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Manually focusing a Canon lens when it is set to AF - yes or no

M

macgrl

macrumors 65816
Original poster
Jul 17, 2008
1,192
5
I have just got a Canon 40D and two lenses - Canon EF-S 55-250mm F4-5.6 IS Zoom Lens and a EF-S 17-85 f/4-5.6 IS USM Lens.

Can I manually adjust the focus when the lens is set to AF or not? Will this damage the lens if I do.

When I was looking at various lenses and Cameras some allowed you to do this but I am unsure about these.

Thank you:)
 
macgrl said:
I have just got a Canon 40D and two lenses - Canon EF-S 55-250mm F4-5.6 IS Zoom Lens and a EF-S 17-85 f/4-5.6 IS USM Lens.

Can I manually adjust the focus when the lens is set to AF or not? Will this damage the lens if I do.

When I was looking at various lenses and Cameras some allowed you to do this but I am unsure about these.

Thank you:)
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I'm pretty sure 55-250 no, 17-85 yes.
 
If you autofocus the lens (shutter button half way down) you can then manually focus if you don't take your finger off.

Taking it off will activate the AF again when you push the button.
 
You can do this with lenses that have a AF motor type called "Ring USM". Your 17-85 falls in this category (generally the more expensive lenses). Other "USM" lenses have the cheaper "micro USM" which does not support manual override. I believe your 50-250 is like that. There is one exception: the 50/1.4 has micro USM but allows manual override.
 
AxisOfBeagles said:
Why? It's a simple switch to change it to MF, and then you don't run the risk of damaging anyhting, or having the AF mess with your manual focus set.
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because AF isn't perfect, and some like to tweak it so it is. AND because when the AF fails, searching for a switch when you're in a hurry is just one more thing to do.

back to the OP: the 17-85 has full-time manual focus, so you can with that. i think all Canon lenses with USM has this feature.
 
On any mid-range to pro Canon DSLR from at least the 40D on, you can decouple autofocus and autoexposure from the shutter half-press assigning these to the AF-ON or * buttons with a custom function. (Cf IV-1 & 2 on the 40D according to http://www.dpreview.com/reviews/canoneos40d/page12.asp)

The 17-85mm EF-S IS does have full-time manual focus - no need to switch into MF mode - but unless you've mapped autofocus off the shutter half-press with the custom function above you may find it annoying to use.

Don't try to manually focus the 55-250 EF-S when it's in autofocus mode! You can damage the lens easily that way. Of course, on the bright side you'd get an excuse to replace it with a 70-200 f4/L which is in every way a far, far superior lens and a relative bargain at ~$600 new.
 
mmrdude said:
You can do this with lenses that have a AF motor type called "Ring USM". Your 17-85 falls in this category (generally the more expensive lenses). Other "USM" lenses have the cheaper "micro USM" which does not support manual override. I believe your 50-250 is like that. There is one exception: the 50/1.4 has micro USM but allows manual override.
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the 55-250 does not have USM.
 
Thanks guys. I have only had them a day and don't want to damage them. :) What happens if you do manually focus if it is set to AF?
 
hmm, I think the USM lenses allows manual override by half pressing the shutter button
 
macgrl said:
Thanks guys. I have only had them a day and don't want to damage them. :) What happens if you do manually focus if it is set to AF?
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I think it's mainly a matter of wearing gears and the motor. You're not going to break it, but it's not good for the 55-250. You're all set on the other one though.
 
I did it on about 6 pictures I took yesterday. I had it on AF and then manually focused with the 55-250. It is still ok today as I have tried it. I won't be doing it again though. Just wanted to check that I hadn't wrecked it!:)
 
vga4life said:
Don't try to manually focus the 55-250 EF-S when it's in autofocus mode! You can damage the lens easily that way. Of course, on the bright side you'd get an excuse to replace it with a 70-200 f4/L which is in every way a far, far superior lens and a relative bargain at ~$600 new.
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The 70-200 f4/l is not far, far superior.

The 55-250 is smaller, lighter, has a greater range, is very cheap and has good iq.

The 70-200 has even better iq and much better focusing and build quality.


Do not assume that the 70-200 is automatically better, it all depends on the OPs requirements ;)
 
Last edited:
toxic said:
because AF isn't perfect, and some like to tweak it so it is. AND because when the AF fails, searching for a switch when you're in a hurry is just one more thing to do.

back to the OP: the 17-85 has full-time manual focus, so you can with that. i think all Canon lenses with USM has this feature.
Click to expand...

Sorry - just doesn't make sense to me. If you're gonna 'tweak', might as well be in manual to begin with. I'm just too old school, I manually focus most of the time - and wouldn't want to run the risk of harming the AF drive in the lens by manually adjusting it while AF is active.
 
I am hoping that I haven't broken it by doing what I did, it seems ok and does what i would expect so don't think that it is broken :)
 
macgrl said:
I am hoping that I haven't broken it by doing what I did, it seems ok and does what i would expect so don't think that it is broken :)
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I suspect that you should not do it again unless that lens is USM, and it doesn't seem to be. I use nikon but I have a bunch of canon lenses too, they would all grind and resist if I tried to focus them manually when they're switched to AF. This is what confuses me about your description.
 
none of them grinded or resisted, I would have picked that up. I am so scared of damaging anything that I pick a lot up :)
Snf
 
And I won't be doing it again, no chance of that. Thank you for you input :)
 
AxisOfBeagles said:
Sorry - just doesn't make sense to me. If you're gonna 'tweak', might as well be in manual to begin with. I'm just too old school, I manually focus most of the time - and wouldn't want to run the risk of harming the AF drive in the lens by manually adjusting it while AF is active.
Click to expand...

on lenses with FTM, you don't run the risk of harming the AF motor. and using it to tweak is faster than going all-manual because the AF will focus faster than your hand, unless the difference in distance is small. one way or another, it's a useful feature to have, though how useful it is varies for different people.
 
toxic said:
on lenses with FTM, you don't run the risk of harming the AF motor. and using it to tweak is faster than going all-manual because the AF will focus faster than your hand unless the difference in distance is small.
Click to expand...

So does that effect what I have done? :)
 
Basically what i did was, focus with af on a subject in the distance, the camera focused on a nearer subject, which is not to be unexpected, i then focused manually whilst on AF setting on the original subject. I moved the rings, no grinding or resistance whilst using my 55-250 lens (see above posts for exact spec) I did this on 5 or 6 pictures and won't be doing it again. I am thinking that I haven't done any damage to it :) I won't be doing it again. I will focus on it manually using rings and setting MF and then if need be set it to AF for the final shot. :):)
 
I don't know about those two lenses, but most Canon lenses allow you to adjust the focus manually while the lens on auto-focus (AF), and as follows: Camera set to One-Shot AF (no AI-Servo, and no burst mode).

-Focus on the subject by pressing and holding the shutter button halfway. Without releasing the shutter button, fine tune or adjust the focus manually, and then take the photo. I can do that with the following three lenses I have: EF 100mm f/2.8 Macro USM, EF 200mm f/2.8L USM, and EF 400mm f/5.6L USM.

The lens manual contains the instructions. Make sure that you read the instructions first, otherwise you can ask Canon or somebody who has the same lens and its manual.
 
Just to re-summarize:

Most Canon USM lenses allow full-time manual focus, like your 17-85. However, your 55-250 does not support it (and it also not USM). Thus, the only time you should be physically turning the focus ring is when it's switched to manual focus. If you turn it when it's switched to AF, you'll be grinding the gears and generally not making your lens happy. However, you can turn the 17-85's focusing ring when it's in its AF mode. Not much more to say.

sangosimo said:
if you have a 40d use LIVE VIEW and dial in the focus perfectly.
Click to expand...
Live View on DSLRs is mostly for tripod work and isn't quite as useful when handholding, especially if you're going to magnify 10x in to confirm focus.
 
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