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PGY 2401C PHOTOGRAPHY I

Depth of field: Correct use of sharpness and blur

New lens designs and the latest films make it all possible: sharpness right where you want it. Creative focusing that's more impressive and easier than ever! As we all know, you first have to choose the right location in order to compose a picture. The distance from the object also has a great influence on the final effect of the picture. The area captured by the effective focal length used and the perspective are equally important parameters.

However, another aspect of skilled composition is often neglected. Some photographers consider the combination of time and aperture to only be a means for metering the light. But there's more to it than that:

• The exposure time creates a dynamic or static impression by using faster and slower shutter speeds.
• In addition to exposure control, the aperture also serves to graduate the image space.

The fully automatic function frequently tempts people to let the camera do all the work. The mediocre photos we've all seen are proof of that. But only a committed photographer knows what the relationship between the sharp and blurred zones will be even before taking the picture and, with enough experience, can even roughly predict the degree of diffuse blur. Use deliberate aperture settings to play with the spatial focal range

The right amount of control

People with SLR cameras can estimate how the effective aperture setting will affect the final photo even before taking the picture. Most exchangeable lenses or high-quality built-in ones have a depth-of-field scale for this purpose. It shows the rough focal range at different aperture settings. In other words, you can see how great the depth of field will be at a setting of 10 meters, for example. The larger the f-number, and thus the smaller the aperture, the greater the range will be in front of and behind the focus setting of 10 meters. Zoom lenses often have several scales because, strictly speaking, they vary depending on the focal length.

Another optical tool some cameras have is a stop-down button. This makes it possible for you to stop down to the selected aperture before actually taking the photo in order to test the effect. You usually take exposures with the largest possible aperture setting, meaning the brightest possible viewfinder image. As the aperture decreases, the image in the viewfinder becomes increasingly dark, while the focal range increases. Experienced photographers can roughly estimate the final definition.

Something you should know...

The term "sharpness" is not explicitly defined in technical terms. There are no precise numbers to represent sharpness per se. Too many factors influence this more visually assessable phenomenon. Illumination and contrast play just as important a role as the contrast rendition of the film. Photos usually appear to be sharper when the sharpness effect is enhanced by contrasts due to a low angle of incident light that is as far as possible from the optical axis. Films with a harder gradation (greater contrast) produce motifs with greater sharpness than softer film emulsions (such as portrait films). The nature of the sharpness and blur is also important. Blur can result from camera shake. Vibrations inside the camera (the action of the mirror) can also lead to general blurring. Partial blurring usually results when an object moves. That's when we speak of motion blur.

What does sharpness do?

The lens aperture not only controls the luminous flux by changing the diameter, but also has a direct effect on the focal range in front of and behind the focusing plane. Focusing is always carried out with the largest, i.e. an open, aperture. Reducing the aperture diameter (increasing the f-number!) increases the depth of field. Since objects are usually three-dimensional, the following situation results:

The depth of field should be thought of as a plane. The image always has an area of high definition and an area which is blurry.

Strictly speaking, sharpness can only be found at the focal point of the light rays, i.e. in the focal plane.

Roughly speaking, the depth of field increases 1/3 to the front and 2/3 to the back.

In the range of 20 times the focal length and with an image in original size, you usually assume a ratio of 1:1. In the close-up range, the ratio switches: 2/3 in front of and 1/3 behind the focal plane. Thus, there's no question that the maximum achievable sharpness is only reached directly in the focusing plane.

There are two factors which limit the depth of field:

• The lens-to-subject distance, i.e. the distance from the motif.
• The linear magnification (i.e. the focal length) of the lens used.

Therefore, the same aperture setting sometimes results in greater depth of field, while at other times it's inadequate. And the absolute depth of field decreases noticeably if you work in the close-up range or prefer longer focal lengths.

In this context, a smaller distance to the object is equivalent to a small depth of field. A telephoto lens produces an enlarged view of the motif. The image is limited by the frame. Getting closer to the object again decreases the depth of field in this case. That's a trick primarily used by fashion and people photographers to make their subjects look like a montage. They work in front of a background that suddenly fades into a complete blur. On the other hand, the small 35 mm lens caused a stir among photojournalists.

It produces greater depth of field at the same aperture setting as the standard lens and at the appropriate distance. Although drastically stopping down the aperture (small apertures) increases the depth of field, the quality of the photo may suffer for it. A photo taken at medium aperture settings (e.g. 5.6, 8 or 11) often appears sharper than one taken with the smallest aperture (e.g. 32). This then results in the general deterioration in the image caused by diffraction phenomena. You should also keep in mind that the depth of field does not depend on the enlargement factor used later on. Only the interplay between sharp and blurry appears more distinctly in the enlargement.

How to get more depth of field

If you manually intervene in the camera program, you can often get more depth of field into the image. You can use several tricks to do so:

• Setting shorter focal lengths (larger frame) at the same distance to the object results in greater depth of field.
• A tripod is generally the best tip for sharpness fans. They can then set smaller apertures, even if the exposure times get longer.
• Using faster films can also help. The jump from ISO 100 to ISO 200 increases the depth of field by one stop, while ISO 400 even increases it by two
• There's not much room for depth of field in the close-up range. If possible, you should increase your distance from the object a bit. That'll give you more depth of field.
• If you can't get close enough to the object: stop down the aperture several steps. This will shift the limit of the close-up setting forward somewhat. If your camera has a stop-down button (SLR models), you can check the effect in the viewfinder.

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