Infinity question

Why some lenses pass infinity focusing? Is it a plus or minus? And should a better lens end tight at infinity?

Three main reasons

• It prevents the AF motor slamming into the stops when it moves to infinity.
  
• Focus can vary with temperature. Infinity focus will be at different points at different temperatures.
  
• Infinity focus varies with the wavelength of the light. Photographers shooting IR will find their infinity focus at quite a different point to those shooting visible light.
  

Thanks Frank. As always you are helpful however few points:

- Not all lenses (AF) have Infinity mark prior to final turn and some end right there. If I have the choice of both, say one ending exactly at Infinity the other passed it, I am better off to buy the second one as it is less likely to fail mechanically?

- When talking of "Temperature" you are obviously talking of Thermal temperature and not Kelvin's color temperature or both. Thermal is in direct relation of Emerging light and Focal point and not expansion of internal lens elements due to increasing temperature. So is this really to that extend that correction is needed? And through experience in different weather I think I have my answer. But then why on manual lenses of olden days, there was hardly any lens with this Pass Infinity feature?

- IR light should fall on focal point just short of visible light (More convergent light) so its mark is to the right of Visible Light on the lens, that is in pre-infinity not post-infinity.
Sample LINKSo that should not be a factor here.

hollis_f wrote in post #16629052
Three main reasons

• It prevents the AF motor slamming into the stops when it moves to infinity.
  
• Focus can vary with temperature. Infinity focus will be at different points at different temperatures.
  
• Infinity focus varies with the wavelength of the light. Photographers shooting IR will find their infinity focus at quite a different point to those shooting visible light.
  

Frank, how about infinity focus with the Tokina 11-16 with astro work. I recently picked one up and haven't used it all that much but it seems that the very end of the focus dial is infinity.
Perhaps the DOF at 11mm has something to do with it?

alliben wrote in post #16629278
Frank, how about infinity focus with the Tokina 11-16 with astro work. I recently picked one up and haven't used it all that much but it seems that the very end of the focus dial is infinity.
Perhaps the DOF at 11mm has something to do with it?

I find mine focuses stars best a bit short of the infinity mark. Doing manual focusing using 10x mag in LiveView makes you realise just how critical the focus is, even at 11mm.

It's OK to have stuff slightly blurred in a landscape image. But the more blurry a star image, the less it looks like a star. With any shot there is but one plane of focus, DoF is all about how much blur is 'acceptable'.

I adapt, modify many older lenses. It is always nice to start with one that has a lot of focus past infinity. Makes it easier to have infinity when I am done.