Why am I allowed to turn my focusing ring beyond infinity?

The manual focus lenses I've handled (M42 Takumars) actually have stopped right at infinity; it's the AF lenses that have slush beyond. My understanding is that there's two reasons for this:

1. Conditions (mostly temperature) can move the focus point slightly
2. The autofocus system needs to be able to "miss" infinity a bit without slamming into a mechanical stop, which would cause excessive wear.

If you shoot infrared, the "infinity" point for IR is actually farther than the infinity point for visible light. A lens that did a hard-stop at the infinity point for visible light wouldn't be useful for IR imaging.

Similar... red, green, and blue light focus at different distances and if you use filters (especially narrow-band filters) you would want to be able to compensate for this.

There's a little margin for error in each copy of a lens or a camera where the sensor might be shimmed fractionally closer or fractionally farther.

There are even thermal expansion properties that can shift focus ... metals contract when they get cold.

So there's no single reason, but rather a lot of reasons why it's a good idea to give the lens a little wiggle room. And think about this from the financial standpoint of the lens maker... if they did a hard-stop and for whatever reason that "copy" of the lens happened to be just shy of focus on your particular camera body copy then that lens has to be returned as "defective". But with the wiggle room... that saves them a warranty claim.

Lumixs1DW Yes, I had the same question, I mean how stupid, isn't infinity just infinity, the farthest distance into the future that you can possibly focus on, apparently not. So, Why are you even allowed to wind 2mm beyond the infinity symbol?

The logical answer is that if you can wind the lens beyond the infinity symbol, there has to be a reason why. So, what is it?

my Discovery... I discovered one evening at Hanmer Springs while shooting star shots on a clear night that my carefully set to infinity symbol lens on my Lumix was giving me off focus shots, slightly blury. i tried different shutter speeds, apertures, ISO settings but it made no difference till I wound the lense beyond infinity. Then everything changed. Shots became bright and sharp.

My conclusion was that there are two kinds of infinity, 1: The one that is reserved for our planet, is far as the eye can see, set on the infinity symbol, and 2: the one that searches the stars. set to fully wound left to a hard stop, it's not hard to appreciate that there is a million klms difference. I recommend you try the difference and see what you discover. My settings were fully open lens, iso 6400, shutter 60 seconds, 12mm lens, and 16-35mm lens. but you need to try different options here, for example, increase the depth of field on a smaller aperture, say F8 and increase shutter speed length to compensate,, take lots of shots and record each one, so you know what works for your camera and lens. Happy snapping.