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Depth of Field (DOF) and Hyperfocal distance
In this article I am going go outline some basic concepts of Depth of Field and Hyperfocal Distance. As a little extra you can download my Hyperfocal Distance Calculator to create your own tables for use in the field and for experimentation.
Depth of Field is defined as the range in front and behind the focus point that is still in focus and sharp. Since perceived sharpness depends on print size, viewing distance and lighting conditions as well as your eyesight.
Therefore I will use quantifiers like shallow to describe the Depth of Field.
Focusing and Depth of FieldIn Landscape Photography I often try to achieve a maximum Depth of Field (picture of El Capitan). Getting such a deep DOF requires planning.
You need to set a very small aperture (large f-stop number). I therefore use Aperture Priority Mode (Av on most Cameras) and place the camera on a tripod, since a small aperture usually results in a long exposure time.
Since I want to keep the foreground and the background in focus, I usually use manual focusing and set my focus distance to the Hyperfocal Distance (see below).
Some lenses (usually prime lenses) offer a Depth of Field Scale. This scale will show you how far the range of the DOF extends at a certain aperture. Although this is a good reference, it is normed. Since perceived Depth of Field also depends on print size and viewing distance, those values are not always accurate in all situations.
Digital SLR usually have a Depth of Field Preview Button that is located somewhere near the lens. Unfortunately most Digital SLR with crop factor have a relatively small viewfinder, limiting the usefulness of this preview. Since my Eyesight isn't what it used to be anymore, I don't use this very much and I rather depend on experience or a cheat sheet I created.
An easy rule to remember is the 1/3 - 2/3 rule: For any f-stop, the depth of field extends about twice as far behind the focal plane then in front. Therefore the saying: Focus 1/3 of the way into your image.
Example: If your DOF is 30 meters and you want to have everything between 20m and 50m in focus, you need to focus at about 30m (1/3 of the distance between 20m and 50m). Depending on your nearest point, you can replace infinity with approx. 50m to 300m (if you focus close, use 50m).
When you focus on a subject at close range (macro) this rule doesn't hold true anymore (the distances become approx. equal).
DiffractionA small aperture (small lens opening) creates another problem, diffraction. I don't want to go into too much detail (just follow the Wikipedia Link). Suffice it to say that Diffraction will scatter the light rays to a certain extend. The smaller the aperture, the more scattering you get, until the diffraction pattern becomes as large as the pixel site of your camera. For the Canon 20D (crop factor = 1.6), this aperture is about f/16. Although I have gone as high as f/22 for some photos, keep in mind that you might lose sharpness again due to diffraction.
Lens SharpnessThe lens is the most important aspect in creating "sharp" images. Unfortunately lens quality is directly reflected towards their prize. This is especially true for zoom lenses. I try to stay away from the so-called super-zooms, because it is very hard to obtain sharpness through the entire zoom range for those. I rather use multiple lenses for different purposes. The sharpest lens I have is a cheap 50mm prime from Canon. A prime is a lens without a zoom.
Lenses lose a lot of sharpness at wide apertures, since they require a larger area of the glass to be honed to perfection. A good lens will retain decent sharpness down to very wide apertures.
Many photographers recommend shooting near f/8, for maximum lens sharpness, as this is roughly in the middle of the aperture range for most lenses.
Generally I would recommend not shooting at the widest aperture (smallest f-stop number) your lens supports, unless you have to (low light conditions, no tripod)
RecapLets recap this Section so far:
Limiting Depth of Field to draw attentionMaximum Depth of Field is not always the way to go. DOF can be used as a creative element, especially in portrait photography or when you want to draw attention to a certain part of your photograph.
In those cases the background will only distract the viewer from the actual subject.
Putting your subject in focus and everything else out of focus is a good way of drawing attention to the subject.
Make the aperture just wide enough to put your entire subject in focus and focus just in front of the subject or on its closest point (remember the 1/3 rule !). If the aperture is too large, portions of your subject will be out of focus and if the aperture is too small you will have distracting background elements in your picture.
Point and Shoot cameras usually have a very small sensor with a large crop factor (CF). This makes achieving a small Depth of Field harder, since the Depth of Field is CF times larger then with a 35mm camera. (The opposite is true for large format cameras).
This increase in depth of field can be very useful when you are shooting landscapes (like El Capitan) and can be somewhat of a hindrance for portraits (like the deer picture).
Another thing to keep in mind is how the out of focus background looks. Bright spots will also distract the viewer, even if they are out of focus. The eye is magically drawn to these spots. The quality of the Lens will determine how these spots look like and if they become very distracting. This is called Bokeh and explained pretty well by Brian (check the link).
To maximize Depth of Field, you can focus on the hyperfocal distance. Switch your lens to manual focus and focus on the range you have calculated.
Everything from 1/2 of this distance to infinity will be in focus.
I have used this technique for the photo of El Capitan, to have everything from the snow covered stones to El Capitan in the background in focus. I had printed out some values from my calculator and I had those "cheat sheets with me". I found a good spot and then I focused on the stones to measure their distance (reading the distance from the lens). Then I picked the hyperfocal distance from the chart (twice the distance from the nearest stone). Knowing that the focal length of my lens was at 24mm (Canon 24-85 USM Lens) I read the correct aperture setting from the chart. The large aperture mandated the use of a tripod.
Hyperfocal Distance CalculatorInitially I made this excel sheet to print out some cheat sheets for my personal use. I have added some more camera crop factor / sensor sizes to the excel file now.
The Excel Spreadsheet has two tabs. Open the Tab I called "Setup" first.
Usage is pretty straightforward. Don't change anything except the values in the Green Field to set the calculator to your camera crop factor, chose a viewing distance and two print sizes (labeled small and large). You can also modify the eyesight parameter if you are not happy with the results you are getting. To start I would recommend leaving this parameter untouched.
Then go to the second tab labeled "Calc". This is the actual calculator. It will calculate two tables (one for large prints and one for small prints).
I am mostly shooting with a focal length between 24mm to 200mm. You should pick ranges that suit your personal needs (careful: some Point and Shoot cameras are specified with 35mm equivalents in mind).
You might also want to modify the f-stop parameters (its the range I use mostly).
The values in the blue fields will be calculated for your. Those are the hyperfocal distances for your parameters, focal length and specific f-stop. Everything from 1/2 of this distance to infinity will be in focus.
If you own a Pocket PC you can also upload the calculator to your device. It will let you change values in the field and give you a bit more freedom experimenting.
Download the Hyperfocal Distance Calculator
Achieving infinite Depth of FieldAt this point I should probably mention that there are also ways of achieving infinite Depth of Field in situations that go beyond the capabilities of straightforward photos and hyperfocal distance.
Tilt and Shift lensCanon offers so-called Tilt and Shift Lenses. With these lenses you can control the angle of the focal plane and put most of the elements in your image (foreground and background) into the same focal plane. This is probably the sharpest image you can shoot with a large DOF (everything is in the same focal plane). However these things have a pretty hefty price tag:
I don't own them although I'd love to give one of those babies a try.
Helicon FocusHelicon Focus is a Software product that lets you stack multiple exposures with different focal planes (or different areas that are in focus) to create a single image with increased DOF.
You can find it here. I haven't tried it, so I can't speak for the product, but I have heard good things about it. I am somewhat turned off by the amount of work one has to put into creating those images and by the fact that you will have to change the focus for each image. This means you will have to touch your camera and potentially create images that won't stack easily by changing the position of your camera (I always use a wired remote when I have the camera on a tripod).
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