Question regarding focus problems with Canon EF 50mm f/1.4 USM lens

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default - 28 Dec 2006 03:09 GMT

At first I thought that my EF 50mm f/1.4 USM lens was not as sharp as my
50mm f/1.8 II lens but what I have now determined after many photos where
the focus seems off, is that is is about the same sharpness, but the plane
of best focus seems to be behind the selected point with the 1.4 (the 1.8
seems to get it just slightly forward). Pictures of an angled tape measure
finally prove it. At apertures larger than f/2.8 the 1.4USM back focuses
enough to be a fairly consistent problem whereas the 1.8 pretty much always
nails it or focuses just slightly ahead but not enough to matter. I'm using
the center focus point to check this.

This is on a Rebel XT (350D). Is this something that would improve with a
XTi (400D) since the 400D has the higher precision cross sensor on the
center point for f/2.8 and faster lenses?

Should the lens be returned to Canon for repair? Is this something they can
adjust?

I'm getting kind of annoyed with the 1.4 with it's focus accuracy problem
and with the fact that it needs about 1/3 stop more shutter time to reach
the same exposure as the 1.8. Since the 1.4 is only 2/3 stops faster than
the 1.8, the extra 1/3 stop more shutter time makes the 1.4 only really 1/3
stops faster than the 1.8. At nearly 5 times the price for another 1/3 stop
and less accurate focus, I am thinking I should sell the 1.4 and just keep
the 1.8. Photozone.de seems to think the 1.4 is a better lens than the 1.8
but I am finding it worse.

Has anyone else come to this conclusion that the 1.4USM is not worth the
money at all?

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Aad - 28 Dec 2006 09:17 GMT

> At first I thought that my EF 50mm f/1.4 USM lens was not as sharp as my
> 50mm f/1.8 II lens but what I have now determined after many photos where
[quoted text clipped - 24 lines]
> Has anyone else come to this conclusion that the 1.4USM is not worth the
> money at all?

Well, it's better built so it should last longer.
USM should make it a faster and less noisy lens.
If thats worth the money? That's up to you.
It is certanly more difficult to use than the 1.8.
About the backfocus problem. You shoot return it and get another one.
(or have yours adjusted)
kr
Aad

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default - 28 Dec 2006 10:02 GMT

> Well, it's better built so it should last longer.
> USM should make it a faster and less noisy lens.
[quoted text clipped - 4 lines]
> kr
> Aad

Thanks for your response. The focusing is much quieter, but not a lot
faster. The 1.4 overshoots and hunts more than the 1.8 which usually just
jumps to focus. As for how long it lasts, I could replace the 1.8 many
times for the price of a 1.4 and I have now read many reports of the focus
mechanism failing in the 1.4 so it may not really be more reliable. The 1.4
is wonderful in low light if I can get the focus to be accurate at and below
f/2.

It is interesting having both and comparing, but it looks like I'll need to
send the 1.4 in to Canon to be recalibrated. Hopefully it is something that
they can do.

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Stephen M. Dunn - 28 Dec 2006 15:48 GMT

$ At apertures larger than f/2.8 the 1.4USM back focuses
$enough to be a fairly consistent problem whereas the 1.8 pretty much always
$nails it or focuses just slightly ahead but not enough to matter. I'm using
$the center focus point to check this.

The aperture at which you shoot has nothing to do with AF; AF is
always done with the lens wide open.

Canon's specs say that AF should get you to within DOF (with the
lens open, since that's how AF is done). Not necessarily exactly on;
just within DOF. If you're not getting that level of accuracy, then
yes, your equipment is not functioning correctly.

$This is on a Rebel XT (350D). Is this something that would improve with a
$XTi (400D) since the 400D has the higher precision cross sensor on the
$center point for f/2.8 and faster lenses?

Maybe; the high-precision sensor is supposed to get to within 1/3 of
DOF (again, with the lens wide open). Maybe also because AF depends on
the combination of the body and the lens (you've already seen half of
this, by getting different results from two lenses on the same body)
and by switching bodies you may find that both lenses focus correctly,
or that the 1.4 now works better and the 1.8 works worse. And you
might get a similar experience by trying a different 350D, too.

$Should the lens be returned to Canon for repair? Is this something they can
$adjust?

Canon can indeed calibrate lenses and/or bodies that are not
performing within spec.

See http://www.photo.net/learn/focustest/ for some more info,
a test procedure, and a whole bunch of comments from people who do
or don't have focus issues with their cameras.

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default - 28 Dec 2006 22:48 GMT

> The aperture at which you shoot has nothing to do with AF; AF is
> always done with the lens wide open.

Thanks for your reponse.

The selected aperture doesn't affect the focus distance that the AF picks,
but stopping down does increase the depth of field enough that the selected
focus distance is now within the depth of field which makes the AF good
enough to use from about f/2.5 and smaller. At f/2.0 the depth of field is
small enough at close distances for the focus plane to be too far from
optimal sometimes.

> Canon's specs say that AF should get you to within DOF (with the
> lens open, since that's how AF is done). Not necessarily exactly on;
> just within DOF. If you're not getting that level of accuracy, then
> yes, your equipment is not functioning correctly.

It might have a focus distance problem, but it might just be a consistency
problem. I took another 15 pictures of the tape measure and found that
sometimes it does get it close enough. It appears that the actual focus
point might not be exactly under the dot in the viewfinder.

If I use the AF assist light from a 380EX flash, then the AF seems to be
more accurate and consistent and it is easy enough to turn off the flash
before taking the picture. CF4 is set to use the * button for AF so it
won't refocus when I press the shutter.

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Stephen M. Dunn - 29 Dec 2006 03:28 GMT

$It might have a focus distance problem, but it might just be a consistency
$problem. I took another 15 pictures of the tape measure and found that
$sometimes it does get it close enough. It appears that the actual focus
$point might not be exactly under the dot in the viewfinder.

It may not be entirely lined up. Keep in mind, too, that the actual
AF points are larger than the boxes shown in the viewfinder; I've
seen claims that they're up to three times the size of the viewfinder
marking. And anything that's within the sensor's area is fair game.
That's why the photo.net page on focus testing has nothing but a
line anywhere near where you put the focus point - to make sure
that's what the AF system locks onto. Your tape measure, on the
other hand, has markings both in front of and behind the spot at
which you're trying to focus, and it's possible that the AF system
is focusing correctly - just not on the spot where you wanted it
to.

A more practical example of how this can be a problem might be a
portrait. You put the AF point on the subject's eye, but if the
eyebrow is within the AF point's area of coverage, even though it's
outside the box in the viewfinder, it's possible that the AF system
focuses on the eyebrow rather than the eye. You see a misfocused
picture and think the camera isn't working properly, but in fact it
is - it's just that the camera's definition of "working properly"
isn't exactly what you had in mind!

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>>>----------------> http://www.stevedunn.ca/ <----------------<<<

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