02-10-2018 02:41 AM
If I focus on my subject at the longest focal length of my EF-S 18-135 and then zoom back to 18 will my subject still be in focus? As I use Tv as my mode I assume the camera will automatically adjust the aperture (I choose the speed) to cater for the different focal length?
02-10-2018 06:47 AM - edited 02-10-2018 06:51 AM
Probably not, a few lenses are parfocal where they retain focus throughout the zoom range but most are not. I haven't got the 18-135 but being a fairly low cost lens then I doubt that it is parfocal although the small amount that it may be out of focus and the extra depth of field when going from long to short may not be obvious at small apertures.
If you set the camera to AI servo and hold the shutter button halfway while you zoom then the focus will adjust itself automatically.
The metering in the camera will adjust for the correct aperture in TV mode as you expected.
02-12-2018 07:52 PM
As Ray points out... VERY few photographic zoom lenses are “parfocal”. “Parfocal” means that the focused distance is the same regardless of focal length. So if you zoom, focus, and re-zoom... whatever was in focus is still in focus (if you have a parfocal lens). This is a common feature of high-end video lenses... but not a common feature of photographic lenses.
LensRentals had a list of lenses that qualified as “parfocal” but they haven’t updated it in a few years and at the time it was a pretty short list (maybe 3 or 4 Canon lenses). A few lenses are very close... just not perfect (technically focus needs a very very tiny tweak).
02-13-2018 09:53 AM
Ken Rockwell says about the new 18-135 nano USM:
"The focus is electronically compensated as zoomed; the camera needs to be on for it to stay in focus as zoomed."
Which was probably added for the motorized zoom gizmo,