January

Why do I have to expose my film one stop more than the manufacturer's ASA rating to get a good black-and-white negative to print from?

In theory, you should be able to obtain correct exposures from any film by shooting at the ISO rating. But this depends on the accuracy of at least three variables: your camera or handheld meter, the method of using that meter and the camera's shutter. Because a difference in any or all of these can result in variations in the amount of light reaching the film, many photographers find that they have to modify the ISO rating and exposure film at a different speed setting or E.I. (Exposure Index). For example, an EI of 80 instead of ISO 125.

In the case of black and white film, the key is to use an exposure that will be enough to capture shadow detail on the film and to then use a development time that will not overdevelop the highlight areas. If the exposure fails to record the shadow detail then nothing can replace it. Highlights, however, can be burned or dodged or better still, controlled by changes in the development time. Hence the guiding principle of "exposing for the shadows and developing (and/or printing) for the highlights."

To do this properly you need to take into consideration all the variables mentioned plus the way you actually process the film. The best procedure to follow is to take the time to do what is called a film speed test. Photographer Tom R. Halfhill has posted a very complete series on this whole subject in two parts that will give you all the information you will need to carry out such a test and interpret the results:
home.pacbell.net/halfhill/speed1.html

www.halfhill.com/speed2.html

Joseph R. Meehan
Senior Technical and New Products Editor
Photo District News

Are depth of field and depth of focus the same thing?


No, these terms refer to different aspects of the focusing process. When you focus your lens on a specific point, the area in front of and behind that point that appears sharp is referred to as depth of field. This area is typically controlled by the aperture setting of the lens. The smaller the aperture opening (e.g. f/16, f/22), the larger the depth of field. That is, the greater the area in front of and behind the point of focus. Opening up the lens (e.g. f/1.8, f/2.8) will reduce the area in focus producing a "shallow" depth of field. The control of depth of field is an invaluable tool in composition since a shallow depth of field allows you to throw backgrounds and foregrounds out of focus to isolate your subject. On the other hand, using smaller apertures will keep the subject, background and foreground in focus providing for a more integrated composition.

Depth of focus also refers to an area in focus but this area is behind the lens occurring around the film plane. In other words, a much smaller area extending just behind and just in front of the plane where the film or digital sensor is located. While changing apertures noticeably alters the depth of field in a picture, these changes have no practical effect on the depth of focus. In other words, with 35mm and most medium format cameras you will not be producing changes in the picture because of any alterations in the depth of focus. The only time depth of focus can affect the picture is when the photographer controls the film plane alignment. This is possible with most sheet film cameras where the film back can be set at different angles other than absolutely parallel to the plane of the lens. The ability to change the angle of the film back while staying within the depth of focus areas along with the ability to change the position of the lens plane are referred to as view camera movements. Together, these give the photographer the ability to make certain corrections in perspective and to increase or decrease depth of field beyond what the aperture can do. This flexibility is one of the main reasons for using a view camera design.

Joseph R. Meehan
Senior Technical and New Products Editor
Photo District News