Back Focus in the 50mm 1.2 and how to correct technique in order to prevent it

[fstopper89]

I just revisited a book I have tonight (Rebel t2i Digital Field Guide by Charlotte Lowrie) I got it back when I first got my Rebel but it has been great to refer back to occasionally. I was thinking of selling it but wanted to re-read it before doing so. Anyways, it said this and it was very interesting:

According to many Canon documents, having autofocus set on One-Shot and recomposing the shot may cause the camera to back focus slightly and account for a loss of sharpness.

I do this quite a bit; I most often have it set on One Shot for portrait work of non-moving subjects as it’s said to offer the best for sharpness and since I like to be able to recompose my shot. But I’ve had instances where I knew I focused (using one of the 9 autofocus points) on the subject’s eyes yet some of my images don’t turn out as sharp. This is with generally shooting wide-open (I usually shoot at 2.8 which is the widest one of my lenses goes) and a pretty fast shutter speed, and adjust my ISO accordingly. So the chance for me having problems with camera shake are pretty minimal, yet sharpness is still an issue. Not all the time, but some of the time. This “discovery” could be the reason. I guess the solution, and one I’m going to practice, is to try to compose the shot how I’m going to press the shutter, but if I feel I don’t like the composition, I can crop it down later in post. Therefore I’d probably have to include more in the frame with the anticipation of trimming it down. That way I can keep one of the autofocus points right on the subject’s eyes (or wherever I plan on focusing). So I’m going to experiment a little. Has anyone heard of this?

And to revisit also some of the questions posted earlier, the closer you are to your subject when shooting wide-open, the smaller focal plane you’ll have available. Like if you’re standing a couple feet away their eyes might be in focus but the tip of their nose will not at 1.4 or even 2.0. If you’re 15 feet away, there is much more leeway. There will be a greater plane of focus.

[cameraclicker]

Think of the plane of focus as an invisible wall, everything on the wall is in focus.  Depth of field is the area in front and behind the plane of focus that is sufficiently sharp to still be considered “in focus”.

 

I have a T2i.  The centre auto-focus point is more sensitive than the rest, so it is the only one I use.  I focus then recompose.  I have not noticed any problem with this method, from the camera as cause.  If you are using a lens at a distance that gives a very shallow depth of field, a problem that sometimes occurs is between focus and shutter release either the photographer moves forward or backward slightly, the subject moves forward or backward slightly, or both move in opposite directions.  This throws off the focus.  See page 69 of the manual, the page title is Shooting Tips.

One-Shot AF, AI Focus and AI Servo, all have their own ways of working.  AI Focus is slowest to operate, so if you know what you will be shooting, choosing One-Shot AF or AI Servo are better options.

 

My other bodies all have stronger auto-focus, so I use them differently.

[Author]

Posts