testing 7D autofocus

Started Dec 30, 2010 | Discussions thread
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padang
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testing 7D autofocus
Dec 30, 2010

Hi all,

I have been pretty pleased by my 5Dii, and the only area of improvement I could really think of is the AF. I thus tested the 7D for a little week, just to see what the best-of-breed AF is nowadays.

In short, I did not see much difference in speed.

I have not used one of the most important features of the 7D: the capability to enable multiple focus points. For the type of photography I am doing, it is never a challenge to keep the focus point on my subject. However I can see how for BIF or team sports some people may need it. I guess it is where the 7D advantage really is.

However the cross-type points make a huge difference in some scenes, and I could make use of it. The feature is also available in the 60D, with same IQ and must smaller/cheaper package than 7D.

Servo Mode

I thought it would be a pretty simple test, a child on a skateboard using the center point. Light was good, ISO200 f/2 and 1/1250s. Speed is pretty consistent, going gently. The target is very contrasty, with b&w zig zag lines. Using 50mm 1.4 USM.

full-size: http://www.flickr.com/photos/ltregan/5306567712/sizes/o/

I configured the camera to use the AF-ON button, and kept pressing on it. Took about one pic every second, which - I think - gives enough time to the AF to refocus in-between the shots.

Only about 50% of the shots were in focus (100% crops):

(the red line is the focus zone as shown by DPP)

It is possible that the 7D, in this configuration, is better than the 5Dii but I could not measure it.

One-Shot

In good light, the 7D focuses really very fast. So does the 5Dii and the XSi. It is mostly in dim light that I have been frustrated with the 5Dii. What is the point of high-ISO if you cannot focus in the first place ?

I am a little mean here, it is rare that my 5Dii refuses to focus using the center point. However the issue is more common with the lateral points in dim light. Here the 7D cross-points have shown to be very beneficial. One could use focus & recompose with the center point, but with a wide aperture there is a focus shift: it is ironic that in low light you cannot use the lateral points, so you need to focus with the center point and recompose, which can give inaccurate focus... To be fair, this focus shift is not really visible at web-sizes.

Anyway: in cases where the 5Dii can focus, how does it compares with the 7D ?

I used the following pretty dim scene:

Both camera will focus quickly on the contrasty sticker. When I rotate my camera a little bit to the right, I get a less contrasty thing to focus on:

These two shots are for illustration purpose: f/2, ISO3200, 1/640s. However I did the test at f/2.8, ISO100, 1/200s and the frames were just black.

So the comparison was to take ten shots with the 5Dii with the 85mm 1.8 and the 7D 50mm 1.4 and measure the time - alternating between each shots between the skate and the pumpkin. Why not use the same lens: because I wanted to compare the systems at equivalent FOV - lens+body comparison.

The 7D took about 12s to take the ten shots, and 14s with the 5Dii. So pretty equivalent in usage.

Spot Focus

Farther point to the right the 7D will focus in normal mode:

Farther point to the right the 7D will focus in spot mode:

The camera was pointed on a tripod to avoid any shake. As per the experiment, the spot focus mode reduces the focus zone surface by about 50%. It makes some improvement, but it is worth noticing that spot is far from a dot - I can think of cases where the eyebrow would be caught instead of the eye for instance.

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