Focusing...what does what? UPDATED

digitalfailure

digitalfailure

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Can someone pleeeeeeeease define the process used by Canon's AF system?

That's a rather random question I hear you say......but read on, the cause of my confusion will all become apparent.

I think I might have an issue with the AF in my 5D as I've never been 100% happy with the images it's produced and the ratio of crud to keepers is very very high....even by my own standards.

I had thought that my abissmal success rate was a combination of user error (come on...it's me we're talking about here!) AND circumstance (low light making focus lock a little harder) as it seemed to affect all my lenses.

However, I bought a 24-70 f2.8 L the other week and thats really showing up some strange image defects :crying:

At the long end of the zoom say....50mm to 70mm things look OK, but at the wide end the images are very very soft and some would even appear to show areas in the back ground to the left as being in perfect focus :eek:

so......what happens.....what decides where the correct focus is because I feel like chucking the whole lot through the window.

The subject last night was illuminated by flash at a shutter speed that was far faster than the focal length and the iso was pretty low, one such example was 1/200th ISO 125 @f4.5 and the subject was pretty static.

Reviewing the LCD showed the little dot to be right on the face too :(

Helllllllllp please.......I'm not sure if I need to get the body checked or the lens and put previous poor images down to be being crap.
 
The AF works by adjusting the focus until it gets best contrast. Test it by using something with high contrast like a newspaper headline with the paper on a table at an angle to you. Get the AF square half on the black and half on the white so it has best possible contrast... then see how the focus looks.
 
I understand the principle Robert, I'm really trying to ascertain (albeit maybe rather poorly) what part of the EOS system does it, is it the body or the lens?

I'm getting soft images with various lenses, but none as serverly soft as the new 24-70.

Also, I can't get my head around why the wide end of the range seems to be affected more than the long end :thinking: my understanding is that both the zoom and the focus are 2 seperate systems. From what I saw last night when using the center AF point at 35mm, my chosen point of focus.....where the little red square showed confirmation on the LCD...was soft to the extremes but an area to the left and 6ft or so further back was pin sharp. The model was in a static pose, so there was no recompostion of the subject.
 
no idea, but i think it the body that does all the focusing, send the cam and lens to canon to adjust to suit???
 
I had my 5D calibrated by Canon as the servo mode was slightly off. Been OK since I think.

Bear in mind the actual focus point isn't exactly where the view finder says it is. Try shooting something behind wire fencing to see what I mean.
 
I'd still do some test shots under controlled conditions with each lens. Then at least you will know if there is a problem rather than think there might be.
 
pearce_jj said:
I had my 5D calibrated by Canon as the servo mode was slightly off. Been OK since I think.

Bear in mind the actual focus point isn't exactly where the view finder says it is. Try shooting something behind wire fencing to see what I mean.
Click to expand...


Out of interest, what sort of costs were involved with the calibration....i'm sure mines never been right from new and it's only got 4.7k clicks on it now......of those I'm sure 4k have never made it off the CF card.


Robert, I'll be doing some tests this evening.....i hope it's the lens tbh and all my other failings are down to me.....but the softness when using other lenses suggests it may be the body :eek:

time for a PX and a 5dII
 
sounds a great idea DF, another new camera to sit and polish at home!!:LOL:

personally, if the 5d only has 4700 clicks, i would get it back to them to fix...

ive heard people say not to use angled test sheets as it can confuse things, three or more compactflash cards facing the cam, but touching the back of the one in front is more useful so i heard:shrug:
 
My service was gratis as I picked it up after only a few months. But I read somewhere it is usually about £70.
 
Without getting into too much detail Canon's AF works as follows...

The AF sensor "checks" the contrast and if it can determine the amount of adjustment needed it tells the lens how much to move. If it can't it will "hunt" until it finds something it can work with. This is a open loop system, once the camera has told the lens where to go it's assumed to be right and not checked.

Servo mode obviously does a lot more as it also predicts where the subject is going to be and adjusts accordingly. Servo mode is still "open loop" but as it's constantly repeated errors can be corrected to some degree.

Focus calibration might need to be done on the body or the lens. The AF assembly might need moving in the body or the lens might be under or over shooting the mark or a combination of the two. One simple check from a tripod in single shot mode is to focus on something and then focus again, if the lens adjusts it could indicate a calibration problem as it clearly didn't get it right the first time :(
 
The supplier (Jessops) returned it to Canon.

Interesting stuff there Pxl8 - seems like an obvious area for improvement then (or is that the preserve of the 1 series?).
 
If it's any help my 1DS just came back from Canon after a focus check ( Free I'm glad to say) and even more important, no problems. According to the invoice it would have cost £30 with £16 shipping. Both credited.

Best give Canon service a ring at Elstree.
 
Switching to a closed system won't really help with calibration issues as the AF would just end up stuck in a loop of getting it wrong. AIUI Nikon uses a different system that's a bit like hunting until it's right but that is inherently slower as a result and a change like that would mean an end of the EF lens system :eek:
 
Hi Guys

Firstly thanks for the replies :)

I've spent the evening messing about with a couple of lenses and a pair of bodies and a flash gun and after countless shots of the same item on my dining room table I have come to the conclusion that my brand spanking new 24-70 f2.8 L is OK

However my beloved and little used 5D body is poorly :(

on one set of shots at 70mm there was a distinct difference between the image quality of the clutter between the 2 images.

to summerise the test set up

I mounted my 5D body on my tripod with the 24-70 and a flash and then focused on an area of text on the face of my kitchen scales. That was nice bold hard edged text in white upon the blue perspex.

with the lens set on f2.8 for minimal DoF i fired 2 shots before replacing the camera body for another 5D that I'd managed to commandeer :D

Without moving the tripod, I swapped the bodies and fitted the lens and flash and made sure "almost :whistle:" all the settings were the same. I didn't notice the flash compensation was dialed down a stop on the donor body. But all other settings i.e shutter, app and iso were the same.

Taking great care to recompose the shot exactly as previous I again fired 2 frames.

The difference was more than I imagined :eek:

The distance to subject from the front element was a fraction over 7.5ft and beyond the subject were a few magazines and a newspaper.
The images from my body ON the focus point zoomed to 100% were very soft but I could clearly read the text on the spines of the magazine and even an area of print on the paper (apparently chester got beaten 2-0 by Aldershot at the weekend) some 15 inches behind the subject !!!!!!!!!!!!

On the donor body the recorded images from the same scene were much sharper on the point of focus and a lovely bokeh threw the background clutter into a blur as you'd expect using f2.8

The images weren't quite so pronounced at 24mm but still noticeable when you look for it.

repeating the test again with my siggy 70-200 f2.8 and a borrowed canon 70-200 f2.8 IS also showed my image of the same point of focus to be softer at 70 and 200

So.......it looks like my 5d is off for a service :(

But, on the bright side......maybe....JUST MAYBE my crap images of late havent really been all my doing.
and finally, the moral of the story

Use your kit every day and you'll notice when it's not working correctly, instead of coming away from an event thinking that lack of practice / technique is to blame.
 
Glad your not as bad a tog as you though DF.

Hope it gets fixed quickly. can we see some of your test pics?
 
:agree: we want pictures
 
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