Infinity focus in manual mode

[MARKOE PHOTOE]

Need some feedback on this:  When I shoot in manual focus mode with any or all of my lenses and I need to max out to infinity focus, I find that the infinity focus is off just slightly and I need to push it back a bit.  This is true for all my lenses being 'L' lenses (24TSE-II, 16-35L-ii, 50 1.2L, 85 1.2L, 70-300L, 70-200 2.8L-II... and shooting with 5D3 and 7D bodies).
 
Do you find the same condition to be true?

As an example, wanted to shoot star trails and needed to focus at infinity. However, turning the focus all the way doesn't necessarily provide infinite focus even at all apertures. And with regard to the star trails, I typically would try F8 or F11.

Your reply very much appreciated.

[keithfullermusic]

I always thought infinity focus doesn't actually focus on anything.  When I'm taking star trail shots I do infinity focus then turn it back just a hair.

I might be totally wrong, but that's what I thought.

[Fotofanten]

Yup, Canon lenses go beyond infinity. I learned that taking star photos at f/1.4 with a 5D mk I which lacks live view.

[chabotc]

On a lot of lenses (ie: all my canon ones, but many others as well) you'll notice that on the distance indicator there's an infinity symbol at the end, and a little white line just before it (about 1/4 inch long often)

The beginning of that little white line is what you want to use to get everything sharp - going all the way to the infinity symbol means that only things at infinity are sharp (ie: nothing:).

[neuroanatomist]

Almost all AF lenses are designed to focus slightly past infinity for two reasons, first to allow for the AF mechanism to include infinity in the focus hunt without bumping up against the end of the traverse, and second to allow for thermal expansion which alters the focusing behavior.

[marekjoz]

(...) to allow for thermal expansion which alters the focusing behavior.

It's even written in a lens manual as I remember. (yes, people might read manuals for lenses as well )

[awinphoto]

It's got to do with heat and expansion.  From what i understand as the lens and housing heats up or cools down, the focus, especially at infinity, can change as elements slightly expand and or contract.  That is why infinity can be off from time to time judging by the distance scale.  That is why canon goes white for the super tele's... so the lens in theory doesn't absorb so much heat and focus is more consistent. 

[RC]

According to the lens' manuals, infinity focus (under normal temp) is set to the vertical of the "L" symbol.   I too find infinity to be a hair shy of the infinity (L) symbol--at least on my 70-200.   Kinda miss my old film lens when shooting MF infinity. 

[neuroanatomist]

That is why canon goes white for the super tele's... so the lens in theory doesn't absorb so much heat and focus is more consistent.

It was my understanding that the need to reflect more light for a cooler lens barrel was primarily because fluorite elements are more susceptible to heat than glass.