AF issue - is this normal?

fingerz · 24-01-2006, 12:07

With my Sigma 18-50mm F2.8, I frequently find that the autofocus at 18mm isn't as sharp as the autofocus at 50mm.

So, assuming I want to take a shot at 18mm, what I usually do is this. I'll frame at 18mm and do a quick autofocus, then zoom in to 50mm and autofocus again. At 50mm it becomes apparent that the 18mm autofocus was quite a way off. I then switch to manual focus, zoom out and take the shot at 18mm.

Is this normal with zoom lenses? I'd specifically like to hear from you 17-40L users, since that's what I'd probably look at getting if I was to replace my walkabout lens.

What causes this? I thought autofocus was down to the camera body, and all it did was tell the lens motor "left a bit, right a bit, that's lovely" (type thing). I don't remember this happening quite so much with the kit lens, although I always zoomed in to check anyway, for good practice.

It's not a huge issue with shots of static objects but for quick 'grab' shots, it'd be nice to be able to rely on the AF at the wide end of the zoom, but I really can't at the moment. I'd do a comparison myself but now I've sold the kit lens, my only other zoom lens is the Sigma 70-300 (the cheap one) and that's probably not a good test.

24-01-2006, 13:16

I thought once the focal length changes, then the need to focus changes.?

That's why at 50mm, your 18mm focus was way off, because it's not correct for 50mm.

So in effect, you're doing the following

Adjust to 18mm
Auto Focus to 18mm
Adjust to 50mm
Auto Focus to 50mm
Switch to manual
Adjust to 18mm and take the shot (with a focus that is correct for 50mm).

Arkady · 24-01-2006, 13:28

That is correct. You must re-focus at different focal lengths because the glass elements within the lens have moved, altering the point of focus.

This happened even on M/F zooms and is quite normal.

Focus - Frame - check focus - shoot.

Ribby · 24-01-2006, 13:34

Quote:

Originally Posted by Marcel

So in effect, you're doing the following

Adjust to 18mm
Auto Focus to 18mm
Adjust to 50mm
Auto Focus to 50mm
Switch to manual
Adjust to 18mm and take the shot (with a focus that is correct for 50mm).

....But the focus still seems sharper than the 18mm auto-focus??

That's not right is it?

Arkady · 24-01-2006, 13:36

You're sayingthat at 18mm the AF isn't precise enough?

Warspite · 24-01-2006, 13:39

I would expect the focus to drift at longer focal lengths - sounds bizarre that its accurate at the longer end and getting worse at the shorter range

dazzajl · 24-01-2006, 14:04

Firstly, I've always found the focusing accurate through the zoom range with the 17-40L and if you have the opportunity to swap to one of them I'd definatley recommend it.

As for why your lens is causing problems at the wide end, I can think of a couple of reasons off the top of my head. One would be that there is a problem with the lens and it is interpreting the instruction from the camera wrongly. The other is that the lens may be giving less contrast at 18mm than 50mm and since the autofocus basically works by looking at the contrast at a certain point it's not as accurate.

fingerz · 24-01-2006, 15:07

So if focus changes depending on the focal length, and I'm focusing for a 50mm shot but then flipping to manual and zooming out again, my shots ought to be quite noticably out of focus, then?

I'll do a test but I can't say I've ever noticed them being out of focus when I've used that method.

Will devise a cunning way to check and do it later tonight to see what it looks like.

Arkady · 24-01-2006, 15:13

At 18mm you have more DoF which might account for not noticing any focus-variance.

fingerz · 24-01-2006, 16:05

True.

Just did a preliminary handheld test with the camera here at work and it did seem sharper using the correct method (ie focusing at 18mm, not 50mm) so looks like my mistake. Perhaps I can rely on it at 18mm after all. Hadn't really used it for grab shots at the wide end because I assumed it wasn't up to it but I may give it a go now.

24-01-2006, 16:55

Yay! I got one right, mister, I gots one!

***Dave*** · 24-01-2006, 19:09

After seeing some of your shots with this lens, I was tempted to purchase a few months back. But then I read reviews on FM http://www.fredmiranda.com/reviews/s...&cat=37&page=2

For me there were too many people getting AF problems, so I decided against it. Have you considered you may have a problem with the lens. Might be worth going back to shop & trying another lens to see if problem is across the board or hopefully just yours, which you can get repaired / replaced

fingerz · 24-01-2006, 21:42

After being schooled on the correct procedure for focusing, I think the lens is actually pretty decent now.

digitalfailure · 24-01-2006, 21:44

What aperture are you on here?

It does sound more like a defective lens to me.

When you re focus using the manual control, your using your eyes and noggin to determine the correct focus. The camera isn't doing anything.

download a focus testing chart from somewhere and set your camera up on a tripod in a room with average lighting. manual focus on the chart at 18mm and take the shot using a remote release or self timer, adjust the zoom out the the end of the lens and focus lock, before adjusting back down to 18mm and selecting AF.
Once the camera has gained focus lock on the same point at the same focal length, take the shot again.

Compare the 2 shots and see which is sharper.

SammyC · 25-01-2006, 09:55

Try this:

http://www.focustestchart.com/chart.html

fingerz · 25-01-2006, 11:17

The problem there is that I have never, ever, in my entire life been able to take a manually-focused shot that was as sharp as an autofocused shot.

In fact, I constantly recoil in amazement when people talk about switching to manual to get better results. Switching to manual is something I only ever do in very low light (less so now I have an ST-E2). All my results from manual focus have been shocking with one exception - macro shots where you focus as close as possible and then just move the object itself nearer or farther from the lens. In those situations only my manual focusing is ok.

CT · 25-01-2006, 11:25

Quote:

Originally Posted by fingerz

The problem there is that I have never, ever, in my entire life been able to take a manually-focused shot that was as sharp as an autofocused shot.

In fact, I constantly recoil in amazement when people talk about switching to manual to get better results. Switching to manual is something I only ever do in very low light (less so now I have an ST-E2). All my results from manual focus have been shocking with one exception - macro shots where you focus as close as possible and then just move the object itself nearer or farther from the lens. In those situations only my manual focusing is ok.

You surprise me Jamie. SLR's used to have a split image fresnel lens in the centre of the focusing screen which was a big aid to focusing. With a DSLR though, and certainly the 20D, you still get the green focus confirmation light when you focus manually. (and the beep if you enable it)

25-01-2006, 11:29

Quote:

Originally Posted by CT

You surprise me Jamie. SLR's used to have a split image fresnel lens in the centre of the focusing screen which was a big aid to focusing. With a DSLR though, and certainly the 20D, you still get the green focus confirmation light when you focus manually. (and the beep if you enable it)

Really? I never knew that, I wonder if that's still applicable with the 350D. (I'll try and do a test later).

I'd love to turn my hand to manual focus, for things like night shots, so this would be a great tool in learning to see focus through the viewfinder, which, TBH without the split image, is bloody hard.

24-01-2006, 12:07	#1 (permalink)
fingerz Forum Regular Join Date: Jul 2005 Posts: 790	AF issue - is this normal? With my Sigma 18-50mm F2.8, I frequently find that the autofocus at 18mm isn't as sharp as the autofocus at 50mm. So, assuming I want to take a shot at 18mm, what I usually do is this. I'll frame at 18mm and do a quick autofocus, then zoom in to 50mm and autofocus again. At 50mm it becomes apparent that the 18mm autofocus was quite a way off. I then switch to manual focus, zoom out and take the shot at 18mm. Is this normal with zoom lenses? I'd specifically like to hear from you 17-40L users, since that's what I'd probably look at getting if I was to replace my walkabout lens. What causes this? I thought autofocus was down to the camera body, and all it did was tell the lens motor "left a bit, right a bit, that's lovely" (type thing). I don't remember this happening quite so much with the kit lens, although I always zoomed in to check anyway, for good practice. It's not a huge issue with shots of static objects but for quick 'grab' shots, it'd be nice to be able to rely on the AF at the wide end of the zoom, but I really can't at the moment. I'd do a comparison myself but now I've sold the kit lens, my only other zoom lens is the Sigma 70-300 (the cheap one) and that's probably not a good test.

24-01-2006, 14:04	#7 (permalink)
dazzajl Been here a while Join Date: Apr 2005 Posts: 317	Firstly, I've always found the focusing accurate through the zoom range with the 17-40L and if you have the opportunity to swap to one of them I'd definatley recommend it. As for why your lens is causing problems at the wide end, I can think of a couple of reasons off the top of my head. One would be that there is a problem with the lens and it is interpreting the instruction from the camera wrongly. The other is that the lens may be giving less contrast at 18mm than 50mm and since the autofocus basically works by looking at the contrast at a certain point it's not as accurate. __________________ "objects become distinctly less photogenic when they are over 500 yards from the car"

24-01-2006, 21:44	#14 (permalink)
digitalfailure Forum Regular Join Date: Jan 2005 Location: on the floor Posts: 694	What aperture are you on here? It does sound more like a defective lens to me. When you re focus using the manual control, your using your eyes and noggin to determine the correct focus. The camera isn't doing anything. download a focus testing chart from somewhere and set your camera up on a tripod in a room with average lighting. manual focus on the chart at 18mm and take the shot using a remote release or self timer, adjust the zoom out the the end of the lens and focus lock, before adjusting back down to 18mm and selecting AF. Once the camera has gained focus lock on the same point at the same focal length, take the shot again. Compare the 2 shots and see which is sharper. __________________ I came, I saw, I broke it!!!

24-01-2006, 13:16	#2 (permalink)
Marcel Posts: n/a	I thought once the focal length changes, then the need to focus changes.? That's why at 50mm, your 18mm focus was way off, because it's not correct for 50mm. So in effect, you're doing the following Adjust to 18mm Auto Focus to 18mm Adjust to 50mm Auto Focus to 50mm Switch to manual Adjust to 18mm and take the shot (with a focus that is correct for 50mm).

24-01-2006, 13:28	#3 (permalink)
Arkady Forum Regular Join Date: Feb 2005 Location: Aldershot, Hampshire Posts: 1,198	That is correct. You must re-focus at different focal lengths because the glass elements within the lens have moved, altering the point of focus. This happened even on M/F zooms and is quite normal. Focus - Frame - check focus - shoot.

24-01-2006, 13:36	#5 (permalink)
Arkady Forum Regular Join Date: Feb 2005 Location: Aldershot, Hampshire Posts: 1,198	You're sayingthat at 18mm the AF isn't precise enough?

24-01-2006, 13:39	#6 (permalink)
Warspite Feet under the table Join Date: Sep 2005 Location: Newcastle Sunderland Middlesbrough (UK) Posts: 1,736	I would expect the focus to drift at longer focal lengths - sounds bizarre that its accurate at the longer end and getting worse at the shorter range

24-01-2006, 15:07	#8 (permalink)
fingerz Forum Regular Join Date: Jul 2005 Posts: 790	So if focus changes depending on the focal length, and I'm focusing for a 50mm shot but then flipping to manual and zooming out again, my shots ought to be quite noticably out of focus, then? I'll do a test but I can't say I've ever noticed them being out of focus when I've used that method. Will devise a cunning way to check and do it later tonight to see what it looks like.

24-01-2006, 15:13	#9 (permalink)
Arkady Forum Regular Join Date: Feb 2005 Location: Aldershot, Hampshire Posts: 1,198	At 18mm you have more DoF which might account for not noticing any focus-variance.

24-01-2006, 16:05	#10 (permalink)
fingerz Forum Regular Join Date: Jul 2005 Posts: 790	True. Just did a preliminary handheld test with the camera here at work and it did seem sharper using the correct method (ie focusing at 18mm, not 50mm) so looks like my mistake. Perhaps I can rely on it at 18mm after all. Hadn't really used it for grab shots at the wide end because I assumed it wasn't up to it but I may give it a go now.

24-01-2006, 16:55	#11 (permalink)
Marcel Posts: n/a	Yay! I got one right, mister, I gots one!

24-01-2006, 19:09	#12 (permalink)
*Dave* Pixalo Crew Join Date: Jul 2005 Posts: 14,892	After seeing some of your shots with this lens, I was tempted to purchase a few months back. But then I read reviews on FM http://www.fredmiranda.com/reviews/s...&cat=37&page=2 For me there were too many people getting AF problems, so I decided against it. Have you considered you may have a problem with the lens. Might be worth going back to shop & trying another lens to see if problem is across the board or hopefully just yours, which you can get repaired / replaced

24-01-2006, 21:42	#13 (permalink)
fingerz Forum Regular Join Date: Jul 2005 Posts: 790	After being schooled on the correct procedure for focusing, I think the lens is actually pretty decent now.

25-01-2006, 11:32	#19 (permalink)
dlh Getting Comfy Join Date: Feb 2005 Location: Lost in Surrey Posts: 132

25-01-2006, 09:55	#15 (permalink)
SammyC Forum Regular Join Date: Jul 2005 Location: Bristol Posts: 1,466	Try this: http://www.focustestchart.com/chart.html

25-01-2006, 11:17	#16 (permalink)
fingerz Forum Regular Join Date: Jul 2005 Posts: 790	The problem there is that I have never, ever, in my entire life been able to take a manually-focused shot that was as sharp as an autofocused shot. In fact, I constantly recoil in amazement when people talk about switching to manual to get better results. Switching to manual is something I only ever do in very low light (less so now I have an ST-E2). All my results from manual focus have been shocking with one exception - macro shots where you focus as close as possible and then just move the object itself nearer or farther from the lens. In those situations only my manual focusing is ok.