Jeepers Creepers, Where'd Ya Get Those Peepers? Part 3.

EUREKA!

Quick show of hands: how many of you use protective UV filters on your lenses?

I'm going on 45 years of using interchangeable lens cameras, and have bought the old saw that it's better to break a filter than the front element of your lens, and there is no degradation caused by a filter.

Right.

Except with a Sigma 120-400mm f/4.5-5.6 APO DG HSM telephoto at full zoom.

To recap (more here and here): I had some unexpected results when comparing a micro four thirds camera (Olympus OM-D E-M10) and an APS-C camera (Sony A77II). All I wanted to know was, how much difference is there between MFT and APS-C. I was surprised that the smaller sensor E-M10 images appeared to be sharper than the larger APS-C sensor images (more on sensors here). But, I was trying to recreate a realistic shooting environment by handholding and using a fairly fast shutter speed. I decided that I needed to be a little more disciplined if just good image quality was my sole objective.

So, I repeated all my tests using a tripod. With image stabilization enabled. And, with image stabilization disabled (as recommended by the manufacturer). And, using the self-timer to further reduce the possibility of causing camera shake when pressing the shutter release. And, oh, my, did I ever get some weird results with the A77II. There were some kind of strange artifacts present when stopped down to f/11. Like a double exposure or something. And, it got worse at f/16! The mystery deepened.

I was ready to blame the lens and throw it under the bus, figuratively speaking. Then, in the middle of the night, it hit me - there was one other variable: the freaking "protective" UV filter that cost all of about $5 sitting in front of my almost $1,000 lens. I know, I know, it is so close to the lens it should cause no discernible degradation of the image.Well, surprise, mateys. It does. And, here's the proof:

Without Filter and With Filter

I would not have believed it if I hadn't seen it with my own two eyes. Multiple times. After doing some research, I've found that you can pay close to $100 for a "good" filter, so maybe that's my problem - a $5 Tiffen just isn't a match for a Hoya or better. In the mean time, however, I think I'll just go filterless.

And, I can get back to comparing MFT to APS-C!

Aye, lassies an laddies, we've uncovered t'culprit, an 'is name be Tiffen...