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robminchin

Learning the E-620: Focussing modes

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The E-620 has three focussing modes: Single AF, Continuous AF, and MF. For S-AF and C-AF, the release priority can be set via the menus and either a single focus point or all seven focus points can be used. Both S-AF and C-AF have a '+MF' option, allowing manual focus to be used.

As has been discussed recently on the forum, the E-620 has a slightly odd behaviour in S-AF mode with release priority off. Once the shutter button is pressed it will take a picture once it finds focus, even if the button has been released again. This seems to happen consistently with both of my lenses, although the camera does seem to give up hunting after around four seconds if it can't get focus.

This four-second hunt and give-up is also what happens if the shutter button is half-pressed. It seems to me that the camera hunts less with the button half pressed than if it is fully pressed (or 'mashed'). This is probably just an illusion coming from the fact that if the lens starts going off in the wrong direction, I'll normally release the shutter button and try again - this stops focussing on a half-press but not on a full-press.

The release priority on/off setting in the menus is set independently for S-AF and C-AF. It defaults to 'off' for S-AF (meaning the shutter will not release until the camera has focus) and to 'on' for C-AF (meaning the camera will take a picture as soon as the button is fully depressed). This works exactly as expected in S-AF.

Changing C-AF release priority to 'off' reveals another slightly strange behaviour. Contrary to the description of the function, this does not stop the camera taking a picture before it has focus. Rather, the camera will perform one C-AF cycle (if it is out of focus) in an attempt to bring the picture into focus, then it will take the picture. The C-AF cycle takes around half a second. It will then (if still not in focus) perform another C-AF cycle after the picture is taken. In single-shot mode, it then stops.

In the continuous shooting modes, things are a bit more complex. The camera delays the first shot to perform a C-AF cycle and then tries its best to get in another focus cycle before the next shot, delaying that marginally. This is with release-priority off; with release-priority on, it makes virtually no attempt to focus between shots as it is concentrating on speed.

I found setting the low-speed continuous shooting to two frames per second gave enough time between shots to fit in a full C-AF cycle. On the high-speed setting, there is no meaningful attempt to focus beyond the initial C-AF cycle - this setting is really only usable for action at a static point. I would not be at all surprised if C-AF performance was better with the SWD lenses - but if I'd bought one of them I wouldn't have been able to afford the camera!

When running at 2 fps, the C-AF tracking is reasonable. It can track cars driving past the house at ~30 mph or two dogs playing. It doesn't have the sophisticated 3D + 3 colour tracking of some recent Nikons, or the multitude of focus points needed for them - it's up to the photographer to track the subject, and it's not possible to do a focus-and-recompose in C-AF mode as the camera will re-focus when you recompose (unlike the D300, which tracks the subject and maintains focus on them - without any confirmation of where the camera has focussed, which can be a bit unnerving in my experience).

In single focus-point mode, C-AF blinks the active focus-point when you start focussing (half-press the shutter button); when using all of the points it doesn't do this. It definitely seems to work better when using all the focus points than when using a single point. There's no indication of which focus point is used on the screen, but the detailed playback review tells you which point locked on. It seems that it tries to track with whichever point got a lock initially, but will shift points if it looses lock on that one.

If the lens is a long way out of focus, it seems like it can take a few seconds (and thus a number of frames) for the camera to even realise it is not in focus in C-AF + continuous shooting mode. Once it does, it takes the 70-300 around 4 frames (at 2 fps) to get into focus at a distance from being manually focussed as close as possible - other lenses will almost certainly be faster. I thought the pause before focussing could be linked to the C-AF Lock setting, which I had set to 'on'. However, turning this off did not affect the behaviour. If the shutter was half-pressed rather than being mashed, the problem did not arise.

It seems that there are very few - if any - circumstances in which it is a good idea to mash the shutter button. It causes a number of odd behaviours - in single-shot modes the camera can take a photo after the button has been released again, while in continuous-shooting modes it can cause the camera to take a sequence of out-of-focus shots before the AF system 'wakes up'.

In S-AF+MF mode, you can manually turn the focus ring to change focus before starting to focus (to speed up the AF), or once focus has been achieved (to tweak things slightly). In C-AF+MF, you can only use the focus ring prior to half-pressing the shutter: once you've done this, the camera is continuously auto-focussing and you can't override it. C-AF+MF simply gives a way to get the camera into near focus prior to starting C-AF (which can be very useful). S-AF+MF mode still requires a focus confirmation from the AF system before it will release the shutter (with release-priority off), but you can defocus the lens to your heart's content once you've got that confirmation beep.

The 70-300 has a switch on the lens to flick it into MF mode. This is a true MF mode, with no option to focus using the AEL/AFL button. When you flick the switch to MF, the viewfinder displays 'AF -L' (presumably for AF locked, or something) and then (when you switch back to AF) 'off'. With the lens in MF mode, the camera has release-priority on.

MF confirmation (green light in the viewfinder) is available. It tends to flicker a bit, I find, but is useful on subjects that you could have used AF on - as I tend to use AF in these situations I'm not making great use of this feature.

MF is also possible with a magnified view in Live View. The magnified view always has 'live view boost' enabled - the setting in the cog menu only affects the behaviour in full-screen mode. While this is normally good, it makes it very difficult to focus on the moon with this method - the entire moon turns white with no details to be seen (although the sharpness of the edges and the terminator gives some guide).

It's also worth noting that the 'infinity' focus to which the lenses return when the power is switched off is not, in fact, infinity. It is the furthest focus point of the lens - which can be beyond infinity. This can be seen in the 70-300 by switching the camera off and on and then locking the focus and taking a picture of the moon or Jupiter (or some other object - that's just the ones I used). This method can be used as a 'quick reset' method to get the lens back to close to infinity after focussing close-up, but should not be used to actually focus at infinity.

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Updated 27th October 2009 at 09:36 PM by robminchin

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