AF phenomenon on the 70D: Can anyone explain?

Started Dec 18, 2013 | Discussions thread
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chbaum
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AF phenomenon on the 70D: Can anyone explain?
Dec 18, 2013

(this is gonna be a very long post)

Hi everyone,

for three weeks now, I've been looking for experts who can comment on a phenomenon that I see with my EOS 70D under specific circumstances. Someone who knows, how a phase AF system works and what the reasons for such a phenomenon could be.

There are quite a few side notes I want to make before I come to the point:

- The phenomenon is discussed wildly and VERY emotionally in two big German DSLR forums

- It is NOT discussed anywhere else in the world! It came up on dpreview, but it was "imported“ by German users and quickly died off because nobody outside Germany seemed to care or seemed to have a problem.

- The debates get extremely heated between Canon bashers who see a problem, a fault, a rip-off by Canon everywhere and others who don’t have any issues with their beloved 70D and try to defend the product and keep potential customers from NOT buying the camera.

- This makes it extremely difficult to see whose camera actually HAS the problem and who just didn’t understand the problem.

- Germans tend to overreact, overdesign and overthink everything. Please don’t blame me for my tests. Just have a look at the results and try to tell me how an AF system might deliver such results.

- Since I’m half a scientist, I readily accept hints on faulty test setups.

- I’m scientifically interested in the whole thing rather than bothered by it. In everyday life, I probably never will see the phenomenon because I lack the lenses and the photography style. Others however might find the 70D completely unusable, depending on their workflow or objects / subjects.

Hypothesis: Some (at least mine) 70Ds deliver a random mix of in focus and extremely out of focus images under the following circumstances:

- fast lenses

- big apertures = low f-stops

- phase AF via viewfinder

- manual selection of the AF field in the middle

- object more than 5m away

I know that many of the lenses tested in that fashion are never actually USED in that fashion (who would take an image of an object in a distance of >10m with a 50mm lens @f1.8?). But that’s not the issue here - there certainly ARE people who use fast lenses wide open on far away objects (e.g. 70-200 @f2.8), and the phenomenon is not supposed to occur, relevant or not. Lenses mentioned by German users are: Canon 50 / 1.8 and 1.4, Sigma 18-35 / 1.8, Canon 85 / 1.8 (or was it even below that?), Canon 100 / 2.8 macro non-IS, Canon 70-200 / 2.8 IS II.

I tested the hypothesis with my Canon EF 50 / 1.8 II at f1.8. I shot from a balcony at daylight. Object was either an empty flowerpot in the yard or a garage roof with pebbles and moss, about 20m away. I used a stable tripod, mirror lockup and 2 sec timer, ISO 200, manual AF select, One Shot mode, mid AF field. I put manual focus on infinity, focused via AF (half-pressed shutter), took the image, manually de-focussed and shot the next image. I took 20 images for each test.

Result: 50-60% of the images were extremely out of focus. The AF did not hit anywhere NEAR the focussed region. Please note that I’m very much aware of the weak image quality of this lens at f1.8. However, the images should have been [i]consistently[/i] "weak“. But they aren’t. Half of them are completely off.

With the next test, I only changed one thing: I manually chose the neighboring AF field, above the middle.

Result: 1 of 20 images was off. All the others showed the lens’s weakness, but the AF had hit its mark every single time. The images are almost undistinguishable if browsed through. Exactly what I would expect from such a camera. This test was made directly after the first, that is 20 images later. Not an hour later or a day. 20 images later.

Believe me, I tested my guts out. I did it three days in a row, two very cloudy, one very sunny. I tested different ISOs, different metering types, AI servo mode. I even tested inside my flat’s hallway (about 5m long) with a stuffed animal as the object. The results stayed the same: The phenomenon (at least 50% unacceptable images) occurs ONLY with the middle AF field at f-stops below f2.8 and objects far away. It NEVER occurs with any neighboring field, never with higher f-stops from 3.2 upwards, never with closeups, never with live view AF.

I uploaded cropped JPEGs of two of my setups on my Dropbox. All my other setups (hundreds of megabytes show the very same results, so this is only one example of many). Please feel free to have a look. The folder names tell you about the used lens and settings. (AF-Feld Mitte = middle AF field / AF-Feld oberhalb Mitte = AF field above middle field).

flowerpot:

https://dl.dropboxusercontent.com/u/37645229/EOS70D_Testreihen_6-10_CBaum.zip

roof:

https://dl.dropboxusercontent.com/u/37645229/EOS70D_Testreihe_11a_11b_12_CBaum.zip

1) Can anyone here tell me, how an AF system might show such a strange behavior?

2) Why does only the middle AF field show it, and the neighboring fields don’t?

3) Can this be a hardware problem? If so, what kind?

4) Could there be a software problem that can be solved with a firmware update?

5) Why is this problem only discussed in two German forums but nowhere else? About 10 people claim to have verified the problem with their cameras at the moment. They collected serial numbers, but there was no pattern except the fourth and fifth digits are always 02 (somebody claimed this to be the second hardware revision manufactured).

There’s so much speculation, you completely lose overview.

Any ideas what the problem actually could be (except for Germans with too much time on their hands - please don’t start with that kind of comment, I also shot real (and really nice) images during the last three weeks ?

Best regards,

Christian

 chbaum's gear list:chbaum's gear list
Canon EOS 70D Canon EF 50mm f/1.8 II Canon EF 100-400mm f/4.5-5.6L IS USM Canon EF 100mm f/2.8 Macro USM Sigma 17-50mm F2.8 EX DC OS HSM
Canon EOS 70D
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