idea to fight the diffraction?

soldevilla · Post by **soldevilla** » Mon Dec 26, 2011 5:52 am

After carefully reading this thread,

http://www.photomacrography.net/forum/v ... highlight=

I ask if anyone has considered to use a microscope objective of higther increase (ej. 20x) with less projection distance to get the magnification that provides another objective (ej. 10x) but increasing the f "effective"?.

Regards

Pau · Post by **Pau** » Mon Dec 26, 2011 6:27 am

Yes, it isn't unusual, in particular with infinite corrected objectives like the Nikon CFI 10X 0.25 and Mitutoyos with tube lenses shorter than the specified 200mm. The main issue is the image circle coverage, so it only works if the image circle can fill the sensor with enough quality at reduced magnification.

Most microscope objectives are designed to cover image circles about 20mm at its stated magnification, so it works only in some cases with wide field objectives and when the good image circle is larger than needed in the microscope, and more likely with 4/3 or APSc sensors.

There are some posts in the forum about this matter, but now I have not the time to search them.

rjlittlefield · Post by **rjlittlefield** » Mon Dec 26, 2011 9:29 am

http://www.photomacrography.net/forum/v ... hp?t=11954 for Nikon CFI 10X 0.25 used at 5.15X on an APS-C sensor. It works well, because this lens has an unusually wide field.

http://www.photomacrography.net/forum/v ... 7815#57815 for Edmund Optics "Nikon Achromatic Finite Conjugate" 10X NA 0.25 at 6.9X on an APS-C sensor. The corners are degrading and the field is pretty curved at this magnification, but I would consider the lens still OK for most purposes.

The same post shows that the otherwise excellent Nikon CF N Plan Achromat 10X NA 0.30 has awful corners at this reduced magnification (6.9X), due to the limited image circle as Pau mentions.

--Rik

Edit: fix typo

mgoodm3 · Post by **mgoodm3** » Mon Dec 26, 2011 10:28 am

Another problem with shoot images at a lower magnification than an objective is designed is that you may start to vignette the entrance pupil. With most high magnification objectives, you can only see the whole entrance pupil when you get very close to the objective (about the working distance). when you move farther away the front of the objective gets in the way and the entrance pupil will get effectively smaller, increasing the f/number.

soldevilla · Post by **soldevilla** » Mon Dec 26, 2011 1:34 pm

I imagined that the vignetting and field correction would make that a solution so obvious was not good... Thanks, I save a lot of time testing it.