Monday, January 26, 2009

DSLR Photography lessons - Lesson 3. Depth of Field / Hyperfocal Distance.

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Hi Pentaxian friends.

Previous lessons: Lesson 1, Lesson 2

Lesson 3 - Depth of Field / Hyperfocal Distance

Depth of Field

The depth of field is the measurement at which subjects, located at various distances from the DSLR, are in focus. Depth of field changes each time you change the aperture. The lens' focal length also affect the depth of field. A telephoto has a shallower depth of field than a wide-angle lens. Assuming that you are using a 50mm lens with a maximum aperture of f/1.4 and a minimum aperture of f/22, using the larger aperture (f/1.4) would give you the minimum depth of field. In other words, the area before and after your focusing point would be shallow. Look at the photo below:

Using the small aperture such as f/22, would give you the maximum depth of field. In other words, the area before and after your focusing point would seem in focus from near the camera to near infinity. Look at the photo below:


On modern DSLR cameras, when you compose your picture looking through the viewfinder, the lens' aperture is always opened to the maximum. To get a peek at what the final picture will really look like, most DSLRs have a depth of field preview button. Some even have a choice of giving an optical preview (in the viewfinder) or previewing the image on the LCD monitor. When using the optical preview, the smaller the aperture (less light in) the darker the preview. The preview is primarily to check the depth of field. However, if your camera is equipped with previewing on the LCD monitor, the image will appear as it would if you press the shutter release button.

Does shooting at the smallest aperture render a sharp image from the foreground to infinity? The answer is: Not necessarily.

Hyperfocal distance

On most lenses, even modern auto focus lenses, a distance scale shows the distance at which the lens is focused. Using a lens focused at infinity and at a small aperture, f/16 for instance, will render a sharp image of far away subjects but the sharpness of the foreground might not be optimum. Depending on the lens' focal length, the closest focused distance of the lens may be 10 or 20 feet away. This is when you want to take advantage of the hyperfocal distance of your lens. The hyperfocal distance of a lens is defined as follow: The closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. Take a look and download this free Hyperfocal distance chart from DOFMaster.com

http://www.dofmaster.com/charts.html

When the lens is focused at this distance, all objects from half of the hyperfocal distance out to near infinity will be acceptably sharp.


For example, using the downloaded chart above, the hyperfocal distance of the Pentax FA 50mm f/1.4 lens used on your K20D and set at f/16, would be 17 feet. To take advantage of the maximum depth of field available with this lens, manually focusing at a distance would assure a sharp image from 8.5 feet (half the hyperfocal distance) to near infinity. Try it, it really works. I'm not making this up!

Hyperfocal distance shooting is mostly used with wide-angle lenses to about 100mm lenses. Lenses with longer focal lengths have a depth of field too shallow to produce acceptable sharpness from the foreground to infinity.

It is commonly used for street photography, as you want to take your picture as quick as possible. You simply set your aperture (as small as light permits), focus your lens manually at the hyperfocal distance, and make sure your subject is within half the hyperfocal distance to infinity. Using a 50mm lens at f/16, your subject would have to be from 8.5 feet to infinity, to be sharp.

Download this depth of field chart from the same source
http://www.dofmaster.com/doftable.html

With it, you can check what the closest and farthest depth of field is for any lens at any aperture. If you look at the comments below, you can see that the Circle of Confusion (CoC) affects the depth of field. If you like formulas, you can immerse yourself in the calculations as shown on the following site.
However, it is far more practical to find the hyperfocal distance from the chart above and then use the depth of field preview button to test the actual depth of field. Or you can always try a test shot as digital photos cost nothing and you can erase it from the card.

Thanks for reading,
Yvon Bourque
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