Introduction
It is frequently said that lens adaptors and extension tubes devoid of “glass” do not deteriorate image quality. It is also rather frequently said that there is no reason to buy expensive adaptors and extension tubes instead of very cheap ones from eBay, Aliexpress or similar sites.
In three separate pages I consider three different deficiencies that affect the performance of macro extension tubes: 1) Alignment and stability, 2) lens-to-sensor distance and 3) glare from internal reflections.
Below I describes tests about image degradation caused by glare and other problems caused by reflections from the inner surfaces of optical “tubes” such as lens adaptors and macro extension tubes.
The test results described here are not for current production items. Currently produced items of the same brands may show improved or degraded performance compared to those I tested a five years ago.
It is also important to be aware that “black coatings” in optical devices are not necessarily black, non-reflective, outside the visible region of the spectrum as I have shown in the page titled Black-anodized aluminium reflects NIR. Thus, the results presented are valid for visible light only.
How reflections are controlled
The cheapest optical devices have a smooth, sometimes even shiny, black plastic inner surfaces. Black anodised aluminium with a smooth surface is also common in cheap devices. These surfaces are prone to cause specular reflections, visible as bright patterns or shapes in photographs or ghosting. Ribbed surfaces, reduce specular reflections and ghosting, but increase diffuse glare if their surface is not matt black.
Special matt black paint that absorbs about 98% of light can be very effective, and even more effective when combined with a ribbed surface. Most effective is a ood black flocking, however, flocking if not of the best quality can give out some loose fluff that is prone to cause problems inside cameras. In addition flocking tends to attract dust particles.
Tests from 2017
I tested in 2017 some then newly bought and earlier acquired devices. I expected some differences, but not as large as I found. Some of the test conditions I used were rather extreme, but they dramatically showed that although extension tubes and most lens adapters are just hollow tubes, if poorly designed or manufactured they can drastically deteriorate image quality.
Not all cheap macro extension tube sets and adaptors are extremely bad, some are not too bad taking into consideration how little one pays for them, and may be good enough for occasional use. The tests also give some idea of what to look for in advertising photographs when choosing an item to buy.
I ran the tests indoors in a darkened room, with the camera on a tripod, mounted on a focusing rail. The light sources, two white LEDs, were firmly attached to the same rail by means of two “magic arms and a magic ball”. The camera shutter was triggered remotely. I used an Olympus E-M1 digital mirror-less camera with an M.Zuiko 45 mm f/1.8 prime lens considered to be one of the best available for Micro Four Thirds. I used as light source warm white LEDs (NICHIA type NS6L183AT-H1, with CRI = 92), driven with a constant current of 700 mA. The test target was an SD card adaptor on a matt grey card with 18% reflectance (Novoflex maxi Zebra card, A4) used to minimize reflections from the table surface.
For the first test I positioned one LED pointing downwards towards the target and a second identical LED pointing sideways and upwards towards the rim of lens at an angle of approximately 45 degrees but in such a way that it was just outside the image frame, i.e., the LED was not visible through the camera viewfinder. First a close up of this set up.
_9112093 I tested several different diaphragm settings, I show here first those obtained at f/1.8 and below those obtained at f/8.0. Kenko 10 + 16 mm M.Zuiko 45mm f/1.8 at f/1.8 with Kenko 10 + 16 mm automatic extension tubes. Auto exposure. Pixco 10 + 16 mm M.Zuiko 45mm f/1.8 at f/1.8 with Pixco 10 + 16 mm automatic extension tubes. Auto exposure. Comix 10 + 16 mm M.Zuiko 45mm f/1.8 at f/1.8 with Comix 10 + 16 mm automatic extension tubes. Auto exposure.
At f/1.8 the Kenko extension tubes together with the M.Zuiko lens suffer from mild glare. This is a very good performance given the set-up used in the test. Contrast is decreased but the image usable. The very cheap Pixco tubes, are not as good as the Kenko ones but probably fine in many normal use cases. The mid-priced COMIX are a joke (pun intended).
The differences at f/8 need some explanation: even though at f/8 the glare is better controlled with all tubes, exposure is wrong for some of them. I think what is going on is that the camera is measuring EV with the open diaphragm, but reciprocity between f-value and shutter speed fails as the glare gets better controlled at f/8. So, the camera auto exposure system gets mislead by the strong reflections!
Kenko 10 + 16 mm M.Zuiko 45mm f/1.8 at f/8.0 with Kenko10 + 16 mm automatic extension tubes. Auto exposure. The black shadow is one of the find from the LED heat sink, not an artefact caused by the tubes or lens. Pixco 10 + 16 mm M.Zuiko 45mm f/1.8 at f/8.0 with Pixco10 + 16 mm automatic extension tubes. Auto exposure. Comix 10 + 16 mm M.Zuiko 45mm f/1.8 at f/8.0 with Comix 10 + 16 mm automatic extension tubes. Auto exposure.
Note: In the photos the SD card adapter accidentally moved. This does not affect the test as the LEDs were attached to the camera and the SD card adapter on a table. Although I did not take another set of images, I moved the LED away from the objective and then returned it to a similar position and checked that the difference between the Kenko and COMIX extension tubes was the same as during the first round. Also during the first round I tested the Kenko tubes twice to make sure that the LED had not moved. There is an out-of-focus fin from the LED heat sink in the image top right region.
CONCLUSION: Extension tubes are hollow tubes, but they may still differ dramatically in how much or little they degrade image quality in actual use! Take this into account when buying cheap pieces of optical equipment.
END-OF-THE-STORY: I never used the COMIX macro extension tubes after buying them. I have used the Kenko ones regularly in the last five years. I did not keep the COMIX ones, but I have kept the Pixco ones as a backup.