This is the second of three instalments on the comparison of three sets of macro extension tubes for MFT cameras: Macro extension tubes (description), Macro extension tubes (glare), and Macro extension tubes (lens mount).
Disclaimer: I have no connection to any of the suppliers of the items compared in this test. I bought them from different on-line sellers. Although some of the products I bought have serious design flaws I have tested only one copy of each, bought in July 2017 (Kenko), June 2017 (COMIX), October 2015 (PIXCO). The items in production at the time you read this post may be of an updated design and different quality. It is also necessary to be aware that in the case of some Chinese brands, cheap and expensive versions of an item may exist.
I did some tests today. I expected some differences, but not as large as I found. Some test conditions were rather extreme, but they dramatically show that although extension tubes are just hollow tubes, they are not all equal on their effect on image quality. It is frequently said that lens adaptors and extension tubes devoid of “glass” will 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. This is not to say that all cheap tube sets are extremely bad, some are good enough taking into consideration how little one pays for them, and may be good enough for occasional use. The tests will also give you same idea of what to look for in photographs when choosing an item to buy.
I run 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. The camera used was an Olympus E-M1 with an M.Zuiko 45 mm f/1.8 prime lens considered to be one of the best available for Micro Four Thirds. Warm white LEDs use were NICHIA CRI=92 type NS6L183AT-H1, driven with 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) as as to minimize reflections from the table surface.
For the first test one LED was positioned pointing downwards towards the target. A second identical LED was positioned 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.
I tested several different diaphragm settings, I show here first those obtained at f/1.8 and below those obtained at f/8.0.
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 setup 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, but exposure is way off. 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!
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.
All illustrations, text and measurements are of my own authorship, and copyrighted.
(c) 2017 Pedro J. Aphalo