f/4 and Autofocus limit? (EF 70-200mm f/4L USM)

[neuroanatomist]

What minimum focus setting did you have the lens set at? 1.3m or 3m. That could have affected your shots in the 10ft range.

I was under the impression that that setting was there to reduce focusing time by allowing the lens to not have to span through the entire focus range?

Correct - the focus limiter just limits the distances through which the AF system will attempt to achieve focus. With the limiter at the longer distance, you simply could not achieve autofocus close than that (meaning an OOF shot in AI-Servo mode with release priority, for example, but I don't think that's an option on a 300D).


RE focus and recompose, you're still probably better off using the closest AF point to the subject that you can, given your desired composition, to minimize the amount of movement you need to make during recomposition.

[TakahiroW4047]

One way to test for AF issues is to use a printable focus test chart, like this one. You tape it to a door (so you can adjust it to a 45° angle to your camera), and mount your camera on a tripod at ~25x the focal length, the longest end of a zoom (so for your 70-200mm zoom, that's ~16 feet from the chart), and test at the widest aperture.

The 25x focal length was a bit too far and with my 6mp sensor ~300% crop was a bit out of the question [:P]


The directions said as close as possible so I shot these with 1ft distance for the Tamron and 5ft for the Canon. I think I can happily say I'm fairly lucky to have a well matched lens/camera set up for both []


...or is there anything here that you guys see??


These are not sharpened. PS unsharp masking filter made the letters mostly nice and crisp []





Tamron 17-50mm f/2.8 non-vc @50mm 1ft away


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Canon EF 70-200mm f/4L USM @200mm 5ft away


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[neuroanatomist]

The 25x focal length was a bit too far and with my 6mp sensor ~300% crop was a bit out of the question

How was it too far? Your Tamron is a 17-50mm, meaning you should be testing at 50mm and about 4 feet. For the 70-200mm at 200mm, that's 16 feet from the chart. Also, I'm not sure where the 300% crop comes from...

The directions said as close as possible

The directions are wrong. Sorry. Canon (Chuck Westfall, actually) actually recommends 50x the focal length. Regardless, performance at the minimum focus distance is going to be different that at typical focal distances. Unless you always plan on shooting at the MFD (e.g. with a macro lens), you should not test at the MFD, but rather at a 'typically used' distance, which the 25x focal length approximates.


At the distances you tested, your AF looks ok (assuming you took multiple shots, at least 3, and the ones you show are representative of them all) - but you should retest at the proper distances.

[TakahiroW4047]

How was it too far? Your Tamron is a 17-50mm, meaning you should be testing at 50mm and about 4 feet. For the 70-200mm at 200mm, that's 16 feet from the chart. Also, I'm not sure where the 300% crop comes from...

I may have a misconception about the "% crop" terminology. I was trying to match the photos given in the directions by showing +/- 20mm range. When I'm trying to fill my screen with that, PS is telling me I'm zooming in ~260% or so. Maybe "zoom" doesn't equal "% crop"??? [^o)]


When I "zoomed in" the photo I took 4ft away @50mm with Tamron 17-50mm, the photo is very pixely (I shoot raw, only convert to jpeg at the last stage from the tiff files).


I shoot with a 300D Rebel, and it's 6mp isn't quite up to the cropping challenge


Tamron 17-50mm shot 4ftaway.


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These are for Canon EF 70-200mm f/4 USM shot 10ftaway (couldn't go 16. I hit the wall [:^)])


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[neuroanatomist]

It looks like the Tamron is front-focusing, just a little bit. The 70-200mm is back-focusing, not horribly but definitely noticeably. The Tamron'sfront-focusingis unlikely to substantially affect images, but the 70-200mm'sback-focusing will probably have an effect in critical focusing situations, (wide open with a close subject).


It's this sort of thing that AF Microadjustment is designed to correct.

[TakahiroW4047]

It looks like the Tamron is front-focusing, just a little bit. The 70-200mm is back-focusing, not horribly but definitely noticeably. The Tamron'sfront-focusingis unlikely to substantially affect images, but the 70-200mm'sback-focusing will probably have an effect in critical focusing situations, (wide open with a close subject).


It's this sort of thing that AF Microadjustment is designed to correct.

Wow!! It's amazing how much impact a fraction of an inch has on the focus!! I'll keep working at this to nail it down. Thanks neuroanatomist!! Now it's a matter of deciding between saving up this Summer for the refurbished 7D, or 50D+Kenko extension tube set...hmmm []