Have you ever thought of the similarity between moving a camera lens farther from the film to focus for a close-up and raising the enlarger for a larger print? The camera and enlarger lenses then project larger image circles at the film or paper plane, requiring longer exposures. Users of in-camera meters never have to give this a thought, since these meters sample the same dimmer image the film receives. However, users of separate meters ignore this at their peril.
The f-stops marked on a lens become progressively less accurate as you focus closer to the subject. This is because an f–stop is defined as the focal length divided by the diameter of the aperture. But the effective f–stop is the focal length plus the amount extended from infinity focus, divided by the diameter for the aperture. For example, when a 90mm lens is extended 37mm from its infinity focus position, the f/8 marked on the lens actually delivers f/11.
It is essential to know (or carry in written form) the threshold lens-to-subject distance at which the difference between the marked and effective f–stop begins to be significant for each lens you use. I call this he close up triggering difference (CTD). It is approximately nine times the focal length of the lens. It is important to keep this distance in mind for each lens in order to note when an exposure correction is needed. Failure to make such correction when the camera is much closer to the subject than the CTD can result in serious underexposure.
Figures 1 and 2 include the CTD for each lens I use. For lenses of other focal lengths, multiply the focal length by nine. Then, divide by 25.4 to get inches and divide the number of inches by 12 to get feet. After dividing you may want express any remainder in inches rather than as a decimal. If so, multiply the number of whole feet by 12 and subtract this from the original number of inches to have the remainder in inches.
After focusing the camera and noting whether the subject is within the threshold distance, the needed additional exposure can be determined in several ways. Since I use view cameras, my favorite method is to measure the lens-to-groundglass distance, then consult the table in Figure 1 (Method A) to find the number of additional stops or fractions f stops if needed. I will also discuss measuring the amount of extension from infinity focus, which is useful with rollfilm cameras (Method B). In either case, I recommend taking care of the adjustment by lowering the meter’s film speed setting, as described in my article in the November/December 2002 issue of PT, along with any adjustments for altered development or a filter, if necessary.
Method A: Lens-to-film distance
For a view camera lens that has a between-the-lens shutter and is not a telephoto lens, use the table in Figure 1. (A true telephoto lens can be identified by the fact that it needs less bellows draw than its focal length. It is not simply longer than usual lens.) Measure the distance from the center of the shutter (presumed to be the optical center of the lens) to the center of the groundglass. Go to the row for this focal length and find the number nearest the measured difference. The number of stops of additional exposure appears at the top of the column. If you own only one or two lenses, you may be able to dispense with the table by writing the exposure corrections directly on a cloth tape measure.
Method B: Extension from infinity focus
Method B is appropriate for rollfilm cameras that use separate meters, true telephoto lenses, and view cameras that focus only by moving the lens sage; i.e., the back might swing or tilt, but the camera has no rear focusing knob. This method uses the table in Figure 2. The numbers in this table are extensions from infinity focus, in millimeters. After focusing and noting that the camera subject difference is CTD or less, find the extension (discussed below). Go to the column in Figure 2 for this lens and find the number nearest the measured extension. The number of stops of additional exposure is at the left end of the row.
Users of view cameras that focus only by moving the lens stage may be able to attach a millimeter scale to the camera bed and place a mark, which points to zero on the scale when the camera is focused on a distant object, n the lens stage. Then when the lens is moved forward for a close-up, the millimeters of extension can be read directly from the scale. (Such a scale is also useful for determining where to set the focus when there is a considerable difference between the lens positions when focused on the nearest and farthest parts of a subject. Note the number on the scale the pointer points to when focused to the two extremes, and set the focus so the pointer points halfway between. Then, when the lens is stopped down, depth of field will extend equally well to both extremes).
Rollfilm users should prepare for Method B as follows. Set each at infinity focus and measure the distance in millimeters from the camera body to the front edge of the lens barrel. Call this the “infinity focus distance” (IFD). Record it in the column for this lens in the IFD row of the Figure 2 table. After focusing and noting the camera-subject distance is CTD or less, measure from the camera body (the same spot that was used for finding the IFD) to the front of the lens. Subtract IFD from this measurement to get the millimeters of extension, then use the Figure 2 table as described above.
Adding other lenses to the tables
Adding another row to the table in Figure 1, or another column for Figure 2, is very easy since the distances lie on a straight line when plotted above an x-axis that consists of lens focal length. If you don’t have plotting paper, simply note the adjacent values on either side of the graph and choose intermediate values, skewed toward one of the adjacent values if the new lens focal length is closer to one focal length than the other. If you want to be more precise, or the new focal length is outside the focal length is outside the focal length range in the table, make a simple plot to find each new value. The x-axis is focal length, the vertical axis is distance in inches for Figure 1, or millimeters for extension for Figure 2. Draw a vertical line up from the x-axis at the new lens focal length. Plot two or three adjacent values (from the same column of Figure 1, or the same row of Figure 2) and connect them with a straight line. The new value is found at the intersection of this line and the vertical line you drew above the new focal length.
I still have in my mind’s eye “the one that got away” nearly 40 years ago because I didn’t apply the close-up exposure correction. I hope it won’t be necessary for you to lose any photographs in order to be motivated in order to keep your close-up triggering distances in mind.