AF issue - is this normal?

fingerz

fingerz

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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.
 
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).
 
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.
 
Marcel said:
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).
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....But the focus still seems sharper than the 18mm auto-focus??

That's not right is it?
 
You're sayingthat at 18mm the AF isn't precise enough?
 
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:confused-
 
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.
 
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.
 
At 18mm you have more DoF which might account for not noticing any focus-variance.
 
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.
 
Yay! I got one right, mister, I gots one! :D
 
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
showproduct.php


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
 
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.
 
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.
 
fingerz said:
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.
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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)
 
CT said:
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)
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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.
 
CT said:
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)
Click to expand...
Now that is somethign I never knew :embarasse
 
I've just done a test with my 350D and I had my Sigma 70-300 APO DG Macro on, and it didn't work.

Now this could either be the fact that it's the Sigma, or that the 350 doesn't do it.

Could it just be Canon glass that does it?
 
DJW said:
So would that work for a Manual lens ?
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Erm.. if I had an adapter to fit one of the manual lenses off the A1 to the DSLR I'd be able to tell you. It seems too good to be true doesn't it? My gut feeling is 'No' as modern AF lenses send info to the camera via the contacts on the back of the lens - focus point isn't just obtained optically (I think) I'd be very pleasantly surprised if I'm proved wrong though. :)
 
Marcel said:
I've just done a test with my 350D and I had my Sigma 70-300 APO DG Macro on, and it didn't work.

Now this could either be the fact that it's the Sigma, or that the 350 doesn't do it.

Could it just be Canon glass that does it?
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I'm amazed Bod. I just double checked with all my Canon lenses and they all do it. We need someone with a 350D and a Canon lens to check this out.
 
I can't speak for Canons but on my Nikon there are two AF/MF switches (1 on the body and 1 on the lens). The one on the lens itself merely allows me to turn the focus ring on the lens, the one on the body however (when set to MF) displays the "in focus" circle in the viewfinder when focus is achieved.

Like Jamey, I can't see how this is any different to auto-focus though :Ponders:
 
fingerz said:
But relying on the camera to tell you when it's in focus is exactly the same as autofocus. It's just that you're using your hand to turn the focus ring instead of the motor.
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Eh? :Ponders: It's not the same though Jamie. For one thing the focus point wont move once you've set it manually. Assuming the camera to be properly set up, that little green focus light is a far more reliable indication of accurate focus than the human eye - all human error is removed. When I use the Manual focus, it's extremely finely set - you need the green light to be on permanently to indicate proper focus and the slightest twist of the lens puts the light out, so it's extremely accurate.

Without this assistance though, I agree it's a beggar to judge optimum focus on a plain screen.

Edit

You can now get focusing screens with a split image rangefinder lens in the centre to aid manual focusing.
 
Steve pointed those out to me a short while back, but for me the cost doesn't justify it really.

I'll have to try with the kit lens on later, see if that works. If not, then it could be that it's a 350D thing.
 
Well I'll be surprised if it's a Sigma thing Bod, but utterly gob smacked if it's a 350D thing.
 
CT: I assume the shutter release needs to be held down half way to achieve the manual focus lock indication? (silly question really or it would be beeping all the time :))
 
dlh said:
CT: I assume the shutter release needs to be held down half way to achieve the manual focus lock indication? (silly question really or it would be beeping all the time :))
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Yes - hold down the shutter button until you get focus lock and you can then release it. :)
 
Yes it does - using a 350D and a Canon lens does beep, illuminate the focus point and give you a green light, upon half pressing the shutter button then manually focus, it will beep when focused correctly. Also it will beep about every second so long as the button is half pressed and it is still focused (ie the object has not moved, or the camera/focus has not moved).

Cheers CT I never new that until you mentioned it until now:thumb:
 
CT said:
Well I'll be surprised if it's a Sigma thing Bod, but utterly gob smacked if it's a 350D thing.
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It's neither, it works.

:getmecoat

That's all I will say on the matter :D
 
Marcel said:
It's neither, it works.

:getmecoat

That's all I will say on the matter :D
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Yes - you do have to switch the camera on! :D

Cool - thank gawd for that! :thumb:
 
Did that lens test, linked to above, on all my lenses.

The two Canons (nifty fifty and 100mm macro) were fine. The Sigma 18-50 front-focused a little on the first test and back-focused a little on the second test but in both cases the focus line was within the acceptable focus area so I'm not going to worry too much about it.
 
CT said:
Yes - you do have to switch the camera on! :D

Cool - thank gawd for that! :thumb:
Click to expand...

I'll admit my mistake, but bear in mind it's purely for the benefit of the members, so they can learn from it...lol

I didn't press the focus button, which would normally be a half-pressed-shutter button, but I have Cf4 set so the AE Lock (*) on the rear of the camera is my focus button.

Held that down, and bingo, it works a treat :)

You may now mock and laugh hysterically. :D
 
Marcel, i'm surprised you missed that function. even the older 300D did it :D


Consider your self mocked
 
I always wondered why all the Canon-shooting hacks I work with went beep every so often.

Useful for blind photographers, I suppose, but I'm a little surprised we're spending this amount of time discussing how to focus a lens...:Ponders:
 
And as well you should wonder m8...........;)

I even found the ON switch on my camera the other day, can't see much through the big glass bit yet though :D
 
Just had a play with my 350D and Sigma 18-200. It certainly gives the red light if it thinks you have focussed correctly manually at longer focal lengths, but as you near the 18mm end it is reluctant, possibly as I was aiming at the screen and it did not find it as easy to get "edges" to lock on to. I do miss the split and frenzel screens to focus as on my A1. I find at the 18mm setting and a wide depth of field, it not easy to focus manually on anything close on the 350D screen anyway. Blame the bifoculs!
 
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