Help about a 40x objective

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soldevilla
Posts: 578
Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less

Help about a 40x objective

Post by soldevilla »

Hello. I need some help... :(

Today I have received a microscope objective, a 40x IOS LWD OPTIKA. First, until now I don´t know that IOS means infinity... and I can´t mount it in my setup, only a empty tube, a body camera in an end and the microscope objective in the other end.

Well, I have made an adapter in order to attach my thread ring in front of my 55-250 ES-S Canon lens. Is not a great lens, but is light and after carry it in a two weeks hollidays is a good idea sacrify some quality...I only take familiar pictures...

I return :D I need to go until 250mm FL if I want not have vigneting in the picture. The FOV is the expected for a 40x but the quality is bad, with low contrast and very low definition. At least I expected to obtain the same quality that give me the telelens,but no...

And the LWD... I feel it have half of working distance of my 20x lenses (JML, Nikon and Leitz) but no... only 2 mm. ¿is it normal this small distance? I have the posibility of retour to the seller this objective but before I want to be sure I no forget any test...

Thanks!

Chris S.
Site Admin
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Joined: Sun Apr 05, 2009 9:55 pm
Location: Ohio, USA

Re: Help about a 40x objective

Post by Chris S. »

Josep, I think you should return this objective. Even if you could eliminate the problems with image quality, you would still be left with only 2mm of working distance, which is very difficult to use for macro work, and would limit your subject choices. As references, my Nikon 40x ELWD finite objectives have 10.1mm working distance, and my Mitutoyo 50x infinite has 13mm WD.

When testing other people's lenses, I've taken a few stacks with objectives that had 2mm or 3mm of WD. It was not easy to squeeze light into that small gap, and doing so greatly limited my options for controlling the quality of that light. This was with fairly flat test targets--three-dimensional subjects would be harder.

Optika brand--do we know whether or not these objectives are corrected for chromatic aberration without a compensating eyepiece? If not, the objective may never produce good results for macro work.

The 55-250 ES-S Canon lens is not one I'm familiar with, but most infinite objectives are not all that persnickety about the converging lens you use. Some camera lenses will probably work better than others, but the differences should range between "good" and "great"--not "quality is bad, with low contrast and very low definition." That suggests to me that the fault lies with the objective, not the 55-250mm lens. The vignetting might be the fault of the camera lens, or be due to the objective's possibly having a small image circle. In a quick glance at a sales site just now, I got the impression that the image circle might be 20mm--though on the page I looked at, this wasn't entirely clear. If true, that's a rather small image circle and could produce vignetting on a DSLR.

If you continue to test this objective, you might want to make sure it is mounted with as little distance as possible between it and the front of your camera lens--in one such setup I tested, a few additional millimeters here brought a large degradation in image quality. And while you have probably done this, you might want to stop the camera lens' aperture down to a middle f/stop (too small an aperture causes vignetting, while bigger apertures may permit some internal flare).

Also, one of the small-WD lenses I tested looked terrible at first, but I realized that this was due to stray light bouncing off the subject directly onto the front of the objective, causing flare. While this was not exactly a flaw of the objective, it was a very difficult thing to avoid when working with it.

If I were in your shoes, I would return this lens without the bother of further testing, and look for another 40x objective.

Good luck!

--Chris

soldevilla
Posts: 578
Joined: Thu Dec 16, 2010 2:49 pm
Location: Barcelona, more or less

Post by soldevilla »

Thanks, Chris,
Today I have retourned the objetive.

I will wait a bit more for my 40x...

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