Focus at infinity

I am trying to determine how the following works.

Lets say I have a lens that its maximun focus distance is 30 metres after that

it is infinity.

Does that mean if I am shooting a subject that is 50 metres away that I should

just turn the focus to infinty and leave it at infinity? I think this is right.

Now does that mean that subjects that all fall into the infinity range (longer

than 30 metres)will be in focus?

I am taking a 300/4 to the footy and have never shot with all this manual

focus gear, previously I would just continue to follow focus regardles of the

subject distance and the pics would be sharp. Given that I can't do this with

manual gear how would you ensure photos are sharp with subjects at varying

distances in the infinity range??

Please help I am doing my head in trying to work this out.

INFINITY - Distance from the camera that is far enough away that any object at or beyond it will be reproduced sharply when the lens is focused on its infinity setting.

See also

http://en.wikipedia.org/wiki/Hyperfocal_distance

Hyperfocal distance is nice in theory, but the fact is, players at the footy are not going to be infinity metres away from you. You still need to focus at that range.

I spend most of my time when using telephotos in the last few cm's before the infinity value, way past the 50m number, and although it requires a feather touch because the focus moves so much with the slighest adjustment, it basically has as much effect as you'd get at the beginning of the focus range.

Test it out yourself. Go find a power pole that's further than 30 metres away, set the focus to infinity, and see if you are happy with the results.

You will need to focus each shot to achieve the sharpest photo's. you will need to use a shutter speed of at least 1/250th if your going to try hand holding. with a monopod you will get better results with a tripod and ball head you can shoot down to about 1/90th any slower and you will start to see movement blur from the players.

You will be able to see what is in focus looking through the viewfinder and that will tell you if you need to turn the lens to focus.

I may be a bit late getting in with this but it may help you in the future. Use the distance scale on your lens. That is what it is for. <P>

<center><img src=http://www.geocities.com/dainisjg/hassey2.jpg></center><P>

The lens on the left has been focused to infinity. On the scale, 18 (feet) is above the 22 (f/22). At f/22 everything will be in acceptable focus from infinity to 18 feet. On the lens on the right, the the focus has been adjusted so the infinity mark falls above 22 and now 9 feet is above 22 on the left side of the lens. Everything will be in acceptable focus from 9 feet to infinity.

<P>Look at the distance scale on your lens. If you use fast film in bright sun, you should be able to shoot at f/22. You should be able to get the 200 feet above 22 on one side and 70 feet above 22 on the other side, more or less. If the action is between those distances you can fire away without having to try to adjust focus on the screen. Bad at judging distance? Then focus on something and read the distance off the distance scale. That is what it is for. <P>

Indoors at parties or other dimly lit areas, I prefer to use the distance scale rather than to waste time fiddling with the focus off the ground glass. People are about 10 feet away? Move 10 to the center pointer, bring up the camera, compose and shoot. What could be easier or faster?