MicrobeHunter.com Microscopy Forum

I just bought my second junky second-hand microscope: an Accu-Scope 3004 (discontinued in 2010) which included a 4x and a 10X.

So, I've been experimenting with swapping objectives with my Konus "College" (model 5302) and discovered that the A-S has a parfocal height of 45mm while the Konus has a parfocal height of only 36mm.

I have a diagram (that I found here ) that calls this measurement "b = object distance of objective = 45mm"


I don't remember seeing either "parfocal height" or "object distance" when shopping around for cheapo objectives.

Is there some other term, or specification, I should be looking for?

Are the lenses I bought on AliExpress (https://www.aliexpress.us/item/2255800378094152.html) likely to be 45mm or maybe something else?

Often the parfocal distance is not mentioned in the specs of objectives on eBay, probably the same with Aliexpress.
It is quite easy to estimate the parfocal distance when you have a face-on close-up photo of a 40X or higher magnification objective, and the thread is known. The thread diameter of an RMS objective for example is 20.2mm. The length of such an objective is very nearly 33 or 36 or 45mm or whatever the parfocality distance is - because the working distance is <1mm. So from the respective dimensions of the objective image, and the size ratio, one can estimate the length and that is almost equal to the parfocality distance.

50-60 years ago, both "short" (33-36mm) and "long" (45mm) parfocality distance objectives were made. Even from the same manufacturer (during different periods of time).
Later on the 45mm became more standard. Today 45mm are ubiquitous, especially for cheap objectives. But there are also 60mm objectives (for example, modern Nikons).

I believe that the 1989 version of the RMS dictionary calls this measurement
“parfocalizing distance (of the objective)”

See: https://www.rms.org.uk/rms-dictionary-o ... scopy.html

MichaelG.

The objectives you're likely to find listed on various Chinese sites are commonly designated by a length that includes the parfocal distance AND the tube length up to the point the eyepiece picks it up.

The DIN standard for a finite objective has a 45mm parfocal distance, and a 160mm tube length - with the eyepiece picking up the image back down 10mm inside the tube. So: 45mm + 160mm -10mm = 195mm.

The objectives you bought (link above) have a 185mm buried in the description. These would be the short barrel versions - 10mm too short for your DIN-standard scope.

Thanks all!

Luckily, I've only spent $12 or $15 or $18 at a time for "microscope stuff"......un-luckily, I've done it half-a-dozen times or so....oh, well.

So now I'll be a bit more careful and buy a "195" objective or two (and no more, that's it, that all I need). (for now)

And when I donate the Konus to a local school or young scientist, there'll only be one or two duplicated lenses.

So, cheapskate that I am, I didn't want to abandon my new too-short objectives.

I found these super-cool little RMS-RMS adapters on ebay -- the black things -- they screw into the turret and the objective screws into the adapter. They come in a ton of different lengths. I bought a 9mm and a 10mm. Once I had a chance to try them, I bought two more 8mm ones. I think the aliexpress lenses will be almost-exactly-but-not-quite parfocal with the lenses that came with the scope.

Rlw, when you're using finite objectives, those parfocal extenders aren't the cure-all they might seem. The good news is that the focus will be a bit closer, though still not quite "parfocal." The bad news is that your objectives will be peering into something more like a 169mm (160mm & 8-10mm extension) tube length rather than the 160mm they were designed for. This will add some aberration. You might not initially notice it in viewing - a bit of spherical aberration is more likely to show up as reduced sharpness in photos.

In days past, makers like Leitz made special parfocal extenders with a compensating lens inside it to allow old "short barrel" objectives to be used with their DIN standard (45mm parfocal distance) objectives.

The extenders you've bought can work very well for "infinite" objectives, which are designed to handle a pretty wide range of reference tube lengths.