Using a microscope objective on a bellows

[johnellerman]

I recently decided to try a PL4/0.1 160/- objective from a "Prism Optical" brand microscope mounted on a bellows mounted on my Sony A7R2. I'm getting images alright but resolution is not great when I zoom in. I have it on silent shutter but there is audible shutter click when I run the setup via a remote mobile phone app. I am wondering whether I have vibration issues or whether the lens is not up to the task. I am also getting pronounced chromatic aberration but suspect there may be a series of issues that I need to address before I can achieve those sharp images of the individual components of an insect's eye. I would like to upgrade my manual Velbon Super Mag Slider to one of those automated ones but need to be sure that I have addressed the other limiting factors first. Any ideas about which factors are likely to be limiting would be appreciated.

[Charles Krebs]

A couple big things come to mind immediately.

I am not familiar with a "Prism Optics" 4/0.10 objectives. So I can only make some guesses in this regard. A 4X with a 0.10 NA is usually a very basic (can still be decent) or low end (??) achromat objective. Very often these were never intended to form a "quality" image circle larger than about 18mm diameter at best. With many the reality is considerably less. You may get light that covers a 24x36mm sensor, but that sensor has a diagonal of 43mm, and unless you have a high quality, "overachieving" objective the image quality away form the center can be severely lacking in quality. How is it dead-center? A higher end 4X will have a larger NA and may be designed to work with "super-wide" eyepieces that require a 26mm image circle. These will very often provide very good imaging out to the edges of an APS-C sensor, but even a majority of these will be questionable towards the corners and edges of a 24x36mm sensor.

Be sure there is 150mm distance between the shoulder of the objective threads and the camera sensor.

Vibration is always a concern, but I am not familiar with the operation of your camera, and whether or not the noise you hear might indicate a vibration concern. Typically Sony cameras have modes you can set that will not cause vibrations.

[johnellerman]

Thank you Charles for your prompt response. Since I posted I have experimented further. I have found that the Sony A7R2 silent shutter is not supported by the remote embedded app so I changed to a 5 second self timer setting and bolted the whole set up to my desk to remove any vibration. I also set the ASA to 100 instead of auto which gave a longer (10sec) exposure. I also found that one multi-shot pass through the subject was better than back and forth on the slider. (I had used back and forth because the adjustments on the Mag Slider were too coarse to ensure proper overlap. The results are better, though I think the lens is now the major issue. The following are two images of a long-dead native bee. The distance between the threads of the lens and the sensor was 190mm. Not sure why you suggested 150mm but would value your reasoning. Note chromatic aberration on the lower left of the zoomed image but the ommatidia to the top of the eye are starting to have some semblance of form to them. Yes, they are poor quality images and the bee has been dead for a week but I think I am now moving toward making a decision on a new lens. Again, your comments would be appreciated.

[ChrisR]

A major problem here is that the focus increments are much too big. You need something about 0.04 mm, = 40 microns.
That caused the fuzzy banding, and indirectly the colour fringes.

The objective is designed to work 150mm from your sensor (for 4x). 190 shouldn't kill it, though. You'll get about 4.8x.

Clearly it doesn't cover the full sensor, which isn't too surprising, though a lot do better than this at 4x, and especially at the nearer 5x you're using it at.

[johnellerman]

Thank you for your response, Chris. I knew that the increments were too large and expected comment about that. I knew it causes the fuzzy banding but didn't realise that the colour fringes were caused by that too. Regarding the image circle, the vignetting is caused by the inclusion of a pair of extension tubes between the bellows and the camera. That is a function of the mirrorless camera design. If I take them out I get full sensor coverage. I am encouraged that the ommatidia are reasonably well defined where they are in focus which tells me that a major problem that I had previously was in the area of vibration. I will be visiting a company nearby that sells Olympus microscope objectives and will take my camera with me and check some out. I will go for a 10x which should be more interesting that the 4x. Does anyone out there know anything about which Olympus lenses would work best? I am also considering buying the automated slider to allow for smaller increments. Does anyone have any comments on the lighting? I am beginning to feel that very diffuse lighting is best. Do people use flash? I do for normal macros but the fly eye was taken with LED lights.

[Pau]

I knew it causes the fuzzy banding but didn't realise that the colour fringes were caused by that too.

In fact they are due to the objective achromatic (low level) correction, but because this longitudinal chromatic aberration mainly shows at defocused areas, focus stacking eliminates it in a big extent, here you can see an example:
http://www.photomacrography.net/forum/v ... hp?p=59281

Old Olympus objectives finite corrected are not adequate for this kind of setup because they need complementary corrections done at the compensating eyepiece. Newer UIS series infinite corrected are full corrected but need to be used with a tube lens

Take a look at
http://www.photomacrography.net/forum/v ... hp?t=12147