Digital Camera World Forum Hyperfocal Focus

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#1
27-09-09, 01:06 PM
 youngd Junior Member Join Date: Jul 2009 Posts: 14
Hyperfocal Focus

A topic re-visited!!

First I understand the theory behind hyperfocal focusing. However, a question has been perplexing me.

It is the difference between using hyperfocal tables (which appear to give absolute values), and the idea of focusing a third way into the picture.

For example, a table gives a hyperfocal distance of 9 feet at f11 for a lens focal length of 24mm. A question is, is this always the case irrespective of where the horizon (or farthest away focusing point) is? Using the focusing a third into the picture approach, mathematically this example would work if the most distant focusing point is 27 feet in front of the camera. However, what happens if, as would often be the case, the horizon is much further away from the camera eg say 100 feet. Does the hypercocal focusing point become around 33 feet, or is it still 9 feet - quite a difference!

Help!!

Daryl.
#2
27-09-09, 03:30 PM
 chris-p Moderator Join Date: Jul 2009 Location: Sussex Posts: 2,455 Images: 21
Hi Daryl

As far as I understand it the idea of focussing about a third of the way into a scene is to approximate the hyperfocal distance. Given that the actual hyperfocal distance is based on aperture, focal length and sensor size, it's usually easier to guesstimate it - hence focussing a third of the way into the scene.

With regards to your specific example, f/11 is a relatively large aperture for generating lots of DoF so you are exaggerating the effect somewhat.
For example (and my apologies for doing this in metric rather than imperial but I've tried to stick to roughly the same figures you've used) with a 1.5x crop sensor and a lens at 24mm, f/11 focusing on a subject 2.74m (9') you get the following:

Near limit: 134.5cm (~4' 4")
Far limit: Infinity
DoF: Infinite

So everything from about 4 and a half foot to infinity should be in "acceptable" focus.
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Chris

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#3
27-09-09, 05:08 PM
 youngd Junior Member Join Date: Jul 2009 Posts: 14
Hi Chris.

Thanks for the response.

The Table I used was for a 1.5 crop, so you didn't need to do that bit of ther calculation again.

However, if I understand you right, it doesn't matter how far away the horizon is. So, if I use the Table, and set the lens focus at the hyperfocal distance, the horizon, wherever it is, should always be in focus?

Daryl.
#4
27-09-09, 05:32 PM
 chris-p Moderator Join Date: Jul 2009 Location: Sussex Posts: 2,455 Images: 21
So long as your focal point isn't too close and your aperture isn't too large, yes the horizon should always be in "acceptable focus"
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Chris

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#5
27-09-09, 06:17 PM
 youngd Junior Member Join Date: Jul 2009 Posts: 14

Thanks Chris.

Good to be "talking" to you on the new site!!

Daryl.
#6
27-09-09, 06:35 PM
 chris-p Moderator Join Date: Jul 2009 Location: Sussex Posts: 2,455 Images: 21
No worries Daryl. Good to see you too!
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Chris