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! []

[scalesusa]

My 70-200mm f4L focuses sharply at all focal lengths and distances. If yours doesn't, something is wrong. It is normal for shots at MFD to be slightly less sharp, but they are still excellent, just not astonishing.


I'd send it back and get a new one for the same money.

[TucsonTRD]

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.


Focus and recompose is not a very good technique either. Even in one shot mode. I don't know your camera at all, so I can't speculate on the settings you may have been using and what you have available.


I was under the impression that all refurbished lenses are tested prior to being resold. In theory, I would imagine they should be more reliable than brand new lenses. I have the 70-200 f4 and I love it. If indeed there is something wrong with the lens, you should send it back and get a replacement as it is a great lens to shoot with.

[TakahiroW4047]

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?

Focus and recompose is not a very good technique either. Even in one shot mode. I don't know your camera at all, so I can't speculate on the settings you may have been using and what you have available.

The 300D is the very first "Rebel" of the series, and although it still takes great photos to an extent, it only has 7 focus points. So there is always some "focus and recompose" involved since I often times leave the subject in the corners.

The actual size of the AF point is<span>largerthan 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.

Most likely my biggest downfall in this situation. 200mm wasn't long enough to have only the subject fill the AF point. Took some photos of my friends playing Tennis earlier this week and they were very good. I now just need a better AI Servo and Spot AF [:P]

[TakahiroW4047]

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?

Focus and recompose is not a very good technique either. Even in one shot mode. I don't know your camera at all, so I can't speculate on the settings you may have been using and what you have available.

The 300D is the very first "Rebel" of the series, and although it still takes great photos to an extent, it only has 7 focus points. So there is always some "focus and recompose" involved since I often times leave the subject in the corners.

The actual size of the AF point is<span>largerthan 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.

Most likely my biggest downfall in this situation. 200mm wasn't long enough to have only the subject fill the AF point. Took some photos of my friends playing Tennis earlier this week and they were very good. I now just need a better AI Servo and Spot AF [:P]