Trials using an external zoom digicam (Sony S75) on a microscope,
with a reversed SLR lens as a relay lens.

by Dave Walker, UK

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1) More detailed notes on use of the reversed SLR lens with the Sony S75 and LOMO pre-DIN optics and one modern DIN achromat.

Sony S75:
Manually set as follows: 20 cm fixed focus gave the least vignetting if an eyepiece was used, infinity focus gave the least if used without an eyepiece. Max aperture (f2.5) at 3x optical zoom (although can be stopped down to ca. f4 without extra vignetting). Sharpness set to '0', jpeg fine. A manual camera white balance was taken on the lamp. (For low-mid power mags. I use a 75W photoenlarger bulb in an old enlarger head with a field diaphragm. Camera exposures with this lamp were typically in the 1/30 - 1/100th of a second range.) The +/- 0.3 - 2.0 stops exposure override button on the Sony S75 is useful for adjusting exposure. The objective working distance was a little higher than for visual to project a focussed image on the LCD screen. The ca. three minutes before auto-shutdown of the S75 on batteries is annoying, using the mains adaptor can avoid this. The self-timer is also on an independent button and was used to minimise vibration.

LOMO optics:
For my two most used objectives, the 3.5x and 9x LOMO pre-DIN planachromatic objectives, they worked well without an eyepiece for minimal vignetting and max. field of view. When used with an eyepiece the compensating 'K' LOMO eyepieces worked best; the non-compensating type gave marked chromatic aberration with the photo set-up. This eyepiece choice is the reverse of what works best for visual work, for reasons I don't fully understand.
Two LOMO pre-DIN apo's (10/0.3, 20/0.65) worked best with the 'K' eyepieces but some chromatic aberration was observed in outer third of field. The 20/0.40 and 40/0.65 LOMO pre-DIN achro's, weren't too fussy but worked best without an eyepiece.
For 20x objectives and above a small hot spot was seen at field centre with an eyepiece. No evidence of Newton's rings has been seen to date, which has been observed with some digicams. A field stop was useful at higher mags to stop tube flare when an eyepiece wasn't used (a 'C' mount adaptor with a 12 mm stop was inserted where the eyepiece usually rests).



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