Steve Perry has a great video on how to use back button AF. I’ve started using it. I started photography long ago with all manual focus gear, so separating the focus operation from exposure seems more natural to me.
That's a stupid software thing and becomes especially apparent under those conditions: the in-camera vignetting correction does not seem to be gradual but compensates in discrete steps away from the center. When boosting the in-camera corrected image (either in post or by extremely high ISO in-camera), these concentric rings become very clear. I believe the in-camera shading compensation is baked into the RAW-file if you do photography, which renders an otherwise nice photo useless.
I always disable in-camera shading compensation (= vignetting correction) for this very reason. And because I often like a bit of vignette in my shots. If I don't want vignetting for some reason, I stop down the lens or correct in post.
I'm using filters on my lenses for the following reasons:
- If you buy good quality filters, the impact on the optical quality of the lens in my opinion is negligible (like 0.3% light loss and almost zero additional ghosting/flare).
- I clean my lenses quite often. Even when first blowing/brushing off dust before a wet wipe, you'll inadvertedly cause abrasion on the front element. In time, the degradation of the coating on this element will have adverse effects on image quality as well. Replacing a front filter is cheap, replacing a front lens element not so much.
- Don't expect the filter glass to protect your front element from a direct impact with a solid object, but having some metal ring protruding beyond the front element certainly helps in keeping it from harm. A lot of impact energy can be dissipated by the deformation of the filter ring before wrecking the front of your lens. Always keeping the lens hood on also helps here.
- Resale value: a minor scratch on the front element (e.g. by accidentally rubbing a grain of sand over the lens while cleaning it) won't affect image quality much, but it severely affects resale value.
Have a read here, you might find it interesting: