How to "Pre-Focus" in a blank space?

[do_u_c_wat_i_c]

suppose i am shooting sports with a telephoto lens
i'll be shooting to freeze the frame.

i have read some books where pros pre-focus a area to get a great shot of
hoops or touchdowns.

amateurs like me who started in digital age is having trouble with that.

how do you focus in a blank space (or know if something comes in to the frame it will be shrap/"in focus")? do you notice the ground or what?

does this trick only lies in "Practice" or there is some technique that i missed ?

One more Question:: Do new AF systems(like nikon's 3D tracking) come close to match this age-old technique

[kencaleno]

If you drive a Nikon,try This:

“Trap” focusing:

This is a neat way to use auto focus especially for fast sports/wildlife action, I don’t know other maker’s settings but these are the settings for Nikon users:
Custom (pencil) menu:

Autofocus set to AF-S
AF area mode set to single
AE-L/AF-L set to AF ON

Compose your shot and set the focus by aiming the centre focus icon at a definite target at the precise distance you want, (Say a tree branch where a bird is about to land, or on second base where the baseball player's foot will land) and pressing the "AE-L/AF-L" button near the viewfinder. This will focus the lens, let go of AE-L/AF-L button. Turn away,
press and hold the shutter button all the way down. Point camera at pre-focused point when subject approaches, and your camera will take the shot,automatically-
Much faster than Human reflexes.


Regards, Ken

[LoveDSLR]

Originally Posted by do_u_c_wat_i_c

One more Question:: Do new AF systems(like nikon's 3D tracking) come close to match this age-old technique

Nikon D3 servo tracking with 51 focusing points using AF-S lens could do it.

[kencaleno]

Originally Posted by LoveDSLR

Nikon D3 servo tracking with 51 focusing points using AF-S lens could do it.

Not everyone can afford a D3-In New Zealand you are looking at $8-9000 for just the body alone

[LoveDSLR]

Originally Posted by kencaleno

Not everyone can afford a D3-In New Zealand you are looking at $8-9000 for just the body alone

Wow! Is it the exchange rate or it is just expensive?

[kencaleno]

Originally Posted by lovedslr

wow! Is it the exchange rate or it is just expensive?

nz $ = us $0.65

NZ Average wage (Overall) $67,000 Per annum; but regional median wage = $23,000 per annum

[LoveDSLR]

Thanks Ken! Interesting facts.

[kencaleno]

For original poster-You could always lock focus: auto focus on the spot you want (shutter button halfway down)then switch to manual focus, which will lock the focus,then when item hits pre-focused spot,press shutter button all the way down.

[Mr Guy]

Trap focus or "Catch in Focus", depending on brand, is a great way to do this. That doesn't FULLY answer your question though. There's a few other tricks that will help.

First, how long your camera takes to autofocus depends not only on the speed of the motor and type of lens, but how far out of focus it is in the first place. Depending on what sport and what camera, your autofocus may be perfectly able to keep up, but you can always help it by focusing on something nearby, say the edge of a basket, a yardline, a nearby player, or even just by manually focusing to a certain distance, if your lens has distance markers on the focal ring.

Secondly, autofocus may not be necessary anyway. Catch in focus is better for single shots than multiple, and focus confirmation can slow you way down. Manually focusing can be much faster, once you're locked in.

Thirdly, you have to know how "in focus" you really need it to be anyway. Using the highest ISO to achieve acceptable results will allow you to bump up the shutter speed and the aperture. You need the shutter speed to lock motion, and the aperture to give yourself enough depth of field to allow room for error. That's part of why pros use flashes even in brightly lit auditoriums.