AF points..... besides autofocus what are their other uses??

[jks]

hi,


just thought somebody out there can help me with this.


Sure we use AF points for the Autofocus part but do they help in the metering of the image even when you are in manual mode or have only the center AF active???


I'm asking coz i'm wondering why top of the Line DSLRs like the 1series have about 50 Af points.


Lets say you take a 5d mkii and 1dsIII , all things being equal i.e. sensor, low light capability, noise control, digic processor and the othre bits and pieces, what stays different is the 5dmkii will retain it's lower number of AF points while the 1series will have it 50 something AF points, which will consistently take better pictures???/


I guess bottomline to my question is -- "do more AF mean better Image quality?"





anyone....... maybe Daniel can enlighten me on this??

[freelanceshots]

This is a pretty good question because i don't know the real world answer. When I started shooting at the newspaper the long term photographers there told me in the begginning to set my my camera up where you select the center focus point only and then use the back button for focus. Since then I've shot this way where its a no-brainer. I choose what's in focus and then recompose when ever I shoot anything. I don't see how any person, professional or non professional could rely on the camera auto focus with all the focus points selected. At the newspaper organization we all shot with Mark II's and then Mark III's so it didn't seem to matter that we had all these extra focus points.

[elmo_2006]

@jks...


I came across this link, though not detailed, may explain the rationale behind the number of autofocus points...


http://www.digital-slr-guide.com/digital-slr-autofocus.html

[Ehcalum]

On the 1D series you can spot meter with a specific AF point. You can also take 3 or so separate spot meter readings in different parts of the scene using AF points and they will show in your exposure bar as well as the general exposure.

[piiooo]

AF points..... besides autofocus what are their other uses??

Besides autofocus Iuse them to:


1) confirm focus when in manual focus mode


2) avoid wonky horisons by aligningthem with rows of AF points


3)help my composition, by placing objects where some AF points are



I'm surethere aremore uses of AF points.

[neuroanatomist]

One of the key advantages to more AF points, mentioned in the article that elmo_2006 linked to, is for tracking moving subjects in AI Servo or AI Focus modes - more AF points means better directional prediction and more coverage of the field.

[freelanceshots]

I've shot thousands upon thousands of fast moving motorcycles, cars and every sport under the sun for that fact where I still continue to use the method that I described using the center focus point only selected. Whether I'm shooting AI-servo for tracking objects or single shot this is how I get the best and most consistant results every time. You just have to make sure that when you are tracking a fast moving object that you pan with it and try to keep it centralized in the view finder where the center focus point is. This is the basic concept that any photographer should be following anyways for traking movement of a single object with any focus point setup. The only time that this technique starts to struggle is when your tracking more then one object where they are ahead or behind each other. A common example would be marathon runners. This is a pretty common problem where it does not get fixed buy just making all your focus points active. Most of the time when you do this your camera still chooses a part of the image that was the last thing you wanted in focus like the background. The only thing that you can do to increase the focus area is to of course increase your depth of field which can eat into your shutter speed. Try capturing multiple maraton runners at dusk with no flash or exterior strobe lighting and have all their shinning faces in clear, sharp focus with no motion blur.

[Ehcalum]

Marathoners, and triathletes. Been there, done that got the medal. I shoot them almost every weekend. I tend to use the top 2 focal points when in Vertical Position so I get their heads. This gives me a less than 1% OOF rate for shots in all types of light and weather.

[neuroanatomist]

You just have to make sure that when you are tracking a fast moving object that you pan with it and try to keep it centralized in the view finder where the center focus point is. This is the basic concept that any photographer should be following anyways for traking movement of a single object with any focus point setup.

That works wonderfully for the examples you listed above - but something they all have in common is linear motion. Panning to follow a fast-moving object like a toddler at close focal range, zigging and zagging through a room or the yard, is a different story...

[freelanceshots]

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<p style="margin-bottom: 0in;"]<span style="font-size: medium;"]How is that different? If
I were to shoot an infant at close focal range that was running around I'd still have it
set up just like I explained. I would say that getting a tight shot
or shots of a medium to small sized hyper active dog in a limited space
would be more difficult to track then a young toddler and I used to
have to do that all the time for business needs. Of course I'd be in AI-servo mode with
my rear AF focus button pressed <span style="font-size: medium;"]continually <span style="font-size: medium;"]and the front shutter button set up to capture exposure only at the moment of pressing that button. As I track the young toddler around while continually auto focusing I snap off the images.
Sure some of the pictures would miss sharp focus but from my
experience I would have a much better hit rate in a shorter amount of time. You know I'm
just expressing how I'd do it where I'd never say that my way is the
only way. Five years ago when I got my first real DSLR Camera the guys at Canon told me to set up my camera this way as well as the older guys from work like I mentioned before. I've just always kind of wondered why people always
think that having a bazillion AF focus points makes a camera better If even the guys from Canon told me to set up my camera this way. That said of course the 1D series cameras have the best auto focus
system but I would say its better because of the more expensive
design and the type of AF points used rather then the number of AF points.
In my opinion again I'd rather have a camera that focus well with one or
two focus points then a camera that's advertised to have 100.