Opticallimits

Hi All,



When I tried to photograph the full Moon last night using the following combo 24-70/2.8L + 350d, I did some comparison shots using the following settings -



Setting 1. Set lens focusing mode to manual and moving the lens distance scale to the infinity mark



Setting 2. Set lens to auto focus and point to the Moon. When the Moon appears to be in focus the lens distance scale was shown to be less than infinity.



What turns out is using setting 2 the image appeared to be much sharper compare to those taken with setting 1. And the Moon was already way high up in the sky (i.e. almost at right angle)



That leads to the following questions -



1. Does your lens exhibit the same behaviour?



2. Under what situation(s) would you use / trust the infinity scale on the lens?



Thank you in advance for your input and I look forward to read your opinions on this.



/cheung

Personally I wouldn't trust the infinity scale unless you have checked it and proved it yourself that it is right. For example, I have manually adjusted my Samyang 8mm fisheye as the lens scale was way out when I got it. The other time I might guess at infinity is if I am using small apertures so the depth of field will mask any errors.



Given even small shifts in the focus setting can affect the sharpness greatly, a focus on the subject at the time is always safest.



Note the lens characteristic changes with temperature. What is at infinity at room temperature might not be when it is freezing. I have observed this in my early astrophotography days. I took out and set up a lens, focused it exactly, and set a sequence of exposures off. When I looked at them, I could see the slowly defocus as time went on, due to the temperature changes. Now I make sure the lens cools down before I start and it has not happened again.

Thanks for your reply Popo.



I will keep that in mind and try to let the lens cool down a bit before taking night pics next time. With the full Moon being so bright I guess it's relatively easy for the 350d to have a focus lock on it. Anyway, do you have any focusing tips for tracking not so bright stars given the AF of 350d is pretty hopeless in moon-less nights (and theview finder is really not bright enough for seeing the stars through it anyway).



Thanks again.



/cheung

To clarify, letting the lens temperature stabilise is only needed if you manually focus one and leave it alone. If you are frequently checking focus it isn't necessary.



For focusing on stars, I find that hard too. If there is something far on the horizon with light maybe that could be used, as it would be close enough to infinity.