Inner focusing system

[Liz]

Hi all

Looking at buying another 'longer' lens for 450D, as are quite a few others by the looks :) .... alas budget wont make it for the L series, but seem to be a few around for about $500. Some lens are quoting an inner focusing system, would this suit astrophotography?? (camera on tripod setup) Dont you need to focus?
This is an example
http://cgi.ebay.com.au/Sigma-28-300mm-F3-5-6-3-DG-Macro-Lens-28-300-f-3-5-NEW_W0QQitemZ110279383820QQihZ001QQ categoryZ141509QQssPageNameZWDVWQQr dZ1QQcmdZViewItem

Have searched on IIS, and some some good data already, but this focusing thing had me wondering.



Thanks in advance.

[Liz]

feeling silly now, after more reading :whistle: ..... i thought that it was like fully automatic, but not the case, and still has AF/MF options. How is this lens anyway? Just looking ATM :)

[Liz]

and ....

http://youshop247.com.au/store/product.php?productid=23996&cat=464&page=5

[AlexN]

the 28-300 is not a bad lens, although I personally dont like lenses that go from wide angle's through to long focal length, the sigma 70 (or is it 80) to 300 is a better lens..

Internal focus (IF) simply means that the element that is moved in order to focus the lens is inside, as opposed to some of the older (cheapo) lens's where the whole front element would rotate and move in and out in order to focus.

IF makes use of specialized filters (like circular polarisers) possible, and also usually speeds up auto-focus, because the motor drive is only moving a small internal lens cell as opposed to the entire front section of the lens.