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PostPosted: Wed Oct 05, 2011 12:28 pm 
How important are high censers? Just curious as I went to take a photo today of a bird in a bush, and the auto focus was in and out but not perfectly focus for me to take the shot? Missed it took to long, I had to alter it to manual focus!
Blast and all those all singing all dancing censers did not do much? I can see that if you wanted to take photos of a moving object with not a lot in front of you, that's ok, Do we rely on technology to much! Or should I have used the technology in my Head and thought about what I needed to do first? :oops:


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PostPosted: Wed Oct 05, 2011 1:47 pm 
What do you mean by a high sensor? High quality or high resolution? These coukd be different

I'm not sure what in the sensor has to do with focusing

One of the things I need to learn is exactly how my camera achieves focus and what help and hinder this. If I know this info them maybe I'll either be able to help prevent this kind of problem or at leat know its likely to be a problem and change my strategy to help


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PostPosted: Wed Oct 05, 2011 3:38 pm 
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Echoing what was just said... learning how autofocus works will help you use it more effectively. And in a nutshell, AF works by finding the area with the largest amount of contrast within the AF sensor area and then drives the focus to maximize the contrast. It's why you can't focus on solid-coloured walls that have no texture or contrast, and its why focusing on a bird in a bush won't work when there are branches and leaves in the way... Think of AF sensors as a magpie - the "oh shiny" factor is what they gravitate toward. If there's something more "interesting" (contrasty) in the area, they'll focus on that instead.

Knowing that, and knowing what type of sensor (cross-type, vertical, horizontal) will also help you achieve better results. Each one is more sensitive to contrast in certain directions, with the cross-type being the best because it can sense contrast in two directions at once.

When AF fails for you, having a lens with instant MF-override is a help, and you can even MF-assist your camera/lens into focusing on the right area by getting the focus "almost there" with MF first, and then AF'ing the rest of the way to nail it.

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PostPosted: Wed Oct 05, 2011 9:12 pm 
Sorry guys, what I meant was the AF sensors, they lit up and was bouncing one to the other.


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PostPosted: Wed Oct 05, 2011 9:16 pm 
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Try setting `single point`, it should help greatly.

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PostPosted: Wed Oct 05, 2011 9:24 pm 
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What you said is only partly true, Plymer. You're talking about contrast-based autofocus, which is only used in Live View on a DSLR. Contrast-based AF is generally the "default" on P&S, bridge and mirrorless (EVIL, CSC, Micro 4/3 or whatever you wish to call them) cameras.

However, the cross-type, horizontal or vertical AF points are used for phase detection, which is the conventional means of AF on a DSLR.

Regarding your issue, blindasabat, it's possible that the AF point that was in focus was focused on part of the bush rather than the bird. I don't know much about using the a580 but I'd imagine you can preselect a particular AF point/group of points so if you have that point aimed at the subject, it's nigh on guaranteed to be in focus at that when you half-press the shutter button.

edit: oldCarlos beat me to the punch.

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PostPosted: Wed Oct 05, 2011 10:00 pm 
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Ah, see, that's what I get for spouting tech ;)

The idea is the same in practise though, because the more contrasty the target, the happier the AF (at least through experience), and is also why they build the red AF-assist beams into certain cameras and flashes to create contrast.

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PostPosted: Wed Oct 05, 2011 11:36 pm 
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Not wanting to sound like a troll but I wouldn't agree that it's the same idea in practice. You can get the same results with them but there are key differences in the ways that each type of AF works. Phase detection can focus accurately on low contrast subjects provided that there is enough light - even with sufficient lighting, contrast-based AF may struggle on such subjects. The purpose of the flash is to illuminate the subject and in many cases perform metering. That you get contrast using a flash is almost incidental as a result of the illumination.

I would say your experiences may just be regular coincidence.

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PostPosted: Thu Oct 06, 2011 9:48 am 
Thank you Rorschach & oldCarlos, I just went through the settings and found it, the bird has gone but picked out a leaf and kept it in focus :)
A bit more knowledge to store away, and thanks to everyone else for there input.


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