Premium Beat has a story about lens whacking. The name is weird enough that you might want to click over to it and check it out. Basically, you remove your lens from your camera and hold it in your fingers at the lens flange, allowing light to spill in. Then, you just continue photographing. It does make for some interesting photos. Check out the article.
Here’s my attempt to whack my lens, but in very subdued light. I didn’t see a lot of fogging (maybe that’s a film only problem) and didn’t get the kind of flare a more direct light source might provide. But, I did get a very different close focus point.
Lumix GX8 with Lumix-Leica 15mm F1.7.
You can see the whacked image on the left. The “Yankee Candle” is much closer to the lens and somehow still in focus. On the right, with the lens mounted normally, the closest focus is on photo of the red candle. I can’t say with certainty what F-stop the whacked lens was set to because it doesn’t have a manual aperture capability. I’m guessing wide open as the aperture is set by the camera electronically and, since the lens isn’t attached…no control.
I think I got a close focus because of the Scheimpflug principle. I purposely held the lens at an angle knowing the tilt would help in this regard.
I’ll have to experiment later with some of the manual control lenses.
The only thing to worry about is dropping the lens. Your fingers are pretty busy. Still, getting an unusual image may be worth the risk.