Autofocus and auto metering – some things to consider to get better results

Written by Gary on November 5th, 2009

Most novice photographers set their camera to the default AF and metering modes, and for the majority of their photos, this probably works good enough for them.

But what happens when you want to compose your subject aesthetically instead of having them in the centre of your frame?

The larger the sensor in the camera, the less depth of field you potentially have, and the more precise your focus needs to be – the problems are many, but one of them is that depending on how you choose to focus, your camera’s exposure system may give quite variable results.

If you have a Panasonic GH-1 or other Micro Four Thirds camera set to face recognition AF and it will AF on your subject’s face, then all is probably well because the camera will meter for the face no matter where the face is in your scene.

The problem is quite different if you use an AF method other than face recognition, or you are using a dSLR such as a Nikon, Canon, Sony, Pentax, or Olympus though.

Let’s first look at focusing assuming you do not have face recognition AF available:

If you have a camera with lots of AF points and you have the patience and can remember how to do it, you could set one of the AF points to the position of your subject within the frame. If you do this, then your camera has a reasonable chance of giving you good focus and metering if your camera’s metering mode is set to Evaluative or Matrix Metering.

However, my preferred AF method for relatively stationary subjects (ie. not in continuous AF action mode) is to ONLY use the centre AF point and have a AFL or Function button on the rear of the camera set to autofocus lock.

This allows me to lock the AF by moving the camera so the centre AF is on my subject and pressing the AFL or Function button, and then I can recompose, wait for the moment I want to capture it without having to keep the shutter half pressed (which results in accidental shots and finger fatigue, particularly with a heavy camera kit), then take the shot.

HOWEVER, there is one big problem with this technique – and that is the camera’s metering.

Most cameras have a sophisticated Evaluative, Multi-Zone or Matrix Metering which breaks the scene into many zones, provides emphasis on the selected AF point(s), looks up a database of many thousands of images, then via a secret algorithm determines the exposure with the aim of converting your subject into a mid-grey tone.

Now this should ring alarm bells – firstly, what if your subject is a white stallion – you don’t want it looking a muddy grey tone, so no matter what metering system you use – you will have to make some exposure compensation to adjust this, or use manual metering. This also applies to people, metering on dark skin will give ~1 stop over-exposure, while metering on light skin will give ~1 stop under-exposure.

BUT, the other alarm bell that is not so obvious, is that it will give priority to the selected AF point – all very well if the AF point is aimed at your subject, but will give extremely variable results if you have focus locked (or use MF and the centre AF point is used by default for metering) and recomposed because the selected AF point may be aimed at a very bright or dark element in the scene which will obviously result in under or over exposure.

One solution to this is to set the AFL button to do AFL and AEL concurrently, but this then does not allow for changes in ambient lighting such as a cloud coming over, and there are individual camera implementation issues.

  • Canon 1D mark III implementation:
    • AF-ON button itself cannot be programmed to also lock exposure, you must also press the AEL button (the asterisk button) but you must also set the desired C.Fn IV setting – the options are:
    • Custom Function IV-1 = 0 results in half-press shutter over-riding AFL – not what we want!
    • Custom Function IV-1 = 1 results in deactivating AF-ON button and thus no AFL – not what we want!
    • Custom Function IV-1 = 2 results in half-press shutter deactivated – what we would like but you must go back into menu system to re-enactivate half-press shutter functionality
    • Custom Function IV-1 = 3 results in half-press shutter over-riding AEL – not what we want!
    • Custom Function IV-1 = 4 results in deactivating AF-ON button and thus no AFL – not what we want!
    • unfortunately this camera does not allow you to disable either the acquired AFL or AEL and resort to half-press shutter by just hitting the AF-ON or AEL button a 2nd time as you can with the Panasonic GH-1
    • and worse, once you have acquired AEL, it becomes deactivated if you try to adjust exposure compensation and even worse, you lose the AEL (along with viewfinder information) after about 6 secs of no activity unless you keep your shutter half-pressed
    • a pretty lame implementation of such an important function, but then Canon’s implementation of other functions such as mirror lock up are just as lame
  • Olympus E-510 implementation:
    • my preferred setting is to set focus mode to MF, wrench menu item AEL/AFL to S:1 C:2 M:3 which then means I am forced to use the AFL button to AF while half-press shutter locks exposure.
    • the nice aspect of this setting is that I only need to set focus mode to C-AF and a novice can use the camera as usual – half-press shutter for both AFL and AEL
    • but you can’t use a button to do both AEL and AFL concurrently as you can on the GH-1
    • and unlike the GH-1, you can’t dial in an exposure compensation whilst your finger is on half-press shutter to hold AEL
  • Panasonic GH-1 implementation:
    • this has the easiest to use functionality and by far the best implementation – its awesome!
    • just set C wrench menu item AF/AE Lock to AF/AE and set AF/AE Lock Hold to ON, then you use the AF/AE Lock button as a toggle to lock or unlock, and when unlocked, the half-press shutter button takes over as usual.
    • and, even better, you can rapidly change the exposure compensation using the front wheel AND see the exposure effect in the EVF AND see a live histogram in the EVF all without taking your eye from the camera – fantastic except, be careful you don’t accidentally change the exposure compensation as it is easy to do!
    • of the 3 cameras, the GH-1 is thus the only one you can lock both AF and AE and put the camera down waiting for the right moment, make changes to exposure compensation after locking, see the exposure effect and even better, automatically turn off the EVF while you are waiting, and the EVF will magically come back to life as soon as you put your eye up to it – another nice touch!
    • of course the dynamic range of the MFT and FT sensors is not as good as the larger sensor of the Canon 1DMIII so you have less exposure latitude and thus exposure accuracy is more important, but at least the GH-1 provides you with rapid feedback while you are taking the image – if the histogram is white and part of it is against the far right frame then you will be getting some blown highlights and you may wish to dial in exposure compensation – just brilliant!
    • it also has object focus tracking mode which can be really handy – just half-press shutter with the centre AF markers over the desired subject until AF is achieved, then you can recompose or move in or out from the subject and the GH-1 will re-acquire AF for that subject no matter where in the frame it is, as long as it still matches the initial pattern and is not moving too much. If this is not suitable you just press AFL button to lock a focus and exposure. Very cool indeed.
    • the evaluative metering mode of the GH-1 when not in face recognition or object tracking mode does seem very much centre-weighted so you may need to compensate for this, luckily compensation is simple to do
    • and that’s not all, if you decide to manually focus such as with tilt shift lenses, the MF assist of the Micro Four Thirds cameras (and the GH-1 with its EVF in particular), give the fastest, most accurate manual focus available courtesy of the fact they do not have to put a mirror up to enter live view to do so
    • the GH-1 though is not without its own faults being quirky at times and having a x-sync of only 1/160th sec without ability to over-ride the shutter speed to force 1/300th sec with flash, an AF system not optimised for birds in flight, lack of in-built IS, but the general AF and exposure system implementation is certainly not one of its faults and is fun to use.

The other solution is to turn off the Evaluative or Matrix Metering mode and resort to manual metering – perhaps by putting meter mode into spot meter and exposure mode to manual then adjusting your exposure manually according to the spot meter on your subject prior to recomposing. This gives you the best control but again does not allow for changes in ambient light.

Perhaps this also gives some insights into the variable results one gets with TTL flash and why many pros resort to manual flash exposure for more consistent results.

see also:


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