Improve Your Photo Sharpness
|
 |
 |
Subjectively, an image is in focus when points and edges of the subject are clearly-defined without visible blur. As seen on the Photo Sharpness Introduction page, this perception varies with image size, as well as with viewing distance. An image may appear in focus at a small size, but at some point of enlargement it will begin to appear out of focus.
More objectively, the concept of focus relates to how precisely a lens renders a point of light from an object onto the focal plane, which is the digital sensor of your camera.
An image is perceived as in focus when the diameter of that point of light is smaller than the resolution of the human eye - at a 10" viewing distance the eye can distinguish, or resolve, features as small as 3/1000 of an inch (0.08mm).
Another way to state this is that when a point is not perfectly focused, it creates a spot called the circle of confusion. When the circle of confusion is smaller than the eye's resolution, an image will appear in focus; if greater than the eye's resolution, the image appears out of focus, as follows.
- In the diagrams to the left, the first circle
represents the resolving power of the eye.
- The blurry point of light that falls entirely with
the second circle is perceived as in-focus.
- The blurry point of light that extends beyond the circle is perceived as out-of-focus.
Depth of Field Extends
Photo Sharpness
When you look at the circle of confusion in the diagram below, you see
that the size of the circle varies with the distance of the object from
the lens. Only when the object is at a certain distance from the
lens will the circle of confusion be smaller than the eye's
resolution. In example 'B', below, the point is at the right
distance to be seen as in-focus. |
| Lens at Maximum Aperture Light from the points at left reaches the lens and is refracted, or bent, to meet the focal plane, the plane of your camera's image sensor. The distance from the lens to the focal point is proportional to the object's distance from lens, so the focal point does not always meet the focal plane at the correct place, as in 'A' and 'C'. This lens has a wide aperture, or opening, to let light through, so the resultant circle of confusion seen at the focal plane is only in focus when the lens is precisely focused, as in 'B'. |
 Ted's Digital Photography Tip Aperture impacts focus & depth of field When you use a smaller aperture to
increase
depth of
field and improve photo sharpness, as in the diagram below, you reduce
the amount of light reaching
the image sensor. Conversely, when you use a large aperture to bring in more light, as in the diagram above, you reduce depth of field. |
Your autofocus system attempts to put a specific point of your subject into precise focus, along with all points in a plane at the same distance from the lens. Depth of field controls the range of photo sharpness by controlling how much of the image ahead of and behind the subject plane are also in focus.
You control the depth of field by adjusting the lens aperture, which is a mechanical diaphragm that varies the amount of light that the lens passes. This, in turn, reduces the size of the light cone, as shown in the example below. The smaller cone yields a smaller circle of confusion at the focal plane, making points in front of and behind the main subject also in focus.
 |
|
Lens at Small Aperture
When a small
aperture is created by use of
a diaphragm, the lens reduces the size of the circle of confusion so
that points ahead of ('C') and behind ('A') the main subject ('B') are
also in focus. All three points then appear in focus.
|
Go to Depth of Field to see examples of the impact of various apertures.
When do want shallow depth of field, and when deep?
- Shallow depth of field is useful for portraits when
you want to soften the image and need only a small range of photo
sharpness. Focus precisely on the near eye,
and adjust a wide aperture to soften the rest of the facial features.
- Shallow depth of field is useful when two objects in the frame are at different distances from the camera and it is important to draw attention to either the near or far object. Focus precisely one of the objects, your primary subject, and the other object will be out of focus.
Ted's Digital Photography Tip Learn a different perspective on focus from Rembrandt Fine art painters have many techniques for achieving sharpness, or the illusion of sharpness, with greater flexibility than photographers. Explore a Rembrandt painting to see what is similar to photography, and what is not. |
- In landscapes, it is important to balance the fine,
distant details with close foreground objects. This helps
give your image a sense of depth and perspective. Deep depth of field
helps these images by assuring photo sharpness from foreground to
background.
- In macro (close-up) photography, it is difficult to
bring a complete subject into focus. A large depth of field will help
to mitigate, but not eliminate, this challenge. Refer back to the push-pin photo.
Bokeh, the Perfect Complement to Photo
Sharpness
With all the discussion about the importance of focus and photo
sharpness, it is important to recognize that a large portion of your
image may be out of focus. The appearance of that out of focus
area affects how people see your image.Bokeh, derive from a Japanese word meaning "fuzzy", refers to those out-of-focus areas. Photographers often judge the quality of a lens by the quality of bokeh it produces.
 |
|
Minolta
Dimage 7Hi, 28-200mm @ 51mm macro
ISO 125, 1/250s, f/3.5 Focusing
close, especially in a macro mode and with
wide aperture, improves the quality of bokeh. |
On one hand, a lens with "good" bokeh produces a milky-smooth texture that fades into the background, helping your subject pop out from the background.
Good bokeh is especially important when working with shallow depth of field, where the out of focus background is intended to offset the crisp focus of the subject.
 |
|
Nikon
D300,
18-200mm VR @ 200mm
ISO 800, 1/125s, f/11 A
lens with average bokeh quality will reinforce its mediocrity if the
aperture is narrowed as in this image stopped down to f/11.
|
On the other hand, a lens with poor bokeh may produce coarse artifacts that distract the eye from the photo's subject.
Continue to Photo Sharpness: Focusing Your Camera, to learn about the mechanics of your focusing system and how they relate to the photo focus concepts on this page.
