# star trails etc- - infinite focus



## PicaPica (Sep 1, 2014)

can someone explain why setting my 17-40mm to infinite focusing distance does not result in sharp stars?

i know i have to pick a bright star and focus manual in live view...but why is that?

from logic it seems when you set the lens to infinite focus distance the stars should be sharp.


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## weixing (Sep 1, 2014)

Hi,
I think the infinity focus will change a bit when temperature change, so the scale is not very accurate. Also, with such small scale in modern AF lens, it very easy to misfocus a star... a bit off and the stars is blur... use live view will be easier.

Anyway, to focus a stars to tack sharp is very challenging, so challenging that there are even software for it... ha ha ha 

Have a nice day.


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## Don Haines (Sep 1, 2014)

A lens has to made to focus past infinity (to infinity.....and beyond  ) because of manufacturing tolerances. If it only went to infinity, you would have some cameras that were slightly off spec that would not focus on distant objects. This means that you can not just twist to the end of the scale, but have to go into live view and manually focus.

If the moon is out, you can autofocus on the moon and then flip it into manual.....


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## FEBS (Sep 1, 2014)

Indeed, it has all to do with manufacturing tollerancies. The lens can focus something further then infinity. So real infinity for your combo is a little then the infinity mark on the lens.


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