Canon USM vs Non-USM Accuracy

I'm not sure if this got past my ISPs Usenet service so apologies if you've
seen my question already.

I've got a Canon 50mm f1.8 mk2 and a 28mm f2.8 which are both non-USM.
I seem to have hit and a miss focusing accuracy with them.

I've also got a 20mm f2.8 which is a USM and have no problems with the
focusing.

Are the non-USM focusing lenses less accurate than the USM versions?

I'm thinking of replacing the 50mm and the 28mm with the bigger aperture and
more expensive USM focal length equivalents.

 

Edward,

I get the same hit & Miss with my 50mm f/1.8, but put it down the the wide
aperture and very low depth of field. In fact it only really happens to me
when the subject I am focusing on has low light AND low colour contrast i.e.
it cant determine where the focal point is.

Apart from that I think its brilliant. Maybe USM is quick enough to find
the focal point and stop the mechanism before it overshoots back out of
focus like on the MKII.

Alex

 

What's the price difference generally between the USM and non-USM in a
50, 85 or 105 lens? Is it huge?

 

It's not huge - but when I looked at buying a 50mm f1.4 USM I noticed that I
could buy a 50mm f1.8 mk2 AND a 28mm f2.8 for less than it.

As I didn't need f1.4 over f1.8 and coming from an era when manual focus was
the norm, I thought that two lenses would make more sense. I think that USM
lenses focus faster, but I'm not bothered about focus speed as my shots are
primarily landscape. My autofocus option on the camera is set to One Shot AF
using only the centre autofocus point to focus then recompose.

I have found that the Auto Focus/Manual Focus switch on my copy of the 50mm
f1.8 mk2 is poor quality leaving me in doubt most of the time what mode I'm
in.


wasn't aware of any focusing issues - hence the questions here.

 

According to Canon USA, the USM technology means faster and quieter AF
operation but should have no impact on the lens' optical performance.

Henry Posner
B&H Photo-Video, Inc.

 

snips focusing issuses USM vs non USM

i have a old 28mm to 80mm non USM lence and a 75mm to 300mm USM lence
niether seem to be probomatic with regards to auto focus.

the 50mm is a older desgine possibly a evolution which might explain
your issues?

by poor quality do you mean loose? not the case here, the switch is
fairly stiff but we're not comparing like with like. my lences are close
on 10 years i'd guess.

roger