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

[TakahiroW4047]

So I just went out today to a nearby lake to test out my new EF 70-200mm f/4L USM. I realized as I was taking photos of birds that 200mm isn't quite long enough for this purpose. So major cropping was involved : (


My question was, the photos of wild life that were quite close (10-15 ft away) were not quite in focus. I was shooting ISO100 and +1/1000 exposure. Which made me wonder, if the lack of being able to use the center f/2.8 autofocus point had some significant effect?


I shoot with an ancient 300D Rebel so I wouldn't be surprised. But I've taken shots with my Tamron 17-50mm f/2.8 non-vc which have had far better focus seen at 100% crop...


Is this something that could be avoided if I just had a better AF on my camera? Would the difference be negligible at that focal length? Or is this the burden of having a maximum aperture of f/4...? If so I would return this lens in a heart beat, bite my lips for now and save up [:'(]


p.s - this lens was also a refurbished one...odd considering I found out after I bought it Adorama is selling a brand new one for the same price...?

[neuroanatomist]

There are a few possibilities. Use or lack of use of the high-precision f/2.8 center AF point is one of them, but probably the least impact.


Here's the thing - the thinner the depth of field, the more critical focus becomes and the more significant the impact of AF problems becomes. Even though f/4 is not really 'fast', aperture is not the only factor affecting DoF. The other two main factors are focal length (longer focal length for a given subject distance means shallower DoF) and distance to subject (closer subject for a give focal length means shallower DoF). So, with a relatively long (200mm) lens and a relatively close subject especially for that long a focal length, you'll get a very thin DoF. Using the DoFMaster calculator, a 1.6x crop body, 200mm f/4 and the subject 12 feet away, the DoF is around 2" (<5 cm).


So, the first question is, how large was your subject? If it was anything other than a small mammal or small bird, a 2" DoF means that the whole creature is not going to be in focus, even if the AF is spot-on. Perfectly correct AF would get you the eyes (which should be under your AF point) in focus, but maybe not the ears/beak/body/whatever. That means you may not have any AF issues at all!


However, if the exact spot you're aiming at is out of focus (OOF), you may have AF issues to sort out. If you open an image (RAW or JPG) in DPP, you can select AF Point on the View menu to check which point you used (unless you focused then recomposed - but if you did that in the situation above, with a very thin DoF to begin with, you're pretty much guaranteeing an OOF shot).


If you are having bona fide AF issues, it could be the lens, could be the body, or could be the combination of the two. Read "This lens is soft" and other myths on Roger's site for a complete discussion of the issue. The bottom line is that with your camera, your only option would be to send the lens into Canon, and if that tests out fine, you'd have to send both the lens and the body (and if they calibrate the body, you may find that the 'problem' is transferred to the Tamron lens!). AF Microadjustment is one reason why I moved from the T1i to the 7D.


In this case, with a very thin DoF, the f/2.8 center point might help. The f/5.6 points are accurate to within one DoF (whatever that is for the aperture/focal length/focus distance), whereas the f/2.8 high-precision AF point is accurate to within 1/3 of the DoF.


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.


Hope that helps.


--John


Ps. Adorama might be selling a new one now for the same price you paid for a refurb, since there is currently a $45 rebate on that lens (the rebate runs 5/9 - 7/10).

[TakahiroW4047]

As I'm looking back and taking a closer look at the photos, I'm realizing that alot of the close up photos that were out of focus were in darker parts of the woods. So that, plus the very shallow depth of field would explain that. Which would be understandable.


However, the longer focal length with large subject distance seems to have very low hit rates as well. I used your link and estimated that at 200mm and a decent distance away I have well over a foot of DOF. I focus then recompose, but I'm thinking I'm not that big of a swayer [:P]


So I'm thinking maybe the subject was too small? aka. need a longer focal length!! I'm not sure how big my subject needs to be in order for the AF to properly focus... Maybe I'm just asking for the impossible. I've placed a roughly identically sized box around where I aimed my center AF point, then recomposed.


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[TakahiroW4047]

Thanks for the great links!! It was past midnight last night so I didn't get the chance to go through them in detail. I suppose it wont matter whether the calibration between the lens and camera is off for me since I don't have Microadjustment. But it'll be interesting to see how they match up. I'll definitely take the time to check it out!! It may be enough incentive for me to consider my next purchase to be a new body!! []

[neuroanatomist]

So I'm thinking maybe the subject was too small? aka. need a longer focal length!! I'm not sure how big my subject needs to be in order for the AF to properly focus...

The actual size of the AF point is larger than the box you see in the viewfinder (or in DPP). So, in your example it was integrating the bird and some of the tree branches in the focus calculation. My 7D offers 'Spot AF' which effectively uses a smaller portion of the AF sensor for more accurate focus. I have found that to be a big help when trying to focus on a small bird surrounded by a lot of branches (or in this case, a distant large bird).


In the shot above, if your goal was to capture the bird, you'd have benefitted from a longer lens, yes.

[TakahiroW4047]

The actual size of the AF point is larger than the box you see in the viewfinder (or in DPP). So, in your example it was integrating the bird and some of the tree branches in the focus calculation. My 7D offers 'Spot AF' which effectively uses a smaller portion of the AF sensor for more accurate focus. I have found that to be a big help when trying to focus on a small bird surrounded by a lot of branches (or in this case, a distant large bird).


In the shot above, if your goal was to capture the bird, you'd have benefitted from a longer lens, yes.

I learn more and more every day!! Thank you so much for taking your time to answer my questions!! I appreciated it lots! []

[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.