Sept. 2000
President's
Message
Portrait of Karl Leck
Photo FAQs
Photo FAQs
Eric Kissa
Q.: What
is the depth of field?
A.: Not everything is sharp on the film. A lens forms a sharp zone between unsharp regions on either side. The depth of field is the width of the apparently sharp zone from its edge closest to the camera and the edge farthest away. The depth of field is imaginary. Optical geometry dictates that rays coming from two points at different distances cannot focus at one point but form a circle on the film plane. If this circle is very small it looks like a point. The largest circle which still appears as a point is called the circle of confusion. The size of the circle of confusion is arbitrary, depending on the resolution of the eye and the viewing conditions.
Q.: What is a narrow depth of field?
A.: The term "narrow depth of field" means that the sharp zone is narrow. A narrow depth of field works either for the photographer (selective focus) or against (inadequate depth of field). Selective focus is used to separate the subject from a disturbing background or from the foreground as well. Inadequate depth of field is a common problem, specially in close-up photography.
Q.: Do telephoto lenses have a narrow depth of field?
A.: The belief that long focal length lenses have a narrow depth of field is a common misconception. The depth of field depends on the aperture but is independent of the focal length. The misconception has probably arrived from the fact that for the same f number long lenses have a larger aperture and consequently a narrower depth of field. The f number is the ratio of the focal length to the aperture. If we increase the focal length, let us say by a factor of 2, the aperture has to increase by a factor of 2 as well for the f number to stay the same. For those who are not allergic to algebra I give the formula for the depth of field W : L2 W = -------------- (D/k) - L where L is the distance from the optical center of the lens, D is the aperture, and k is the required angular resolution (circle of confusion). The aperture D is the diameter of the opening in the lens that admits light to the film. The equation shows that the depth of field depends only on the aperture of the lens and the distance of the object being photographed, the required resolution k being constant. A large aperture, D, and a short distance, L, give a shallow depth of field. Stopping the lens down will make all faraway objects appear sharp. The distance L = D/k is the hyperfocal distance, because W is then infinity. Everything is equally sharp beyond this distance.
Q.: The selective focus technique requires a large aperture?
A.: Yes, this is one of the reasons pros prefer wide aperture lenses. However, the use of a large aperture is not always straightforward. A larger aperture (a smaller f number for the same focal length) requires a faster shutter speed. If a flash fill is used, the maximum shutter speed is limited by 1/250 s or lower, depending on the SLR camera. The photographer on a bright day has two options: to use a slow film or a neutral density filter. A polarizer helps as well to slow the shutter speed. The A mode (aperture preferred) allows to select a large aperture but when the flash is turned on, most cameras reduce the shutter speed to the synch speed and a gross overexposure may result. When photographing people outdoors with fill flash I use the shutter preferred S mode at 1/250 s to get the narrowest depth of field possible.
Q.: How can you tell that a portrait was shot using a narrow depth of field? If the background is out of focus but the foreground is not visible, the depth of field may have been quite wide?
A.: Yes, this is correct. However, let us consider the practical side of it. To shoot a portrait by using the selective focus technique, the photographer selects a wide aperture to narrow the depth of field and focuses on the eyes of the model. If the eyes are not at the same distance, the camera is focused on the closer eye. This does not mean that portraits are always shot with a narrow depth of field. Imagine a portrait of a person eating a lobster and drinking wine in a restaurant. The disturbing background can be eliminated by selective focus but the depth of field in front of the person must be sufficiently wide to keep the person, the plate and the wine glass in sharp focus. The point of focus is somewhere in front of the person and the depth of field is adjusted to get the person, but not the background, in sharp focus. When a model is positioned against the sky or a smooth backdrop, a selective focus is not needed, unless desirable for a certain pictorial effect.
Q.: What is the technical difference between regular lenses and macro lenses? What are their pros and cons in close-up photography?
A.: The true macro lenses are single focal length lenses corrected for a flat field, a high resolution and low distortion at close distances. Some macro lenses can produce life-size images without extension tubes. Cons: Expensive. In New York the prices are: Nikon 105 mm/2.8 $650, Canon 100mm/2.8 $580, Minolta 100mm/2,8 $600. Macro lenses made by independent manufactures are more reasonably priced. Both the Sigma 105/2,8 $389 and the Tamron 90/2.8 $400 are very good. Some zoom lenses can provide a 1:2 life-size at a lower cost. Their optical performance is adequate for nature photography but not for copy work requiring a flat field and a low distortion. A close-up lens and an extension tube (for single focal length lenses only) are low cost options to be considered. Erik ekissa@aol.com
Another e-mail question was sent from a Club member to Bob Coffey.
Q: Where can I have my negative print film push processed, since I had my camera set on the wrong film speed?
A: To my knowledge, push processing the C-41 process is not available. All is not lost, however! With negative film you have much more latitude than with slides, so you can be one stop over/under and still make very acceptable prints. It is preferable to be over exposed as it provides a denser negative. In your case, you were one stop under exposed and the negative image will be thinner (less opaque) than it should be. The print processor will automatically compensate with shorter exposure time to achieve the 18% neutral density gray standard and you should get acceptable prints. If you are not satisfied, the operator can reprint with a manual override to adjust per your request. (Response by Bob Coffey)