03-27-2017 08:28 PM
Why do your experts recommend using the center point, especially with cameras with a handful of AF points, to boot?
Because the center AF point is special. The center of the viewfinder is traditionally where you focused a classic SLR camera. The focusing screens of old film cameras were designed that way. The center of the viewfinder is where you focused.
DSLR manufacturers have made digital cameras to emulate the feel and functionality of classic film cameras. The center AF point is typically the most sensitive AF point in a DSLR, and the most accurate. In many cameras, the center AF point is the only AF point that can consistently and accurately lock focus with narrow apertures, like f/8.
Your camera only goes to f/5.6, which is typical for the average consumer DSLR. Bear this limitation in mind when you purchase and use lenses. Your camera's AF performance falls off with lenses with a maximum aperture greater than f/5.6, or greater. Most of the time, the camera firmware will simply disable the Autofocus under those circumstances, because the engineers know that AF accuracy falls off dramatically above f/5.6.
Word of note, though. Your camera lens focuses at maximum aperture, under most circumstances. In other words, if you have an f/4 lens, but dial in an f/16 aperture setting, the camera will autofocus at f/4, or whatever the maximum aperture might currently be for your lens. When you take the picture, then the camera will stop the lens down to your f/16 aperture setting prior to activating the shutter.
03-28-2017 05:05 AM - edited 03-28-2017 05:27 AM
sorry i worded it poorly, i do understand the surroundong af points take over the tracking.
i cant copy/paste to this reply which is a pain. regarding ur second to last para. am i to think that if my largest aperture is f2.8 the firmwater may acually disable the AF ? , because this sounds like a good reason to abandon AF...i must be misunderstanding you
you use the term "greater" and "above" presumable they both nean "larger aperture" [just so i can follow you]
its very intereseting what u say about the centre af point being better than the others
from what u say i'm asuming that if u use a surrounding af point for the initial focus lock and the subject remains staionary that the accuracy of the focus will be as good as if u had used the center af point. Is this right?
since these posts i am contantly using the camera/len and experimenting with the af/tracking using a tameron f2.8 35-70mm and eos 5d mk 2
03-28-2017 06:21 AM
Summer is coming. Get out and use your camera. Good-bye, and good luck.
03-28-2017 07:22 AM
03-28-2017 09:47 AM
i asked this question to a person here who advised me a couple of posts back but he will seems to given up , can anyone oblige
"am i to understand from what you say that if my largest aperture is f2.8 the firmware may actually disable the AF ? "
No, f/2.8 is fast enough for any AF system. But some cameras can't focus above f/5.6, and very few can focus above f/8. So you could have a problem if you added a lens extender that cost you three or four stops.
03-29-2017 05:38 AM
03-29-2017 07:37 AM
ah its clear now, thanks. Im assuming that "above f5.6" means smaller aperture
i spoke with canon tech dept uk yesterday , interestingly they said they had no knowledge of the surrounding af points being less accurate. But then i suppose if its just a general poor design they wouldnt admit it LOL
It's more a cost effectiveness issue than it is poor design. The better cameras have better AF systems, but they cost more to make and are therefore more expensive. And to take full advantage of the better AF systems, you need faster lenses, which are also more expensive. You get what you pay for.
And yes, the "f" number is the ratio of the focal length to the diameter of the aperture, so a higher number means a smaller aperture.
03-29-2017 11:17 AM
That is because the fact that they only work at certain apertures does not make them "less accurate". Because of the large depth of field it is difficult to autofocus at small apertures.
Imagine a wide angle lens at f/16 or so. Where should autofocus set itself? Everything appears to be in focus for its circle of confusion.