This post is to report on my recent experiences with an MJKZZ tube set.
First, I think it's important to say up front that the set of tubes I'm working with is not a random sample
. In fact they belong to another fellow, who contacted me because he was not able to make them work well despite lots of assistance from MJKZZ corporate. I was curious to see what the problem was, so I offered to take a look at them for him.
Earlier, mawyatt reported that
The tubes in the MJKZZ are the standard extension tubes but of higher quality than you can generally get on eBay unless you know the vendor. I already had a bunch of these and most were very sloppy fit, the Nikon F mount adapter & lens adapter were mostly loose and had to hand pick the better ones and toss the rest. MJKZZ are better tubes & Nikon adapters than I had generally found on eBay.
Sadly, the particular set that I'm working with is quite different. In fact these tubes have turned out to be the worst
that I've ever seen. They even have a type
of manufacturing defect that I don't recall having seen before.
Skipping forward to the good news, the final product of my labors is a very nice set of tubes.
This is what the final configuration looks like. It's rock solid and focuses perfectly at infinity with the Raynox DCR-150 alone. It's also completely free of internal reflections. Loosening two capscrews on the clamps allows the whole tube assembly to rotate, for switching between landscape and portrait orientations. I'm pretty sure I would enjoy using this set, but I'm going to send it back to the other fellow so he can.
So, the final state is the good news.
The bad news is that the whole process was not a great out-of-the-box experience. In fact it took quite a lot of fiddling to get all the parts to play together well.
The biggest problem, and one that I did not expect, was this:
The above is a GIF animation, and if it's displaying correctly, you're seeing about 1 mm of free play between the female bayonet and the extension tube that it's fastened to.
Here's what the problem turned out to be. In every one of the three sets of tubes, most or all of the screws would not seat fully. The underlying reason is that the screw holes were simply not tapped to full depth, although they were drilled plenty deep enough.
In addition to that problem, there was also an issue that the male bayonets did not snugly fit the female bayonets. In most cases the male flange was not sized to engage the springs at all, leaving the joint to simply flop around. Illustrating this second problem using leftover parts:
Both of these problems could have been addressed with tape, which I understand was MJKZZ's corporate suggestion to the other fellow. I like tape, but I don't consider it to be a good long term solution for parts that are not supposed to move.
So after considering options, I decided to just use my old friend epoxy. I removed the four screws holding the female bayonet loosely in place, cleaned all the parts with a cotton swab soaked in acetone, put a thin layer of slow-set epoxy on all the mating faces, reset the screws to where they had been before, put another thin layer of epoxy on the front of the female bayonet and the back of the bayonet-to-M42 adapter, assembled the set, and clamped it to about 50 pounds overnight to ensure that any spare epoxy got squeezed out before the stuff set.
The resulting sandwich is rock solid and measures flat to within 10 microns from side to side. I'm not anticipating any problems, but in the unlikely event that some of the epoxy eventually debonds, the screws and bayonet flanges will guarantee that nothing actually falls apart.
Now, about that VLE module (variable length extension). It consists of two tubes, one internally threaded, the other externally threaded, both M42, both 20 mm long. You might expect that due to normal manufacturing tolerances these two parts cannot possibly fit together snugly, and you'd be right. To address that problem, the kit I received contained a roll of PTFE pipe thread tape, with the suggestion to wrap the threads with enough of that to make the fit snug but not too tight.
PTFE tape is a pretty clever solution here, but again, I'd rather not go that route if it can be avoided.
What I would do instead, if I needed the VLE to actually be adjustable, is to use this approach:
It's attractive to consider skipping the epoxy and just using two locking rings. The problem with that approach is that the tube is only 20 mm long. By the time you engage two turns at each end (1 mm per turn), plus two locking rings in the middle (6.5 mm per ring), there would be only about 3 mm of adjustment left. Since the fixed tubes come in units of 7 mm, 3 mm of adjustment is not enough to reach all possible lengths. One ring and some epoxy would work better, leaving almost 10 mm of adjustment. Or two rings would work, if you could find suitably thin ones which I'm pretty sure would have to be purpose-made.
But in fact I didn't have to use either the tape or a locking ring, because adjustment turned out to be unnecessary. After determining by experiment exactly how much extension was needed to infinity focus the DCR-150 (stopped down to mimic the rear of an objective), it turned out that I could reach that length by simply choosing proper parts from the kit and screwing them tight together. The VLE in the final configuration is essentially used as just a 20 mm fixed length M42 extension tube.
That left the issue of glare from internal reflections.
According to the mjkzz.com website, "The benefit of using Nikon extension tubes is that it has large enough inner diameter and grooved inside to prevent internal reflection." That sounds good, and I'm sure the grooves make these better than smooth shiny tubes, but test images clearly show that "prevent internal reflection" is way
too strong a statement. Without added flocking, there is an easily seen amount of veiling glare.
Here is the demonstration, in the spirit of what I did at the start of this post. The subject is a "black hole" that emits virtually zero light, imaged through a 10X objective (one of MJKZZ's 10X NA 0.25 achromats, in this case). The left column shows the view looking into the tubes with a point-and-shoot camera, the center is the image captured by a Nikon D800E fullframe camera attached to the tubes, and the right column is the histogram of the captured image.
The flocking needed to accomplish this consists of three pieces of Beetle Black cardstock, formed into tubes and inserted into the assembly. The thin one in the camera end is glued in place, the others are just held by cardstock stiffness.
All in all, an interesting exercise. The MJKZZ custom parts are beautifully machined, the ones originating elsewhere were not. Clearly MJKZZ is vulnerable to quality control issues in their supply pipeline too.
Edits: to correct typos