17-40L Focustest (replace or not)

[Reservoir_Dog]

Hi all

I just have bought a 17-40L Lens for my 20D.
Did some focustests, and i notice a backfocus...
What do u think?
Here are the links

FOCUSTEST (http://users.pandora.be/reservoir.dog/)

Then an aspirine box, Focused on the R, with both lenses (17-40 and kitlens)

17-40L (http://users.pandora.be/reservoir.dog/Focus1.jpg)

Kitlens (http://users.pandora.be/reservoir.dog/Focus2.jpg)

Seems to me that the kitlens is dead-on (confirmed with testchart)
What should i do?

Opinions much appreciated!

[Reservoir_Dog]

Thank u all for the informative answers!

[felix21685]

seems like the L lens backfocuses a little and the kit lens front focuses a little..
how many pictures did you take like that..were all the results consistent?

[blue_max]

I was going to answer tonight, but I guess you got personal messages.

Glad you got it sorted.

Graham

[Reservoir_Dog]

I was going to answer tonight, but I guess you got personal messages.

Glad you got it sorted.

Graham

???
Got no personal messages

[pierrot]

I can't understand where the problem is, or what you call backfocus.

You focussed dead on the "focus here" text of the chart, right? Then you notice that there are more millimetric lines in acceptable focus behind the target than in front of it. Is it your point?

If yes, it is not at all a matter of backfocus or whatever, but just a very basic optical law of the depth of field:
When somethoing is in focus, the depth of field (the distance in which out-of-focus objects will appear as "in acceptable focus") is parted 1/3 in front of and 2/3 behind the focussed target.

In other words, imagine that your distance-aperture combination gives you a DoF of 3 feet when shooting your old grand aunt from 10 feet away ; everything situated 1 feet in front of her face and 2 feet behind will also appear as well focussed. This means that you'll get a perfectly focussed picture of your dear auntie, including her legendary welcoming breasts and her not less legendary huge backside.

That's what I can see from your test pictures, not more. ;)

[Cadwell]

What pierrot said. No focus problems there.

[aam1234]

Reservoir_Dog, I didn't see your test but would like to state a general opinion. It's not about your question or your lens.

IMO, the best test one can do to a lens is a real world test. The person ought to go out and take the pics he/she normally take. if they come out good...all good & dandy. If the person notices an obvious problem (after making sure it's not an operator's error) then the lens is kaputt.

That's just my opinion of course.

[ed2day]

Looking at your focus test charts, the line where it says "focus here" looks perfectly focused to me. That should be sufficient IMO. That's supposedly the criteria Canon uses. People will complain that the DOF is off-center from the focus point by a mm or two which is ridiculous. The 1/3-2/3 rule doesn't hold at close focus distances--approaches 50/50. As for the aspirin box picures, the experiment doesn't look controlled well enough to draw conclusions. The DOF looks different. They are framed differently. Was the distance the same? The target is too cluttered. You don't know what the camera actually picked to focus on--lots of high contrast content in the focus field. Did you use a tripod and remote shutter release(or timer)? If not, throw the test away. Make sure lighting is bright. Take multiple shots since focus will vary slightly shot-to-shot. Shallow DOF experiments at close focus range are very hard to control and interpret. Better yet, just take the camera out and use it like you intend to use it and see how it turns out.

[Tom W]

From what I can see, the 17-40 looks good - focus is well within the range of tolerance on the chart. You could try reversing the chart so that the top part is closer to the camera than the bottom and see what happens.

It is often difficult to get an accurate test when you're shooting at an angled object like that because the actual autofocus points are much larger than the indicators in the camera show. The box images aren't a real good test since the camera could have chosen a number of high-contrast boundaries on which to determine focus.

I really think that this lens is fine.

[JulianL]

You have some nasty lens shadow in those pics....

[pierrot]

The 1/3-2/3 rule doesn't hold at close focus distances--approaches 50/50.
You're right, I made it short for a quick "understanding" purpose.
1/3-2/3 rule applies when setting the lens at the hyperfocal distance ;)

[Reservoir_Dog]

Ok
Thank u all for the answers, and yes, it was all taken on a tripod.
Now i know i must not worry with this lens.
Indeed, every day pics are nice sharp, and has nice colors!
Greetings

[SeanH]

Honestly, I have not seen one of those test that I would consider the be spot on. Some times it bugs me too when I get a shot I thought should be sharp and it's not, but overall I think I get many more sharp shots than I did in the manual focus days. However I think a split/microprism screen would be vey cool to have on my 20D. Anyone know if they make them for auto focus cameras???