Rugby_Nut
member
Reged: 08/07/2008
Posts: 181
Loc: High Wycombe, Bucks
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The e520 has 3 focussing points available in normal view. I assume this is the same for the other e4xx and e5xx models. If, for instance, you have composed your image and you have an object/person covering each AF dot and these objects/people are all different distances away, how does the camera choose what to focus on? Am I correct in assuming that in Auto, A, and S modes it will try to alter the depth of field to get everything in focus. Is that correct or does the centre spot (if selected) get priority? I'm historically a centre spot focus on half shutter depress and re-compose merchant but when using a tripod this is not so easy. I appreciate you can use focus lock and recompose on a tripod but I would like to understand the logic behind the AF Area.
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Chris
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Photocracy
The Great Pretender
Reged: 18/11/2006
Posts: 625
Loc: Sunny South Coast
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Hmmm, interesting question which had me resorting to my manual. I think what you're asking is, will these models perform an auto depth of field function of the kind that was on my EOS 30. In the Canon version, you had to select a subject with your chosen AF point, then select another subject further away. The camera would then calculate optimum aperture to ensure depth of field covered both subjects. Having looked at my E-500 manual, I think I would have to conclude that there does not appears to be any similar facility. To achieve what your suggesting, I think you would have to focuss on the nearest subject and guestimate a suitable aperture for sufficient DOF to cover your other subjects.
For example, using aperture priority mode, you could set the aperture to a middling f8 and see if that works. If there's fast action about, this might slow your shutter speed too much though. A little trial and error would probably pay dividends here.
BTW, I never used the auto DOF facility on the Canon as it was too complicated to remember how to control it.
Regarding how your Oly 'knows' which AF point to select, I can only say it seems to have an uncanny knack of getting it right. So much so in fact, I began to wonder if my E-500 had some kind of secret eye-control like my EOS 30 had (I was really convinced at one point that Oly might have left an undisclosed feature in place because they were not 100% happy with its performance). I suppose in reality, in most situations the main subject is obvious by being the biggest or brightest object in the frame.
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Rob
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Rugby_Nut
member
Reged: 08/07/2008
Posts: 181
Loc: High Wycombe, Bucks
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I'm still mildly confused because the e520 manual doesn't really say anything at all apart from imply that having all three points on is AUTO mode. I'm just intrigued to find out how auto mode actually decides what to focus on. It must be a similar issue for E3 owners because they've got 11 (I think) autofocus points. If you set the E3 to 'auto' what happens then? It must be an interesting algorithm if it seems to get it always right but it makes you wonder whether those occasional pics that are slightly out of focus might be down to not understanding how 'auto' autofocus is working. I think I'll set my camera to centre spot focus for the time being 
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Chris
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beejaybee
Marvin
Reged: 18/07/2007
Posts: 4967
Loc: Really Here In Name Only
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Quote:
it makes you wonder whether those occasional pics that are slightly out of focus might be down to not understanding how 'auto' autofocus is working.
Which is precisely why I prefer manual focus. When I balls it up it's clearly my fault.
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El Sid
Going potty
Reged: 14/04/2003
Posts: 9463
Loc: Sussex-by-the-Sea
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When all AF points are active the camera generally focuses on the closest part of the scene that is covered by an AF point on the assumption that that is the subject. As far as I am aware this appears to hold true for all makes...
In some circumstances the camera may preferentially focus on a part of the scene which has particularly good contrast (especially in low light or with a low contrast subject).
When all AF points are active the onus is on the user to check whether the camera has selected the correct portion of the scene as the subject (I presume the E520 has AF point indicators as per most cameras). You may find that sometimes more than one AF point is indicated as being in focus. I find this seems to depend on the nature of the scene (it's inherent 'depth') the selected aperture, the focal length of the lens (my 10-20mm is particularly prone) and the overall lens/subject distance. The camera is indictating that all the parts of the scene covered by the selected AF points are within what it considers to be acceptable focus. In some cases this may be sufficient for the purpose but if critical focusing is required on a specific subject it's always better to preset the AF to a single point.
Like many people (especially those of us that started with split image focussing) I tend (or at least try...) to leave the AF on the centre point as a default condition as it's much quicker to focus and recompose when that momentary opportunity arises...
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Nigel
Completely BSRIPN 
ElSid Gallery
A camera in the hand is better than one in the cupboard........
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Rugby_Nut
member
Reged: 08/07/2008
Posts: 181
Loc: High Wycombe, Bucks
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That makes sense Nigel. Thanks.
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Chris
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Rugby_Nut
member
Reged: 08/07/2008
Posts: 181
Loc: High Wycombe, Bucks
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For information chaps, good website and explanation of auto focus here
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Chris
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