Depth of Field and Sharpness

For many photographers, depth of field and sharpness are synonyms. Although they share many characteristics the two concepts are not the same.

Depth of Field describes the range of an image which appears to be in focus. Lenses can focus on one distance for each shot. A photo taken with a wide, low-numbered aperture, such as f/2.8, will have a narrow range in front and in back of the focus point which appears to be in focus. The same photo, taken with a small, high-numbered aperture, such as f/22, will appear to be in focus over a much greater range. Several factors, including sensor size and focal length will determine the actual depth of field.

An example: a photo taken from 10 feet with my Canon 30D and 100mm f/2.8 lens
  • at f/2.8 depth of field would be from 9.84 to 10.2 feet. Less than .5 feet would appear in focus.
  • at f/22 the apparent focus would be from 8.9 to 11.5 feet. The depth of field would be 5x greater - over 2.5 feet!
By experimenting with different f-stops at different distances, a photographer can make more or less of an image 'appear to be in focus'.

Photo with shallow Depth of Field
Photo with large Depth of Field

Sharpness is the crispness of an image at its focus point. Sharpness is best evaluated by zooming in on an image on your monitor or by examining a high quality print closely.

An image with shallow depth of field can be very sharp.

Shallow Depth of Field example
Zoomed to 100% actual pixels
An image with great depth of field isn't necessarily sharper than one with shallow depth of field.
Some factors contributing to lack of sharpness:
  • camera shake
  • subject movement
  • 'mirror slap'
  • digital noise
  • optics
Camera shake:
A wide, low-numbered aperture will take less time to make a photo than a small, high-numbered aperture will. As shutter speeds lengthen, the probability of the photographer movement increases. Compare your own steadiness at 1/250 sec. versus 1/30 sec. Traditionally, photographers are taught that the slowest shutter speed they can handhold is 1/focal length - 1/250 sec. for a 250mm lens, 1/60 sec. for a 50mm lens.

Subject Movement:
When photographing in breezy situations, longer shutter speeds will tend to make lightweight subjects move.

Mirror slap:
Mirror slap is the shaking caused by the mirror of an SLR camera popping out of the way to expose the sensor area. The vibrations caused by mirror slap are most evident at shutter speeds between 1/60 sec. and 1 sec. You can avoid these vibrations by using the mirror lock-up feature, which is called the exposure delay mode on many Nikon cameras. Note - this is not the same mirror lock-up feature used to clean your sensor. Refer to your instruction manual for specifics on this feature with your camera.

Digital Noise:
Higher ISO images are generally less sharp than lower ISO images. To counteract low shutter speeds, many photographers raise their ISO. In my own testing, I have found that most SLR cameras begin to show noise above ISO 400, especially in low light. While the noise at ISO 640 or ISO 800 is usually negligible, it can affect critical sharpness.

Lenses do not perform equally at all apertures, nor do they perform equally throughout their zoom ranges. We should expect this, just as we expect that we won't get the same gas mileage at all speeds and road conditions. As with many mechanical things, lenses tend to work best in the middle part of their ranges, rather than at the extremes. In my years of photography, I have found that my SLR lenses are generally sharpest at apertures around f/8 or f/5.6. So, while they have better depth of field at f/22, they are generally not as sharp.

Prime lenses (non-zoom) lenses tend to be sharper than zoom lenses. Zoom lenses that cover either wide angle or telephoto, but not both, tend to be sharper than long range zoom lenses.

There are sharpness evaluations available online. DP Review has tested a variety of lenses. They have interactive charts which allow you to look at the sharpness of the tested lens at different zoom settings and apertures. Here is a link for the test for a Canon 50mm f/1.8 lens - a non-zoom lens of reasonable quality.