Split focus tip

[baoliun]

Sup folks!

An interesting tip I read in K.Malkiewicz book,which I though might be helpful for some of you here:

"Splitting focus " between 2 objects at different distances. They will both be equally sharp if we focus for a point not half way between them but 1/3 of the separation distance from the closer object. For example,two objects at ten and sixteen feet respectively would both be equally in focus if you focus for twelve feet. This is often referred to as the 1/3rd distance principle.

 

[mad_hatter]

I've not heard of this as a principle before. It makes perfect sense, of course. But saying that both objects would be equally focused, it strikes me that, more often than not, neither subject would actually be in focus; both subjects would be equally out of focus.

Am I missing something?

 

[baoliun]

It is a hyperfocal focus,both subjects will be equally sharp,but not as sharp as focusing for either 10 or 16 feet obv

 

[jax_rox]

1/3 in front, 2/3 behind critical focus point will also be in focus, which is how this works.

That's why if you're ever distance judging for focus, you have more chance of getting it spot on if you misjudge a little forward, rather than behind.

 

[A.D.]

Here's a couple of links to some quick guides on hyperfocal distances and dof front/rear distribution (it's not always 1/3 front, 2/3 rear).

http://www.cambridgeincolour.com/tutorials/hyperfocal-distance.htm
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

 

[stef]

I've not heard of this as a principle before. It makes perfect sense, of course. But saying that both objects would be equally focused, it strikes me that, more often than not, neither subject would actually be in focus; both subjects would be equally out of focus.

Am I missing something?

I think the point is that most people would think that focusing halfway in between would work best. It doesn't. DOF shortens as you get closer. This is a standard thing to deal with in photography.

Both subjects won't be out of focus because a critical element was left out - you set the distance 1/3 of the difference between them, and set the iris to bring the first subject into focus. The far subject will then be in focus. Of course, you'll have to have enough light the smaller you make the iris.

 

[A.D.]

The 1/3rd rule is more applicable to it's common usage in landscape photography, shooting deep scenes while using a very wide angle lens. If you check out the first link of the two I posted, you can see a very basic chart about half way down the article. At 10mm it's 70/30 f/r, at 20mm it's 60/40 f/r, and by 50mm it's 55/45 f/r.

So, if shooting between 10 and 20mm, then by all means use the 1/3 rule, but once you get to 35mm you should ease back a little more towards the halfway point, and by 50mm you should be focusing pretty close to (but not over) the halfway point. For shots longer than 50mm there is very little change in the f/r distribution past 50/50, and so we would usually go for the halfway point every time.

Mad Hatter - That depends on what each person considers is 'acceptable focus'. This is affected by things like the final viewing size of the shot/resolution/format/eyesight etc. While the center of the focal plane is the sharpest point still, there is an area either side that is in acceptable focus - the size of that area is down to the points above, and any artistic considerations of course.

If anyone wants to get really serious about dof, then check out one of the dof calculator programs available online. Just google for 'dof calculator'.

If they haven't done so already, Canon users can install Magic Lantern, which can identify what lens and focal length you are shooting at, and displays the focus distance, hyperfocal point, and the near/far focus points.