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Old Feb 9, 2006, 1:42 AM   #1
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Im new to photography but I've learn many new stuff within the past few months of researching about it.

My question is the auto focus on dSLR. Example will be the Pentax ist DS has 11 point area AF while the lower line Pentax ist DL has only 3.

Im not really sure what the area AF really does. I have a rough idea but came someone explain it to me? Why is having more area AF better? And is 11 really different and a lot better then having only 3?
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Old Feb 9, 2006, 3:09 AM   #2
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It depends a lot on your style of photography. I would guess that most people use the center spot only most of the time. The few times extra focusing dots are helpful are when you have many different objects in a scene at different distances, and you want to focus on one not in the center without having to recompose. To be honest, I use the center spot about 90% of the time, since it takes some extra time to switch between spots; --and recomposing (and a slight manual focusing on occasion) becomes second nature. To each his own, but fewer spots is not a deal breaker for me.


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Old Feb 9, 2006, 5:27 AM   #3
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I used a film SLR for years with only a centre autofocus point and it was rarely a problem to focus then recompose or focus manually. I now own a Nikon D70 with 5 focus points and quite often use the off centre points but if I had to go back to just 1 I wouldn't be unduly worried.

I'd look at the other differences and then the price difference and decide what you think you'd miss and how much you are prepared to pay for it.
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Old Feb 11, 2006, 1:31 AM   #4
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Basically, the area autofocus just tries to average the multiple target points, and adjust focus based on this, as well as setting aperture to keep all ponts within focus, assuming you are talking about programmed auto settings. This will usually give pretty good results in most cases.

brian


Edit: Actually, I think most cameras pick just one of the points to use for focus. Depending on the camera, some will pick aperture based on the relationship of the others.

brian
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Old Feb 14, 2006, 4:24 AM   #5
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For me it depends very much on the circumstances.

Focus-recompose can give you out-of-focus pictures if you are working with wider apertures and fairly close distances (depending on focal length).

For "street style" photography I generally use the full array of focus points and let the camera do the work. Particularly if I'm using relatively slow zoom lenses, for primes and telephoto where focus-recompose is likely to get the focal plane in the wrong place I tend to select the focus point I want manually.

At other times, particularly with fast lenses I will use the centre-point only because the 20D has a high-precision AF point in the centre that can take advantage of f2.8 or better apertures.

So I guess it all comes down to learning how your camera works and adapting to the circumstances.

One useful thing about the multi-AF point selections on Canon cameras, and I presume for the other manufacturers too, is that when you get an AF confirmation in the viewfinder ALL of the spots which light up will be within the DOF of the settings for that shot. This can be a very handy way of judging DOF, much faster and better than using the preview button.
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Old Feb 14, 2006, 9:38 AM   #6
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I select a focus point more often than not.

When shooting in low light at wide open apertures, it's best to avoid moving anymore than you have to after a lock (lean after focus lock and you may have an out of focus image at larger apertures when filling the frame with a closer subject). ;-)

Also, according to some, recomposing can cause focus errors. Here are a couple of articles on it:

http://www.mhohner.de/recompose.php?lang=e

http://www.outbackphoto.com/workshop...y06/essay.html

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Old Feb 16, 2006, 11:03 PM   #7
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I've adapted to my camera, with its 9 point AF, and I like it. I dont know how much I'd miss it though, so I wouldnt weight to much on that specification.

I was shooting a product for ebay tonight and manually selected various focus points which was greatly helpful since I used a shallow DOF, wanted sharp focus in off-center areas, and was taking multiple frames on a tripod. Focus/recompose would of been a pain, though I could have just as easily manually focused.

I think the main draw is for moving subjects since the camera can follow your subject through the different points, but I've not used that yet.
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Old Feb 19, 2006, 9:06 AM   #8
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After over 30 years of shooting manual focus SLR's, I have become accustomed to using the central split image to focus, so the new multi point AF systems do nothing for me. I have my cameras set to use only the center AF. If I need to focus on something that is not in the center of the frame, I point the camera at it, focus, then lock the focus and recompose.

So for me, having more than one AF point is moot. I certainly would not let it be the deciding factor when buying a camera. After a certain point, the number of AF sensors a camera has becomes more marketing than practical.
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Old Feb 19, 2006, 12:28 PM   #9
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Thank everyone for replying to the post and help me understand the auto focus. Right now I have the Pentax DS with 11 point AF. I bought the camera for $699 w/lense and a carrying case. I see that bhphoto.com sells the the DL body itself for only $459 which the main difference between the two is 11 pt AF vs 3 pt AF on the DL. But the DL has a bigger LCD screen which I think is more usefull then the 11 pt AF.
Its about a $240 difference in price. I dont mind not having the stock lens that came with the kit, it really does no use for me because I am intrested in mainly with outdoor photography.
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Old Feb 19, 2006, 12:46 PM   #10
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The only time I've run into AF issues with reframing is using bright primes at wider apertures and closer ranges. Then, AF error can be introduced trying to use the center focus point and reframing after locking focus.

When shooting existing light, I typically use the focus point closest to my subject's eyes for focusing to minimize camera movement after a lock.

I used 35mm Autofocus cameras with only the center focus point for years without apparent issues. But, in this day of being able to view digital images at 100% size on screen, it's easier to see imperfections and focus errors that you didn't notice at smaller print sizes with film (and you probably didn't enlarge most shots with film).

In most conditions, it's not going to be an issue, and the center focus point in more sensitive on most cameras, too.


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