Understanding dof Part 2: Time to go Hyperfocal

I think that when the auto focus was a feature of a camera lens more than twenty years ago, a great many photographers never got to understand depth of field and hyperfocal distance.

Take this quick true or false test to see how you go.

1. The only way to have an object in focus is to use the auto (or manual) focus to focus on it.
2. The depth of field does not change with distance from the object.
3. The depth of field does not change when you focus on different parts of a scene.
4. The only way to ensure everything is in focus is to use the largest f-stop possible eg f29, f32 etc, and use a tripod.

How did you go, here are the answers: False, False, False, False ....

let me clarify all this for you.

We know that the depth of field depends on three things: 1. focal length of the lens your are using (17mm, 40mm, 70mm, 90mm, 200mm etc) 2. Where we focus within the scene ie the distance this object is away from us. 3. The aperture setting we have set ie f4, f8, f16 etc.

With today's cameras having auto zoom and auto focus, it is no wonder why it is easy to forget about depth of field (since the dof is changing as we zoom in as well as what we focus on).

Imagine that for a given lens (say 90mm)  it has a "window" of sharpness. When I point at a spot and focus, there is a window of sharpness around this spot. The size of the window (aka dof) will depend on how near or far the spot is from me as well as the aperture setting.

Here is a table of distances (in feet) for my 90mm lens

 

Distance to Subject		f/4	f/8	f/16
10 feet		9.6-10.5	9.2-11	8.5-12
50 feet		41-64.5	34.5-90	26-490

 
A couple of things stand out:
1. As the f-stop increases the window (aka dof) grows.
2. The further into the scene I focus the greater the dof.

 My question now is, what is the distance I need to focus at within a scene to get the biggest possible dof? That distance is called the hyperfocal distance. 

 Here are the hyperfocal distances (in feet) for my lenses.   

 

Lens: 17mm		f/4	f/8	f/16
Hyperfocal distance		8	4	2
	dof	4- ∞	2- ∞	1- ∞

 

Lens: 40mm		f/4	f/8	f/16
Hyperfocal distance		44	22	11
	dof	22- ∞	11- ∞	5.5- ∞

 

Lens: 70mm		f/4	f/8	f/16
Hyperfocal distance		134	67	34
	dof	67- ∞	34- ∞	17- ∞

  

Lens: 90mm		f/4	f/8	f/16
Hyperfocal distance		220	110	55
	dof	110- ∞	55- ∞	28- ∞

 

Lens: 200mm		f/4	f/8	f/16
Hyperfocal distance		1094	547	274
	dof	547- ∞	274- ∞	137- ∞

Some things to note:
1. With a 40mm lens, you can get infinite dof with f16 if you focus 11 feet from you.
2. When focusing at the hyperfocal distance, the dof starts half way between you and the focal point and extends to infinity. Now that is cool!!
3. fast zoom lenses are cool bis of glass but they dont give us much dof.
4. Since there is a nice pattern in the data, I just remember the hyperfocal distance at f/4 for each lens and go from there.

The final part to this study on depth of field will look at how to create super sharp images with deep depth of field when we are close to an image, or the lack of light means we must use a small f-stop say f4 or if our lens gives too much diffraction at large f-stops (f22, f29, f32 etc) but we still need a sharp image with large depth of field.

 If you want to know the hyperfocal distance of your lens, here is a cool calculator I use. hyperfocal calculator

 have fun! ☺  

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