Focusing your camera – Focus-lock & recompose
Following up from the article on focusing modes of your camera – when your subject is off-center, you have two options to acquire and hold focus:
- use the appropriate off-center AF sensor
(but you can run into the problem of it not being a cross-type AF sensor), or - lock focus, and then recompose the shot
A scenario where you might want to use focus-lock & recompose, is when there is strong back-lighting, and your camera has difficulty auto-focusing. The photo above would be a typical example.
Here I was shooting directly into the sun, and was nearly blinded by the bright light and sweat stinging my eyes – so I shot several continuous sequences while moving slightly sideways in the grass. This was to make sure I get at least a few clear images when I finally choose a selection of photographs for the couple. I pre-focused the entire sequence by standing up and focusing more towards their feet, so that the sun in the frame didn’t mess with the auto-focusing. Then, locking focus and recomposing, I was able to get sharply focused images.
With this image, the depth-of-field at f/7.1 was enough to cover any focusing error incurred by the focus-and-recompose method.
And here we get to the crux of a potential problem with focus-lock & recompose – as you swing your camera through that tiny arc to recompose, the plane of focus might shift too much.
This is more problematic with wide-angle lenses, where we tend to swing the camera through a wider arc than we most likely would with a telephone lens.
If your lens has a perfectly flat plane of focus, then as you move your camera with that tiny bit of rotation to recompose, the focused distance will remain the same, but the plane of focus will also rotate through that same arc. So if you subject is off-center, the plane of focus might now be behind your subject, and you get a photo that looks slightly out of focus. (This looks similar to a back-focused image, but the cause is different. Back focusing is usually due to poor calibration between the lens and camera.)
The plane of focus changing with the focus-lock & recompose method, isn’t much of a problem when you shoot with a longer lens for a tight portrait. That tiny arc you move the camera from their eyes to a better composition, will barely affect the plane of focus, and DoF will most likely cover any tiny error in focus.
But with a wider lens, this is potentially big problem at wide apertures – as you rotate your body to change the composition, the focused distance might not be correct still. It really depends on how much you move the camera, and how much DoF you have, and also the curvature in focus plane of the lens. With these variables, it is good practice to be conservative with the focus-lock & recompose technique. If you are sure your non-center AF points are accurate, it would be the better choice to use the appropriate AF sensor for your composition.
Camera settings & photo gear (or equivalents) used for this photo
I envisaged this more of a panoramic image. You can see the final intended crop by clicking on the main image.
- 1/400 @ f7.1 @ 500 ISO … just available light
- Nikon D3
- Nikon 14-24mm f/2.8G / Canon 11-24mm f/4L
Related posts
- Focusing your camera – Focus modes / AF modes
- Using back-button focus (BBF) on your Canon camera
- Nikon focusing modes – Nikon D300 / D700 / D3
Great read again Neil. As a wedding photographer I have always (up until recently) used focus lock and recompose, but couldn’t understand why some of my shots were out of focus, I spoke about this on the forum (I think) a few years ago and was told this was the problem. Since then I have tried to use bbf but still don’t really get the results I wanted so I switched back to focus- lock- recompose, but since switching back I think more about what I’m doing, one tip you gave me a few years ago was to not just shift the camera but shift my body along with it, which helps somewhat. My gripe I’m struggling with at the moment is the fact the 6D under exposes a great deal and I have to compensate, but not sure if I should compensate in camera by exposing to the right(ettl)or during processing in Lightroom? I guess that’s another topic you might post unless you already have Neil?
Jason,
If I’m understanding you correctly where you said, “Since then I have tried to use bbf but still don’t really get the results I wanted so I switched back to focus- lock- recompose . . . ,” it doesn’t matter which button you use to focus and recompose; the recomposing is the issue. Simply, you just might not be able to get away with recomposing as aggressively as you are (with the lens you have been using).
Neil,
Suppose I use the “hold shutter halfway down” technique to lock auto-focus on someone’s eye. In order to recompose and keep that focus lock on the eye’s plane, am I supposed to keep holding down that shutter button halfway while I move myself or the camera to recompose the shot?
That is the way I have been doing focus lock and recompose, but I sometimes find it difficult to keep that shutter button down halfway as I recompose. If my finger slips or I don’t apply enough pressure, the focus lock is lost. I guess using the back button focus would be better in this case for me.
Stephen .. not necessarily. You can configure the AF-ON button to be Focus Lock.
Then you would still focus with the shutter button. But once the camera has acquired proper focus, then you hit the AF-ON button to hold focus indefinitely.
Neil,
Thanks for the reply. I will need to go back to the Nikon camera manual and look at the AF-ON button settings. I have not used the AF-ON button at all ever since I owned my camera. I’ve been totally depending on the shutter button for focus lock.
It goes back to one of your key pointers: know the functionality of your camera.
As Neil said, if your DOF is large, then you can get away with focus on a target using a central focus point and then recompose. If the DOF is very shallow, e.g. f1.4, then use a single focus point and move the focus point around in the frame so you’ve got the target (e.g. eye) in the desired place in the composition – whether you use the AF back button or the shutter button to focus is immaterial then.
If you’re shooting continuous frames, then you have the additional option on some Nikons (D4) to select the shutter release based on if focus has been achieved or just release the shutter even if focus lock hasn’t been achied. Default
Neil,
I think that’ll work when using AF-S mode – if you keep holding the AF/ON button in AF-C mode it’ll re-focus to wherever the focus point is pointing at (which I use for sports)
Thanks
Johan
I’ve got a question on how you determine your preferred exposure. Do you first frame the shot and, based on experience, guess the amount of (what the meter would see as) overexposure…knowing you need to do this in order to expose for the subjects correctly…or do you first take a close up meter reading off of the subject(s) to determine exposure and then move back out to frame the shot with the sun in the frame?
I was curious about the same thing Alan asked about metering. I would like to learn more about shooting high-key images like you have here.
Thanks!
Metering was simple – I pointed the camera to the ground in the direction I was going to shoot in. A few test shots then, and checking the camera’s preview confirmed I was good on the exposure.
Since I’m shooting in manual, the sun and bright sky then had no influence on the exposure.
Very nice image Neil, especially the pano version.
Am intrigued by the amount of different post-processing steps…. will have to have a look at these.
I would never have guessed that it is taken at 15mm – amazing.
Best, P:)
Pasquier .. you have to keep in mind that the image at the top is a crop of the 15mm image. But just for comparison, here is the unedited full-frame image directly out of the RAW converter, versus the photo you saw at the top of the article.