Hyperfocal Distance and Infinity

[lukeap69]

Hi Bob

I've been reading your articles for DOF, Sharpness and Hyperfocal Distance.
Now my (silly) question is should I focus on hyperfocal distance to get the maximum (acceptable) sharpness and DOF?

When to focus on infinity?

What are the difference of focusing on hyperfocal distance and infinity?

Thanks

[Bob Atkins]

You focus on infinity when you want the maximum possible sharpness at infinity.

You focus at the hyperfocal distance when you want the maximum possible depth of field (i.e. usually when you want a sharp foreground extending close to the camera).

For example, let's say the HFD is 6ft. If you focus at infinity, you'll get"acceptable" sharpness from infinity to 6ft. If nothing in the frame is closer than 6ft, you should be fine. If you focus at 6ft, then objects at 3ft and infinity will be "acceptably" sharp (i.e. just on the edge of being "unacceptably" blurred). If there's some important element of your composition at 3ft, then you might want to use HFD focusing.

[KeithB]

Focus is perfect at only one distance from the camera, the focal point.  There is a distance in front and behind that point that has acceptable sharpness called the depth of field.  If you set the focal point at infinity, the focus range extends from in front of infinity some distance to further than infinity 8^) some distance - i.e., the region of acceptable sharpness beyond infinity is wasted.

By using the HFD, you can reclaim that lost area of sharpness and have your depth of field extend from *exactly* infinity to the other closer end of the depth of field.

So if you want to maximize the region that is acceptably in focus, use the HFD.  If everything is way far away, set the focus at infinity.  As for me, I will just use autofocus. 8^)  Especially since my EF-S 17 - 85 seems to have an infinity *range* not an infinity point.  Makes getting star-trails tough - especially with my poor night vision.

[Bob Atkins]

Actually all lenses have an infinity point at a given focal length, not a range. What some lenses do have is a distance marking for infinity that appears to be a "zone". The siginificance of this is that either the infinity focus point for zoom lenses may change for different focal lengths, or (especially in the case of long telephoto prime lenses), the infinity focus point can change as a function of temperature due to thermal expansion of the lens barrel.

With modern AF lenses I would always use AF to get focus and never trust distance markings.

[lukeap69]

Thank you Bob and Keith for very clear explanation.

@Bob
You said, you rather use autofocus rather than distance marking. If you are shooting landscape, where would you normally focus to have maximum "acceptable sharpness"?

[KeithB]

It depends on the shot.  if you are a long way away from everything, just focus on whatever the camera grabs.  This is a case where the scene settings on the camera may be helpful.  The Landscape mode usually tries to set a small aperature for you, otherwise use aperture priority mode.

If there is nearby foliage, or other foreground you need in focus, like the top shot on this page:

http://www.kenrockwell.com/tech/how-to-use-ultra-wide-lenses.htm

you will need to set a very small aperature - which is probably more important than the focus point - and focus a bit behind the detaill in the foreground you want in focus.

Here is where digital shines, take a shot, check the focus on the LCD, and if it is not where you want it, try again.

If you have dim light and no tripod, (the shot referenced above was hand held!), by all means use manual focus.

[lukeap69]

Whew. That's what I was doing before I got some inconsistent shots (probably because I used F22 also on some shots). Then somebody commented about the hyperfocal distance so I read the articles of Bob.

Some of the learning sites suggested focus 1/3 of the scene. Some dispute this is as a always correct method.

Anyway, thanks Keith for taking time to reply.

Cheers

[Bob Atkins]

Focusing 1/3 of the way into the scene is obviously wrong. If the HFD is 6ft and you focus at 6ft, the limits of the DOF will be 3ft and infinity. It's pretty clear that 6ft isn't 1/3 of the way from 3ft to infinity!

The 1/3 rule is a VERY VERY rough "guestimate" of the focus point for "typical" shots which don't involve anything very close or very far away. What is always true is that under such circumstances you get a bit more DOF behind the focus point than you get in front of it.

For closeup macro work, the depth of field splits about evenly in front of and behind the focus point, so again the "1/3 rule" fails (in this case it should be a "1/2 rule").

There's a little known fact that if you point your camera downwards onto a flat plane, the center of the frame will be the correct point to focus on to get both the top and bottom of the frame in focus. You obviously have to chose the right aperture to get enough DOF for this to happen, but if you do the focus point in the center of the frame will be correct.

The snag is that you're not usually focusing on a flat plane when you're doing landscape work :-(

[lukeap69]

I understand now.

Thanks Bob. You've been really helpful.

[KeithB]

One other point I realized, the other important factor is the focal length of the lens.

Other things being equal - as they will be for any given camera! - a wider lens gives you more DOF than a telephoto lens.

For example, my EF-S 17 - 85 at f/8 has a depth of field of 6 feet to infinity at 17 mm and out to 160 feet to infinity at 85 mm!  (I used Bob's calculator to get these numbers.)

As Bob's article states the DOF varies as the *square* of the focal length, so it is a strong influence on DOF.  So, if you want maximum DOF, use your widest lens or the wide setting on your zoom.

[lukeap69]

Yup, I observed that too. Thanks.

[Bob Atkins]

The focal length of the lens only really matters if you're standing in the same spot.

Unless you are focusing close to or further away then the hyperfocal distance, depth of field depends on on magnification. So if your magnification is, say, 1/10, then it doesn't matter if you use a 28mm lens of a 200mm lens, depth of field is the same.

Standing in any one spot, a widenagle lens gives you lower magnification than a telephoto lens, so then the wideangle will have a larger DOF.