focus points/aperture settings + shooting groups & couples

[LeoChanPhotography]

hey guys!

hope this doesn't sound like a stupid question, but here goes anyways. :)

i've got a canon 40d. what focus point do you use to shoot groups and couples? and what aperture settings?

if the couple is in two parts of the frame, how can you lock focus on both of them? does the 40d allow for multiple focus points? if i used centre focus point and say the woman was on the left and the man on the right, it would focus on the background. if i used a left focus point, it would focus on the woman on the left only. if used the right focus point, it would focus on the man on the right only. do you get by that by having a smaller aperture like f/11 or something?

for groups - how do you make sure everything is in focus? just setting the aperture to f/11? where would you put your focus point with so many faces? :)

thanks!!

[tdodd]

Focus is about distance to the subject, measured straight forwards from the lens, not radiating out in all directions from the lens. If the two people are the same distance away just focus on either one of them - not the gap in the middle. If one is in focus then the other one will also be in focus. You can select a focus point off to one side, within the camera, that covers the subject you choose, or focus with the centre point and then recompose your shot once you've locked focus. Often it is a good idea to focus on one eye (typically the nearest one), as that is usually the most critical part of a person to have really sharp. Anything in front of the eye - nose, mouth, cheeks, chest etc. and anything behind the eye - hair, ears etc. will also be in focus subject to there being sufficient Depth Of Field.

Depth Of Field is a range of distances from the lens at which things will be acceptably sharp. The only truly sharp point is at the distance at which you focus, but points in front of the focus point and also behind it will appear sharp if you have sufficient Depth Of Field.

Depth of Field varies depending upon....

- the size of sensor in your camera;
- the focal length you are using;
- the aperture you are using;
- the distance to the subject.

You can find many detailed explanations on the internet, which Google will uncover for you. Here is one....

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

There is an online DOF calculator, where you can play with some numbers to see how DOF varies with different settings. You should also be able to work out what sort of f/stop value you require in order to shoot rows of people, for a given camera body, focal length and distance to the group. You will also be able to figure out whereabouts you should focus for optimum sharpness throughout the group. If the group is three rows deep you may find that focusing on the faces of the middle row will give you the best overall sharpness, or maybe the ears of people in the front row. Play with the numbers and find out....

http://www.dofmaster.com/dofjs.html

[LeoChanPhotography]

very helpful advice. thanks. :) regarding focus with centre point, then recomposing - how effective is that? and how does that work? like i'd focus the camera on the woman on the left first, then recompose my shot with the couple to the composition i like? the camera will know that my first focus was on the woman? :P i've always wondered about that.

[tdodd]

It can be very effective. However, at close range with very wide apertures it can lead to back-focusing, where your really sharp focus point ends up a little bit too far behind your subject, because you are actually changing the distance from the front of the lens to the subject, when you focus and then recompose. There is an article on the problem here....

http://visual-vacations.com/Photography/focus-recompose_sucks.htm

If you have sufficient DOF, or a decent bit of distance to your subject then the focus error gets completely swallowed up by the DOF, and will not be noticed.

Apart from that small caveat, you need to have the camera in One-Shot mode, make sure the camera is only using the single focus point of your choice (green box mode won't work, for example, as it takes away your control to select your own focus point) and then aim and focus with a half press of the shutter button. Keeping the shutter button half pressed you then recompose your shot and fire. The original focus will be held so long as you maintain the half press on the shutter. If you accidentally release the shutter before completing the shot you will need to start again.

One solution to minimising the focus error with the focus/recompose technique is to use a focus point, other than the centre one, which most naturally covers the subject you are focusing on with the composition you want. e.g. If the woman is on the left you might use the top left diagonal focus point and place that over her eye, or even just the top focus point. That will make the composition when you focus closer to your intended final composition and thus improve focus accuracy because you will be moving the camera less after focusing.

There are really many ways to skin this cat, including making use of the AF-On or * button to separate focusing from metering, but really that is stretching the topic a bit far for this conversation.

[Wilt]

Here is a chart showing various subject distances, various FL lenses, and the ability of the DOF to handle focus shift that is caused by focus and recompose with an APS-C frame to various angle changes.

http://i69.photobucket.com/albums/i63/wiltonw/focus_recompose.jpg

Column D shows the subject distance resultant from the recompose shift. If that is a larger number than the Near DOF distances shown in columns E thru L, DOF ameliorates focus recompose focus errors. So shooting portraits with 50mm lens at f/2 at shooting distance of 10' can result in focus recompose errors, as was illustrated in an article on the web. But most other situations are not a problem!

Note that a up to 20 degree shift of composition can be generally accomodated by even wide aperture lenses, unless very close subject distance.