yet another focusing question

KmH · Dec 13, 2011

The Mike Hodson illustration Mike posted is pretty good.

But, the 3rd fame showing the effect on DoF of different focal lengths is mis-leading.

The 'person' 15 feet from each focal length lens will be a different size in the image frame (viewfinder) of each focal length lens. The 'person' will be smallest in the image frame when the 24 mm lens is used, and largest when using the 75 mm lens.

In practice, as Mike mentioned above, with a longer focal length we back up away from our subject to keep the subject the same size in the frame. That makes the distance to the focus point longer.

So to keep the subject the same size in the frame using all 3 focal lengths, the subject would be - 5 feet from the 24 mm lens - 10.5 feet from the 50 mm lens - and 16 feet from the 75 mm lens.

With all 3 lenses in the example set to f/4, and with the subject at the distances I quoted, the total DoF for each lens is right about 2 feet.

Here is the important point to understand about the longer focus point distance using a longer focal length, while keeping subject scale constant in the image frame. The total DoF for each lens is the same,but the magnification of the background isn't.

The 75 mm lens magnifies the background a lot more than the 24 mm lens does. The background of the photo made using the 24 mm lens is blurred, but those blurred background elements are smaller in the frame when compared to the background the 75 mm lens magnified. so the blurred background elements from the 75 mm lens are much larger, making them look like they were blurred more. But, they weren't, they're just bigger from being more magnified.

joel28 · May 3, 2012

Big Mike said:
Excellent answer from analog.universe.

Just to expand on this a little bit. A lens can only be physically focused to one specific distance (or at the long end of the focus range, it's just at 'infinity'). So you could never actually get the lens to focus on each child separately. However, if both of them were the same distance away from the camera, then focusing on one of them, would have them both in focus.

So when talking about the focus points on your camera, there are two 'modes'. There is automatic point selection and there is manual point selection. Manual point selection is easy, you choose one of the points, and that's the only one the camera will use. But with auto point selection, the camera may use any of the points (you probably have 9 or 11 points, I think).
I'm sure you've seen situations where more than one of the AF points lights up. That doesn't necessarily mean that the camera is focusing on all of those points...but it does mean that those points are the same distance away, so they will all be in focus. So for an example like the one above, with people side by side, you may get an AF point of both of them to light up (but don't worry if they don't). But if, for example, you had a group photo with multiple rows of people, you could never get the AF points to light up on people in different rows (unless you were really far away) because they are likely different distances away.

But, as analog.universe talked about. We need to think about the Depth of Field (DOF) along with the focus points/distance. To get both kids in focus (or the rows of people), you might need a deeper DOF, which is done by using a smaller aperture (higher F number).
Other factors that influence the DOF are the focal length and the distance to the subject (focus distance).

So if I would like 2 kids in focus, would i sill focus on one of the kids eyes? Or you would focus somewhere in between them, so the DOF spreads on both sides?

KmH · May 3, 2012

The DoF is an area basically parallel to the camera's image sensor front to back, not side to side.
But yes, if the 2 subjects are not equidistant from the image sensor and the DoF is shallow you could compromise by choosing a point of focus (PoF) slightly behind the neasr subject that is also slightly in front of the further subject, but focus sharpness begins falling off with distance from the point of focus.
If your DoF is very shallow it's possible neither subjects eyes would be sharply focused.
That's when you run out of technical options and have to make adjustments so the DoF gets deeper.