Depth of Field

Depth of field plays an important role in photographs and especially in portrait and landscape photography. While Depth of field can add or detract from a photograph, it is also a creative tool used by serious photographers for adding depth and subject focus to their images.

Note: lens sizes and dimensions mention in this article are for 35mm equivilent full frame cameras.

Depth of Field - What it is

Without getting into technical jargon and details, we will define the term, "depth of field" as, "The distance in front of the lens over which an object appears acceptably sharp." Now, let's dig a little deeper and find out what depth of field is all about, what affects it, and how we can use it.

Classis Example See images below

The classic example for understanding depth of the field is the photograph taken of a white picket fence. The photographer is looking down the row of pickets and is focused on one picket a moderate distance away. The pickets that are closer to the camera become more and more out of focus. In the same manner, the pickets farther away become more and more out of focus. All the pickets on either side of the in focus picket, that appear to be acceptably sharp, are in the depth of field.

Therefore, the depth of field is from the closest picket that appears acceptably sharp to the picket farthest away, that appears to be acceptably sharp. 

I have used the term, "acceptably sharp" which is vague, but is also defined scientifically. Acceptably sharp has to do with the eye's ability to distinguish a circle blur from a point in an image.

We won't go there as that is getting into optical physics and scientific measurements. However, there are other things which play on depth of field which are of more concern to us.

Telephoto and Wide-angle Lenses

Telephoto lens have a narrower viewing angle then wide-angle lenses. To demonstrate, hold two fingers up in front of your face about a foot away. Close one eye. As you spread your fingers apart to include more of the scene between the tips of your fingers,  the "V" angle between your fingers increases. 

A telephoto lens has less depth of field than a wide-angle lens. To try to demonstrate this lets use the fingers again.  As you spread you fingers apart, the tips of your fingers draw an arc. The height or depth of the arc is less when the fingers are closer together(telephoto) and greater when farther apart (wide-angle).  The depth of this arc between the finger tips is simply what creates the depth of field. A fish eye lens with a wide 180 angle of view will have a buldging curvature and great depth of field, while a telephoto lens will be more flat with less depth of field.

Portrait photographers using professional full frame Digital Single Lens Reflex (D-SLR) cameras, will use a lens in the 100mm to 200mm range for their portrait subjects.

While focusing on the subject's eye highlights, the area only inches farther away or closer become soft and out of focus.

The depth of field is further controlled by the aperture opening of the lens. This shallow depth of field forces our attention to the subject's face and not any distracting foreground or background.

In portrait photography, "the eyes have it."

Wide-angle lenses have greater depth of field. Landscape photographers will use lenses in the range of 20mm to 50mm, so objects close to the camera are sharp as well as objects at or near the horizon. Therefore, when looking at landscape photographs, unless there is a specific reason, the entire image from foreground to horizon is sharp and in focus.

In landscape photography, "the foreground has it."

Controllable Factors

In both of the above cases there are controllable factors. The first controllable factor is distance to the subject. The closer to the subject the less depth of field. Conversely, the farther from the subject the more depth of field.

For example, many flower photographers will use a 100mm macro lens. Macro lenses afford focusing at close distances. When a flower photographer approaches a flower and is only a very short distnce away, the center of the flower may be in focus while petals and leafs just an inch or less farther will be out of focus.

If that same photographer moves several feet away and takes another picture the flower and the surrounding petal area would likely be in focus.

If the photographer then moves to 20 feet away, and takes a picture of the flower the entire flower bed maybe in focus.

The second controllable factor is the aperture setting. Landscape

The aperture is the adjustable lens opening that determines the amount of light passing through the lens. Also, because of the curvature of the lense it affects the depth of field. The larger the aperture (opening) the less depth of field. The smaller the aperture, the greater the depth of field.

For our example, let's go back to the picket fence. Our photographer is using a 35mm camera and an 85mm lens focused on a picket eight feet away, and adjusts the aperture opening to its maximum of F-2.8.

The depth of field would be from 7.78 ft to 8.23 ft. or about 5-1/2" Now, the photographer adjusts the aperture to the small opening of F-16. The depth of field is now 6.92 ft to 9.48 ft. or almost 31"; an increase of almost 5.6 times the distance.

Now let's visit our flower photographer, the 100mm lens is set two feet from the subject, and the aperture set to F-4.0. The depth of field is 1/4". Changing the aperture to a small opening of F-16 the depth of field is now 1-1/4".

So, in summary for aperture remember, bigger is less and smaller is more; for distance, less is less and more is more.

Sensor Size

Now, there is one non-controllable factor we haven't talked about.  That is the camera you purchase and its image sensor size. I'm not talking about the number of pixels as in mega-pixels, but I am talking about the physical size of the sensor.

Currently there are about nine sensor sizes ranging from 4.3 x 3.4mm to 36 x 24mm. In loose general terms, the more expensive the camera the larger the sensor. The less costly point and shoot cameras like an older Canon PowerShot A100 have a sensor size of 4.5 x 3.4mm, while the current D-SLR in the professional range such as Canon's EOS 5D Mark II, have a sensor of 36 x 24mm.

Without getting into the physical reasons, the smaller the sensor the greater the depth of field. Point and shoot cameras have very limited control of the depth of field and everything in the picture appears to be acceptably in focus no matter the zoom or aperture setting.

This is desirable and totally acceptable for the nondiscerning snap shooter, but totally unacceptable for the serious creative photographer wanting discriminating creative control over depth of field.

Summary

In this discussion we have found that depth of field is the area of acceptable sharpness, and what affects it.

• We now know that telephoto lenses have a shorter depth of field than wide-angle lenses. 
• The closer the camera is to the subject the shorter the depth of field, while greater camera to subject distance provides greater depth of field. 
• Acceptable sharpness toward the camera from the point of focus, is less than the distance of acceptable sharpness away from the point of focus. 
• Large aperture openings have short depth of field while small aperture openings have greater depth of field. 
• Inexpensive point and shoot cameras have small sensors and greater depth of field, while larger sensors like those used in professional cameras have less, and more controllable, depth of field depending on lens type, camera to subject distance, and aperture setting.
    

     28mm @ 5.6 (left)                                      200mm @5.6 (right)

Canon EOS 5D EF 28-105 and EF 100-300 lenses

Notice how the Row of Pickets become out of focus for 200mm.  Look at 28mm, see pickets in background compared to 200mm pickets in background.  Wide-angle lens has more depth of field. 

 Notice how your eye goes to the in focus pickets in the 200mm image.  The background pickets in the 28mm image detract from the foreground pickets and your eye does not know where to go. 

Use depth of field to focus attention on the subject. Use wide-angle lense for greater depth of field and use a telephoto lens for less depth of field, as the following examples show. Return to Classic Example Top

Sun Flower Bud -100mm Macro @f-5.6              Monument Valley - 28mm @ F22

Sun Flower - Notice hairs in foreground to mid flower are in sharp focus while stem and rear of bud is out of focus.

Shot outdoors surrounded with white translucent umbrella to soften hard sunlight.  Short depth of field less than 1" with telephoto at a close distance of less than 3 feet.

Monument Valley - Foreground to horizon is in sharp focus demonstrating effective use of wide-angle lens at small aperature for landscape photography.

Also as a side note: What rules of composition does this photo break, with regard to subject and horizon placement?  Is it still an effective photograph? Return to Controllable factors

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