.
Try focusing on something genuinely distant - the moon works! It can be hard to find something far enough away.

3) OK, worth being aware of.
Chris R

rjlittlefield
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Post by rjlittlefield »

Macro Photog, you're shooting quartz internal inclusions, right?

So that effectively means that you're shooting through thick cover glass, while your 50X objective is probably designed for no cover glass at all.

Shooting through the wrong thickness of cover glass introduces spherical aberration. That degrades your image quality. With a wide aperture objective, the degradation can get bad surprisingly quickly.

However, it turns out that the aberration can be at least partially compensated by changing the "tube length". See the discussion at MicScape: Tube length, Coverslips and Correction Collars. The direction of compensation is that a too-thick coverslip requires reducing the tube length.

I strongly suspect this is the cause behind your observation. Shooting through quartz is equivalent to an overly thick cover slip, which is partially compensated by reducing the tube length. Alas, the optimum correction will be different for each depth of inclusion. If the inclusion gets sufficiently deep, then you might even be better to ditch the Raynox entirely, thereby effectively shortening the tube length even farther.

In other words... The standard recommendation, that it's best to have the rear lens focused on infinity, implicitly assumes that there's no unexpected material to look through. If there is, then the optimum point will be somewhere else, which you have to determine by experiment.

In the grand scheme of things, you would do best to get an objective with a correction collar built in, as discussed at http://www.microscopyu.com/articles/for ... ction.html .

--Rik

Macro Photog
Posts: 92
Joined: Sat Dec 13, 2014 11:45 am

Post by Macro Photog »

ChrisR Thanks for the suggestions. I am aware of the Raynox reversal properties because I read Ric's test.

Ric, All I have to say is nail - head. You described my experience almost exactly. Yes almost everything I do is internal. I document some of my beginners trials with an infinity objective (link below) with some decent but not great pics. As I'm discovering this is a journey.

http://www.photomacrography.net/forum/v ... hp?t=31083

Some inclusions near the surface are quite clear at the 68mm level but I was unable to get consistency as depth increased. While I am always aware that nature throws a random, usually low quality "lens" into all of my images I was unaware this objective would be so sensitive in that regard. I might have guessed though with all other positive and negative issues being magnified (no pun intended) literally and figuratively at 50x.

Anyway everything makes much more sense now. I don't mind the experimentation and can tailor it now to focus on this issue. Experience will eventually give me increasingly better results. At least now my experimentation will have a direction and I won't be wandering around in the dark just hoping for a good result.

After looking at the Miti G Plan specs I'm still really happy with my choice because the working distance for the G Plan objective is 13.89mm vs. mine at 20.5mm. I also get greater DOF 1.6 vs. 1.1 (G Plan). Working distance is critical for my work and I have through trial and error shot some decently sharp images. I expect this to get better now that I know the issue.

My subjects are usually 1mm or more inside the specimen. After reading the article I wonder if G Plan objective would really make any difference since there is no real consistency in the depth or quality of the "cover slip" in my work. Any additional thoughts would be helpful information for my future purchases and may help others in their considerations.

I will look into correction collars and also am looking into adapters to insert my bellows between the raynox and the objective.

Thanks again. This community is invaluable to my work.

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