# Image Stabilization during back-button-focus



## JustMeOregon (Feb 18, 2014)

While reading CR's new review of the Sigma 24-105mm lens, I saw something that gave me pause...

In the Sigma 24-105, their version of Image Stabilization (OS):


> ...OS is always activated on this lens, something that differs greatly from Canon lenses, which only activates IS when focusing and tracking.



That got me thinking of how Canon's way of activating IS (only when focusing & tracking) affects me while I'm using back-button-focus... In Canon lenses, does Image Stabilization _stop_ once I release the back-button-focus? With back-button-focus, does the _shutter button_ continue to have _any_ roll in IS? How does this IS behavior fit-into a focus/recompose workflow? Do I really have to keep the back-button depressed until _after_ I've recomposed & taken the shot(s) in order to keep IS active?

The fine-print in the User's Manuals of the Canon IS lenses states:



> If you set the camera's Custom Function to change the assigned button to operate the AF, the Image Stabilizer will operate when you press the newly assigned AF button.



As I read this, with the buttons on my 5D3 reassigned for back-button-focus (the shutter button set for "Metering Start" and the AF-ON button set for "Metering and AF start"), IS is activated when I press the newly assigned AF-ON button; okay, I get that. But this statement does not _necessarily_ mean that IS immediately stops when the back-button-focus is released, or that IS is _de_activated in the shutter button when reassigned to a back-button. So I'm left wondering what is the correct way to use the focus/recompose technique while still keeping the IS active during the shot. Is there a delay in the IS turning off after being activated by a back-button-focus? A lot if "internet people" claim that this is the case... Does the (half pressed) shutter button still retain its IS function when AF is reassigned to a back button? Or is the only correct way to focus/recompose with back-button-focus is to keep it depressed all the time while shooting?


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## wickidwombat (Feb 18, 2014)

interesting ! I did not know this

I'll have to keep an eye on this in future


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## neuroanatomist (Feb 18, 2014)

With back button AF (which I use and recommend), the IS system is activated by either a press of the AF-ON button or a half-press of the shutter button (the latter activates metering, too). Maintaining the button press (or half-press) keeps IS active. With either button, there's a short lag after release (~2 s) until the IS turns off, and I haven't noticed any difference between AF-ON and the shutter button in the duration of that lag. As a side note, I tried assigning the Lens AF Stop button (found on supertele lenses) as IS ON in custom controls, and with that setting _only_ the lens button activates IS (neither AF-ON nor the shutter do). 

So, you can focus-recompose with back button AF, then half-press the shutter and you'll get IS. Two things to note... 

1) No matter how you activate IS, it's not instantaneous (although Mode 3 on MkII supertele lenses comes close) - it takes ~0.5 s for full effectiveness. So, if you focus, recompose, wait long enough for IS to stop, then just mash down the shutter, you're getting no benefit from IS (in fact, it's usually worse than with IS switched off).

2) Focus-recompose is problematic with fast lenses shot wide open...you'll always get a backfocused shot. 

Ps. The Sigma 24-105 IS is always on? I wonder what that does to battery life??


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## JustMeOregon (Feb 18, 2014)

Thanks Neuro! If I understand you correctly, it makes sense... What your saying is that even though the focus function of the shutter button is re-assigned to a back-button, the shutter button itself _still_ retains it's IS activation capabilities. So if I'm setup for BBF, I can indeed release the back-button (after focus is achieved) and re-activate IS by simply pressing the shutter (slow enough for the IS to settle-down) during the usual course of initiating the exposure... Correct?


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## neuroanatomist (Feb 18, 2014)

Correct.


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## noms78 (Mar 9, 2016)

Is there a way to disable IS on pressing the AF-ON button? I would only like the IS to engage with the half-press of the shutter. I frequently use BBF to focus once, then wait a while before taking a shot. This has the annoyance of having the IS triggered on two separate occasions causing unnecessary battery drain (and its just plain annoying).


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## neuroanatomist (Mar 9, 2016)

Not sure. I know you can assign IS Start to the DoF preview button, not sure if that would remove that function from AF-ON but leave it on the shutter button.


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## MixPix (Mar 23, 2016)

neuroanatomist said:


> Correct.



Interesting and informative.


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