Loss of contrast with macro rings?

I previously tested some cheap auto macro rings, and found them very useful for creating macro images and videos with ordinary lenses. Since this time, some concern has arisen that the macro rings can cause a loss of contrast in the images, due to a design flaw: The insides of the extension rings are glossy, which can cause unwanted reflections.

To test the side effects of the glossy insides, I tried to take images before and after sanding down the insides. Here is the ring inside before and after the sanding treatment:

Without treatment	After sanding the inside

I used the sandpaper from a bicycle tyre puncture repair kit to make the inside more matte. If you do this, please be aware that sanding the inside is going to produce a lot of dust, dust you don't want into the camera sensor. So be sure to wipe it off with a moist cloth, let it dry, and then blow off any residual dust with a rocket blower.

To test the macro ring with and without the treatment, I set up the Panasonic GH3 with the Lumix G 45-200mm lens at 45mm f/10. I used the 16mm extension ring.

To get an even, bright background, I placed the subject inside a transparent bucket, as a simple macro soft box. Outside, I had the Lumix FL360 flash unit in TTL mode, connected with a TTL cable:



Here are the resulting images, before and after sanding down the inside of the macro ring:

Without treatment	After sanding the inside

If the glossy inside of the macro tubes is in fact a problem, the bright white background of the subject would expose this. This brightness would then spread to the rest of the image, causing a loss of contrast.

What we see, is that the image taken after making the ring more matte, is in fact slightly better: The red colour of the dynamite sticks is more saturated. And we can notice somewhat better contrasts. But the differences are fairly subtle.

My conclusion is: The lack of contrast due to the glossy macro ring insides is not a big issue. And if you are worried about it, it's a simple matter to fix it by sanding the insides a bit. Just make sure you don't get any plastic dust into the sensor.

If you prefer to buy a set of rings which don't have this problem in the first place, then the good news is that Kenko is producing some which have a proper matte, ribbed surface inside the rings. They are a bit more expensive, though.