Cross-type AF sensor question

This is slightly OT as it relates to my EOS 3 film body but I'm sure the technology is the same as EOS DSLR's so hopefully someone can help.

The EOS 3 has 45 AF sensors of which 7 are cross type (dependent on max aperture lens used). I've just noticed that with my EF50 f/1.8 fitted, only the top sensor of the group of 7 is working as a cross type. All the others work fine as vertical sensors i.e. locking onto horizontal lines.

Is this likely to be an expensive repair, given the value of the camera is only around 100 uk pounds/$160? I would like to have it fixed but I suppose I could live with it and use focus/recompose with the good sensor when focus was critical but that sensor won't operate as cross type with my 24-105L where because of the f/4 aperture, only the centre sensor can be used as cross type.

Someone else may be able to weigh in here to contradict this, but I think it's possible that there is nothing wrong with your camera. At times as technology has improved, new sensors have been added into cameras that require certain lenses to activate those sensors. Although they often require a wide maximum aperture to activate the sensors, that is usually not the only requirement. Some lenses even with wider apertures don't activate them for some reason or another.
The most recent example of this is the Canon 5DmkIII. "Group A" lenses will use all of the diagonal cross points. Group B will use the center diagonal and the rest of the cross points. Group C lenses will use cross points but none of the diagonals.

Canon lenses are grouped A through H. An "L" designation does not automatically make it a Group A lens.

So what you need to do is first check the lens and see which "group" Canon puts it in. Then refer to your owner's manual and see which lens groups activate which AF points.

I just re-read that and realized you mention the "top" sensor of the group. This is indeed odd. I thought you were referring to the center point when I read it the first time. If you are certain it is the top point working as a cross and none of the others are, and it's a pain in the rear for you I would send it in to a camera repair shop. It isn't warranty so any shop should be fine. They should give you an estimate and then you decide if it is worth it or if you want to just buy another camera to replace it. If you do that you'll likely have a diagnosis fee to pay and that's something only you can decide if it is worth it or not.

When it comes to repairing an "obsolete" camera, I wouldn't worry about it being obsolete. If you are shooting film, you need a camera and the EOS 3 is a nice one. Don't get caught up in the "market value" aspect. That market value is based on the number of willing buyers and the number of used ones there are on the market. I still shoot film from time to time with my EOS 7e and I wouldn't hesitate to repair it if needed and I would pay any cost to repair it up to what it would take to buy another from someone else. At that point I would just replace it with another. Canon doesn't make film SLRs any more so these cameras represent the peak of the technology and from a market perspective they are terribly undervalued which is a terrific thing for a film shooter.

I'd be more inclined to get another camera, instead of trying to repair it. Repair mean labor, and labor means cost, regardless of the market value of the device you are trying to repair. Buy another used camera, and have this one as an alternative to it.

That assumption unfortunately is not valid. There is a good thread on the topic at http://photo.net …gital-camera-forum/00axVT.

Isn't it the same person asking the question, then? Seems identical.
Besides, that thread doesn't say anything but that focusing is identical with digital cameras.

Yes, you are correct, I guess the guy is just looking for answers but I'm not convinced he has a camera issue.

One thing, cross-type autofocus points can detect both horizontal and vertical lines simultaneously but often the sensitivity in the two directions differ. In some of the latter DSLR models the sensitivity is the same in both directions but not sure if that was the case back in 1998. There is a good explanation of the focus point type at http://photo.stackexch​ange.com …ing-cross-type-or-high-pr.

I've carried out a more detailed test using a white target with a solid black vertical line. I used the EF50 f/1.8 because that should use all 7 cross-type sensors as illustrated in the diagram below. What I found was that the top sensor achieved focus 100% of the time. The two on the next row down got a focus about 60% of the time. The centre sensor achieved focus about 30% of the time and the last two rows seldom if ever achieved focus on the vertical line.

So the vertical sensitivity is reducing from top to bottom (horizontal sensitivity is fine on all sensors). Now I'm only speculating but I think varying sensitivity is due to the distance between the secondary mirror and the AF sensor module. The sensors that are the most sensitive are the ones that are closest to the angled secondary mirror. Incidentally, when I tried my 24-105L which should only activate the centre crosstype sensor due to its max aperture of f/4, it didn't work at all. I need to repeat the test outside in natural daylight to see if things improve.

There isn't dirt or anything blocking part of the light ray paths towards the AF sensor optics, is there?

apersson850 wrote in post #15187493
There isn't dirt or anything blocking part of the light ray paths towards the AF sensor optics, is there?

Its difficult to say. I don't want to prod around too much in there. When the primary mirror is down, I can lift it a little, enough to see the secondary AF mirror and it looks clean. If I raise the primary mirror up completely with a bulb exposure (mirror lockup doesn't work with lens off), the af sensor module seems to get covered with a small flap which closes as the mirror goes up. All I can do is blow some air around with my rocket blower but without any improvement.

50mm f1.8 isn't exactly known for accurate focus.

Interesting. I say send it to a repair shop and have them take a look. If it costs too much to repair, buy another.

tkbslc wrote in post #15187787
50mm f1.8 isn't exactly known for accurate focus.

No, but regradless of that, the camera's AF points should be able to detect an obvious contrast in the direction they are supposed to.

I have one, perhaps peculiar, question: Does it work any differently if you hold the camera upside down?

apersson850 wrote in post #15188082
No, but regradless of that, the camera's AF points should be able to detect an obvious contrast in the direction they are supposed to.

I have one, perhaps peculiar, question: Does it work any differently if you hold the camera upside down?

No, it doesn't work differently when I hold it upside down. I will try it outside in natural light at the weekend.

OK. Then I have no clue. Looks like something is skew or malfunctioning in your camera.