Now one of the four microscope objectives my friend lent to me was a bit of a mystery to me. The outer casing was missing, so I was unsure what it actually was except that it was supposedly a good performer for extreme macro. It turns out that it is the JML 20x microscope objective.
As I wanted to see what this objective was capable of, I decided to mount it as far from the sensor plane as possible. I used my Sony A6000 for this test. This was mounted to the Hawk Factory M42-NEX focusing helicoid and onto my longest bellows. The total distance of the objective mount to sensor plane was 30.5cm. The working distance to the subject was about 11mm.
The subject I used was the iPhone 6 plus retina display screen which has 401 PPI (pixels per inch). That works out at 63 microns per pixel (25.4mm/401=0.0633mm). As there were 14 pixels captured across the entire Sony A6000 sensor width, that works out to be 0.88mm across. As the sensor width is 23.5mm, that works out as about 26.5x magnification.
Now I did not think that the image captured is very sharp. There are a few possibilities. As I was using ISO 200 to reduce the noise, the shutter speed had to be 1/20 secs even with the retinal display set to the brightest. So any slightest movement will result in blurring of the image. Now this was minimised by my using electronic first shutter, and standing very far away from the subject and triggering with an infrared remote. So I did not expect image movement to be contributing. The more likely reason I thought was that I had a screen protector on the phone and that was making the image less sharp.
Now to prove this, I had to charge up my wife’s old iPhone 4S as it did not have a screen protector on. I cleaned the screen as best I could and then put it through the same test as before. As you can see, the pixels are very different from the iPhone 6 plus screen indeed. But it does look sharper when cropped in.
Now the iPhone 4s retinal display has 326 DPI, which means each pixel is 78 microns. So working out the 11.5 pixels across the entire sensor width of the A6000, means that it is about 26.2x magnification.
This just proves that the JML 20x microscope objective is pretty impressive for high magnifications above 20x. But care has to be taken to ensure that there is no vibration as any tiny movement will be visible in live view. I had to take extreme precautions to ensure I did not adversely affect the final image captured.
The JML 20x microscope objective will fill the entire APS-C sensor particularly at 20x magnification and above without any vignetting. But at such magnifications, one does require a very sturdy imaging setup to prevent shake. It is definitely a good perform for visible light imaging. I might just have to keep my eyes out for one.