This will give you a lot of information, it is quite technical but everything is there http://toothwalker.org/optics/dof.html

Going onto the other question about using Av or M mode, that is probably my advice.

It is a very common mistake to think that shooting Tv would be good for sport as you can set a fast shutter speed (this is assuming we are talking about freezing action, not creating motion blur). The problem is that in doing this you can easily set something that is too fast for the available light so everything will be under exposed. Or initially things are exposed OK then the light reduces and then you are again in the under exposed situation.

Another element of shooting the majority of sports is you want a shallow depth of field. If you are shooting Tv and there is a lot of light then the aperture will shut down increasing your dof which isn't desired. So there are 2 instant problems with Tv.

If we now look at AV you are going to set the widest aperture (low f number) which will ensure you are getting the most light into the camera thus allowing fastest shutter speed along with correct exposure and also the narrowest dof so you isolate your subject. If the light increases you get a faster shutter speed so you still get a good exposure, if it decreases you then would have a lower shutter speed but still exposed well. You just need to ensure you have set an ISO setting that allows you to obtain a good shutter speed.

Here is an example of freezing the action and also how the shallow dof helps get rid of a nasty background.



A time when you would want to use Tv in sports shooting is when you want to create a lower shutter speed so you can bring some motion blur into the equation. For example shooting vehicles this is often desired.

Here is an example of using Tv and shooting at 1/60s to get the motion blur (this shot was just off of the start line so not a huge amount of speed at this point so blur is not extreme).



Going on from Av and Tv if you are able then shooting in M is better as you are not letting the camera guess the exposure. For example if something you are shooting is very light the camera will under expose a little as it trys to darken things up and if it is a dark subject it will over expose. This is why you see nearly everyone posting aircraft shots has them under exposed as the sky messes up the metering.

So its the aperture that determines the dof? The wider the aperture (lower number) the shallower the dof and the more narrow the aperture (higher number) the deeper he dof is that correct? So to shoot a pic with the subject and background in clear focus you need to have a smaller aperture and offset that with a faster shutter speed and or higher iso (depending on light)? I'll try and l read the link you provided if I get a couple minutes at work today.
Thanks

Almost but you have a slower shutter speed not faster to get the right exposure.

To make exposure brighter you decrease shutter speed, increase ISO or use a wider (smaller f number) aperture.

To make exposure darker you increase shutter speed, decrease ISO or use a narrower aperture (bigger f number).

If you make a change to one element and want the exposure to be the same you need to adjust one or both of the other variables.


Yes if you don't change other criteria. There are two other main variables which are focal length and distance to subject.

Take a look at http://www.dofmaster.com/dofjs.html and play with the settings to see the changes.

That's part of it. Focal length also makes a difference as does distance from you to the focal point (i.e. the point in space you're focusing on). For example, 400mm f5.6 at 20 feet has shallower dof than 35mm f2.0 at 20 feet - even though the 35mm shot was taken at a wider aperture.
In addition to mark's link, here's a calculater you can play with to see how DOF changes when you vary focal length, distance, aperture:
http://www.dofmaster.com/dofjs.html


Also - another important point - you talked about your subject and background both being in focus. That also depends on how close your subject is to the background. If your subject is in front of a wall and the wall is 5 feet behind them they're both in decent focus even with only a few feet of DOF. Now, if your subject is 40 feet in front of the wall, even with the same DOF value suddenly the background is no longer in focus. So whether the background is in focus or not is partly controlled by DOF but also controlled by how close the subject and background are to each other.

OK I stop typing for a minute and Mark beats me to it

I hate it when that happens LOL.

And that must have been nearer 15 minutes

My brain hurts and I'm just getting started.

Don't try to do too much at once, also when you start playing and tyring things out it will make more sense.

I've been doing more reading than taking pics because by the time I get home I would have to use a flash and I don't know but I suspect that just complicates things more. How much will using a flash change what we are talking about her?