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I use the ‘center square only’ as the reference on subject for focus. Another problem arises here in that, if using the shutter button to achieve focus, the camera also uses that region to determine exposure upon complete travel of the shutter button. So any shift in composition after focus can seriously compromise a final result in exposure.
--- Don M
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Aspects of this issue were covered in a thread on "focus and recompose" on the FM Canon forum. The majority of responses indicate that most use the older techniuqe of focus and recompose succesfully. However as a balance, I pointed out that Canon does not advise this approach as errors in both focus and exposure can occur (as you also point out).
This text is taken from pages 11 and 18, referring to the 1D series cameras, but many of these points refer to the 5D, 20D too:
http://www.photoworkshop.com/canon/EOS_Digital.pdf
Page 11
The 45-point Area AF sensor covers a significant portion of the picture area, especially
with the original EOS-1D and EOS-1D Mark II. However, many photographers never use
the off-center focusing points, preferring the old-fashioned method of locking focus with
the center point and then recomposing for the actual image. Before multi-point AF
cameras were available, “Focus Lock and Recompose” (FLR) or manual focus were the
only choices available. That’s no longer the case.
FLR is sufficiently accurate for photographing distant subjects, but it can cause focusing
errors, especially backfocus, when photographing subjects within about 15 feet of the
camera. This is often the case during portraiture. For optimum focusing performance
with close subjects, we recommend avoiding the FLR technique. Instead, use an offcenter focusing point or focus manually.
EOS-1 class digital SLRs are equipped with an extensive variety of Custom Functions
(C.Fn) and Personal Functions (P.Fn) that allow photographers to tailor many aspects of
camera operation according to personal preferences. Although there are minor
differences in the quantity and variety of C.Fns and P.Fns according to individual camera
models, most of them are consistent across the product line.
Page 18
Background
Traditionally photographers have used the FLR (focus, lock, recompose) method when
interfacing with their camera and their subject. This is how it had to be done with
manual focus cameras and the early auto-focus models with a single focusing point.
Almost everyone knows how to use FLR, but it can cause problems. First, it’s not nearly as fast as some of the methods we’ll discuss here. Metering is less precise, particularly flash metering. Additionally, it can lead to backfocusing problems when shooting at wider apertures within 15 feet of the subject.
The fastest and most accurate way to work is to compose first and then use the benefits of the 45-point Area AF sensor (11 point with CF 13-1 on 1D series) to get your focusing and metering settings.
By using this Event combination, the photographer obtains many focusing options, all
without having to take attention away from the subject. It also allows the scene to be
composed in the viewfinder prior to choosing a focusing point. This is helpful because
in shooting these types of events, people are unpredictable in their movements and
with a quick flick of the thumb the photographer can be ready for any rapidly changing
shot without having to lock focus and recompose. Compose the shot, and then just pick
the point that works best.
Another advantage is that when using this combination of settings with Evaluative
Metering, the camera will give more precise metering for your intended subject and do a
better job of automatically compensating for backlighting and unevenly lit scenes.
For EOS-1D and EOS-1Ds cameras that use E-TTL (not the newer E-TTL II), this
combination will also give you much more reliable flash exposures when you’ve chosen
a focusing point that lands on your subject, rather than using FLR.
It will take some practice to break the habit of FLR, but once you learn how to use the
45-point AF system effectively, it can really improve your performance as a
photographer.
Operation
Turn the QCD on the back of the camera to “roll” from AF point to AF point along the
outer periphery of the AF Area ellipse. Tap the Assist Button to immediately go to the
center AF point (this combination has automatically assigned the center point as the
HP). To get off the center point and back to an outer AF point, simply use the QCD again.
This combination affects the focus point selection method only, not the focus activation.
Actual focusing is done by pressing the shutter release halfway.
Hidden Feature: To get back to AFPS immediately… while holding the shutter release in
the halfway position (SW-1), tap the AF Point Selection Button and release. The camera
will stay on AFPS until you either use the QCD or tap the Assist Button. This is useful
because generally, in well over 50% of shots in a wedding or event type environment,the
main subject is the closest and most prominent object in the frame and, in
One-Shot, this is where AFPS works best.
The Bonus Feature is that the QCD is now part of your focusing system rather than your
exposure control. The exposure controls are relocated to the AF Point Selection Button.
Hold the selection button in and turn the Main Dial on top of the camera to adjust
exposure compensation in AE modes, or f/stops in Manual mode. The disadvantage of
having these exposure features on the QCD was that it made it very easy to change
exposure settings accidentally. Now, it’s just as fast to adjust your exposure settings,
and you are far less likely to change them accidentally.
I would propose that for those using a tripod, selection of an AF point is clearly optimum and would be a normal part of setting up the shot. For those having to work very quickly and hand held only, you can proabably get away with focus lock and recompose but there is some risk depending upon the foreground/background contasts, etc. The risks of missing the best shot may far outweigh the exposure/misfocus risks.
On the 5D the use of CF 13-1 allows quick use of the joy stick to set the AF point and along with CF 4-1 dissonects the AF point selection from the AE. On the 1d series cameras CF 18-1 switches the AF point to the registered point (often set as center) by pressing X button. I don't know if the 5D has a comparable function, but the 1 series cameras, PF 13-1 limits the AF point to 11 and spot metereing is then linked to the selected point. Thus 13-1 and selecting an AF point adresses many of the pontential mishaps of FLR. I know this is not a solution to your requests, but may shed some insight into how Canon has set up their dSLR flexibility.
Mike K
Linear Mode
