Can't hold focus lock when zoomed & wide open

[Lazy Photographer]

I've got a tamron 17-50 F2.8 VR on my T2i and have noticed for the past while that at 50mm (fully zoomed in) and F2.8 (wide open), I'll lock focus onto my subject with a half shutter press but when I attempt to recompose, the focus slips to where my centre point has been repositioned. I shoot with only a single centre focus point. I catch it right away, of course, because with such a shallow depth of field it's readily apparent the moment it happens. I suspect this isn't normal but don't know if it's the lens or the camera or me. Since the AF motor is in the lens I'm guessing it's a lens issue. Anyone else experience anything like this? Or is there something I'm doing wrong that I'm not aware of?

 

[Phranquey]

Sounds like you are set on continuous focus.... Not familiar with Canons, but mine has a toggle selector for manual, continuous, and single lock focusing. Check your manual under focus modes.

 

[Irrok]

I agree, however you specified that it takes place when you are at 50mm - f2.8
have you confirmed that this is the only time it happens? if so You may have a problem with the lens... However I believe it probably the continuous focus setting.

 

[Lazy Photographer]

Yes, I have it set in continuous shooting and in AI Focus (switches between single shot focus and continuous shooting of moving objects focus on the fly). This mode allows me to shoot single shots or a burst of shots of a moving object without any trouble. But if I'm trying to focus on a static image and something in the background (cars, people) are moving by, are you saying the camera is automatically trying to find focus on those moving object? If so, it makes total sense. And it's a huge relief, since all I need to do is switch the mode.

 

[Lazy Photographer]

I agree, however you specified that it takes place when you are at 50mm - f2.8
have you confirmed that this is the only time it happens? if so You may have a problem with the lens... However I believe it probably the continuous focus setting.

To be honest, I think it's happened at other settings too but it stands out for me when I'm trying to shoot really shallow depth of field.

 

[Derrel]

I think you need to read the manual and get a better understanding of the nuances possible on that button and what it can do,and how it can be programmed...

 

[Josh66]

Are you focusing, then recomposing and adjusting the zoom setting?

Both od those will change the focus point...

Don't zoom in to focus. Set the zoom to whatever you want it to be, then focus.

 

[Lazy Photographer]

I've read the manual cover to cover plus two books dedicated to the T2i. The one thing they all agree on is AI Focus works for both single shot and continuous shooting modes. None of them get into any real detail. The manual's the worst of the bunch. It's very light on complete explanations, let alone nuances.

Thanks everyone for your help. Much appreciated as always.

 

[Phranquey]

I think you need to read the manual and get a better understanding of the nuances possible on that button and what it can do,and how it can be programmed...

Slightly more eloquent than what I was thinking....

 

[Lazy Photographer]

Are you focusing, then recomposing and adjusting the zoom setting?

Both od those will change the focus point...

Don't zoom in to focus. Set the zoom to whatever you want it to be, then focus.

Yes, that's what I'm doing. I set the zoom to 50mm and the aperture to F2.8 up front because I've decided I want a very shallow dof, I then aim the centre focus point at the subject and half-press the shutter release to focus. Then, while still half pressing the shutter, I recompose and fire. But before I can fire, the focus slips over to the new location of the centre point. Get me?

 

[Lazy Photographer]

I think you need to read the manual and get a better understanding of the nuances possible on that button and what it can do,and how it can be programmed...

Slightly more eloquent than what I was thinking....

Really helpful, thanks...

 

[RobWyse]

Yes, that's what I'm doing. I set the zoom to 50mm and the aperture to F2.8 up front because I've decided I want a very shallow dof, I then aim the centre focus point at the subject and half-press the shutter release to focus. Then, while still half pressing the shutter, I recompose and fire. But before I can fire, the focus slips over to the new location of the centre point. Get me?

It might be an idea to set the autofocus to the button on the back of the camera, rather than the half-shutter. That way you can focus, release the back button, recompose and press the shutter without any chance of the autofocus acquiring a new focus target, as the auto focus will no longer be active once you release that rear button.

 

[reznap]

I don't know why you wouldn't just use one-shot AF. You're leaving too much up to the camera imo, which leads to unpredictable results - whether we're talking autofocus, autoexposure, etc...

Are you getting a beep when focus is achieved? You're supposed to, even in AI focus mode. I don't think this mode was meant for "focus, recompose" and if that's what you're trying to do, switching to one-shot AF is as easy as pressing 3 buttons which are right next to eachother.

It might be an idea to set the autofocus to the button on the back of the camera, rather than the half-shutter. That way you can focus, release the back button, recompose and press the shutter without any chance of the autofocus acquiring a new focus target, as the auto focus will no longer be active once you release that rear button.

That's what's known as "AF Lock" and it's a major PITA I think. Very awkward. Yeah, it might work but I don't see many situations where it would be very practical... maybe sports?

 

[RobWyse]

It might be an idea to set the autofocus to the button on the back of the camera, rather than the half-shutter. That way you can focus, release the back button, recompose and press the shutter without any chance of the autofocus acquiring a new focus target, as the auto focus will no longer be active once you release that rear button.

That's what's known as "AF Lock" and it's a major PITA I think. Very awkward. Yeah, it might work but I don't see many situations where it would be very practical... maybe sports?

I use it for everything, never found I had any problems. To me it is practical in every situation, mainly because I prefer it to the half-shutter.

I think that it is mainly intended for sports and wildlife though. Or any situation where you want/need continuous AF, but want to tell the camera to settle down for a minute so you can trigger the shutter.

 

[Lazy Photographer]

Thanks guys. Yeah, I'm familiar with back button auto focus, but I agree with Paul that I think I might not be a good candidate for this sort of thing. The easiest solution would be to simply set the AF to single shot and switch it to continuous when necessary. I started out that way and only recently tried using the AI Focus. It's no big deal for me to go back to it.

Oh yeah, and yes I do get a beep confirming focus.