Optimizing a JML lens

soldevilla · Post by **soldevilla** » Mon Mar 26, 2012 2:30 pm

Hey. I have a JML objective. I have done some testing of distance between the lens and the chip of my camera, some tests clearly excessive (more than 30 cm.) But I don´t know which is the proper distance to work with the best quality possible that this objective can offer. Does anyone know the recommended distance?

Thanks!

rjlittlefield · Post by **rjlittlefield** » Tue Mar 27, 2012 10:10 pm

If you're talking about the JML "20X", then I can tell you that it delivers 20X on 222 mm total extension, from sensor to the shoulder of the mounting threads. I have no idea whether that's the nominal design value. It could also be that the seller says "20X" just as a general guide to power. I would use framing as my guide -- simply use the lens on whatever extension does a good job filling the sensor. The lens appears to be around NA 0.40, so +-20% of tube length should be no big deal in terms of added aberrations.

--Rik

soldevilla · Post by **soldevilla** » Wed Mar 28, 2012 2:26 am

Thanks, Rik

I own the JML 26mm f 4.5 lens and perhaps offered to me a little small increase. I have a Nikon x20 LWD also but sometimes its working distance is small for to take pictures of minerals, because small quality crystals are protected in geodes.
For this reason I follow with interest the thread Not All that Glitters is Gold about the JML x20.
The results of my 26mm away more than 30 cm. are not horrible, but there is a noticeable loss of quality and I´m demanding to my images to allow printing of 40x50cm. My camera is a Canon 350D, so the resolution is not very high...

Spanish cacoxene FOV 1,3mm aprox. JML 26mm and 30cm. tube extension.

Craig Gerard · Post by **Craig Gerard** » Wed Mar 28, 2012 4:25 am

Spanish cacoxene FOV 1,3mm aprox. JML 26mm and 30cm. tube extension.

That is a stunning sample and a beautiful photograph!

Craig

shrek · Post by **shrek** » Wed Mar 28, 2012 10:53 am

Very nice minéral and photo !

jp

rjlittlefield · Post by **rjlittlefield** » Wed Mar 28, 2012 12:58 pm

soldevilla wrote:The results of my 26mm away more than 30 cm. are not horrible, but there is a noticeable loss of quality and I´m demanding to my images to allow printing of 40x50cm. My camera is a Canon 350D, so the resolution is not very high...

Just running the numbers, a 26 mm f/4.5 lens on 30 cm total extension would be around effective f/50, a bit more or less depending on pupil factor. I suspect that most of the "loss of quality" you're seeing is just diffraction blurring, not added aberrations due to using the lens away from its design point.

The "gold standard" for 20X and long working distance would be the Mitutoyo at NA 0.42 and 20 mm WD. The JML 20X is only about 12 mm WD and probably not as sharp, but it's much less expensive.

--Rik

soldevilla · Post by **soldevilla** » Wed Mar 28, 2012 3:18 pm

rjlittlefield wrote: The JML 20X ...but it's much less expensive.

This phrase is music to my ears

rjlittlefield · Post by **rjlittlefield** » Wed Mar 28, 2012 6:25 pm

Sorry, I forgot to include a link to my mini-report about the JML 20X. See HERE.

--Rik