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Besides being most excellent macro diopter lenses, a Raynox can also be used as a tube lens for an infinite objective.

Mounting A Raynox Tube Lens

For best IQ, the Raynox must be mounted at one of its own focal lengths from the sensor, so that it focuses at infinity. The separation between objective and Raynox that may change the image circle but not the IQ.

The Raynox DCR-150 is (250mm) and the Raynox DCR-250 is (125mm) -- the equation is 1000/diopter to get the mm. So ideally, a Raynox DCR-250 should be located 125mm from the sensor, with the CFI60 10X positioned somewhere between 140 and 200mm (maybe more) from sensor plane. It seems to play well in this region.

Raynox Reversed?

my image quality seems to improve if I have the Raynox very close to the front element of the microscope objective

Tests at PM.net show that the image quality does appear to be improve if the Raynox is mounted in reverse. The Nikon MicroscopyU site recommends distance between lenses for better optical performance and in at least one setup the 150mm distance range seems to work well, which is in the middle of the Nikon recommendations (100-200mm).

That said I've personally found that my image quality seems to improve if I have the Raynox very close to the front element of the microscope objective so there is possibly some more empirical testing to be done with respect to these apparently contradictory conclusions.

The DCR-150 at 208 mm will give very close to rated magnification with the tube lens focused at infinity; the DCR-250 at 125 mm will give about 62% of rated magnification, with a correspondingly wider field.

Raynox As Closeup Lenses

Besides making a fine tube lens, Raynoxes are of course intended as macro diopter lenses and I must mention this here, as their quality is excellent.

For Canon folk, the Raynox DCR-150 is a good choice to start with a kit 18-135 IS lens. It is a +4.8 diopter lens, focal length about 210 mm. Used in front of a 18-135 IS lens, it will go up to about 0.65X, so a field width of about 35mm on a 550D.

Should you seek higher magnification, you might go up to the DCR-250 at +8 diopters, which will get you to a little more than 1:1, about 21 mm field width.

A nice, but discontinued diopter set from Raynox, is the Raynox CM-3500 set. The most powerful of the three is identical to the excellent MSN-202 diopter. The +12 lens in that set works well in combination with a 100/105mm macro lens and an APSC sized sensor.

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