Macro Photography

Definitions

Macro is close-up photography in which the image recorded on film is from 1/2 to 20 times the actual size of the subject (1:2 to 20:1 image size to object size).

Magnification is shown as a ratio and readable as a fraction. It takes one focal length for infinity and then 1 for each increase in magnification; two focal lengths distance results in 1:1, four focal lengths results in 3:1.

The closer a subject is to the camera lens, the longer the lens aperture to film distance must be for the image to be in focus. The closer the lens is focused, the smaller the depth of field--so that even with the lens set to its smallest aperture, the span of distance in sharp focus may be only a fraction of an inch.

Macro Means

Plus diopters ("close-up lenses") are simple lenses with long focal lengths. The +1 has a focal length of 1000mm/1meter; the +2 is 500mm, the +3 333mm, and the +4 250mm. Plus diopters attach to the front of a camera lens and work on the principle that combining lenses actually reduces focal length. Since the aperture ratio increases as the same rate as light decreases because of image magnification (that is, since the lens diameter doesn't change, the size of the opening relative to the focal length gets bigger when the focal length is reduced), no bellows factor adjustments are necessary. In addition, diopters do not change the angle of view of the lens.

A teleextender is a negative lens systems that intercepts the image and enlarges it before it reaches the film. A 2x makes the image twice as large, leaving the depth of field unchanged (though 2 fstops more light must be put on the subject to compensate). A 3x triples image size and requires 3 fstops more light. These gizmos work very well in macro applications because the distance from lens to subject is so small that there's very little image deterioration. Using a teleextender with a lens results in more space to work between camera and subject, greater magnification, and greater depth of field.

A reversing ring allows a lens to be mounted backwards, thus increasing distance from lens aperture to film plane. A small gizmo typically used with a "normal" lens (50-55mm angle of view), a reversing ring has the effect of enabling closer focus and greater image magnification.

Extension tubes are tubes of various widths that can be placed singly or in combination between the camera body and the lens to increase the distance from lens aperture to film plane. These gizmos are useful in macro applications to increase magnification of image when the lens is very close to the subject. And they can also be used with longer focal length lenses (like a 200mm) to achieve a closeup shot from a considerable lens-to-subject distance.

Macro Flash

Within the realm of closeup/macro photography, the correct flash exposure does not vary with distance from subject. The flash needs to be positioned the same distance from the subject as the optical center of the lens, and that may take some experimenting. But if the flash is at the same distance from the subject as is the aperture of the lens, exposure is constant as distance decreases.

Flash settings for Vivitar 285 (guide number 80) for closeup/macro images
Note: These camera settings already incorporate bellows factor adjustments!

  ASA 25 ASA 50 ASA 100 ASA 200 ASA 400
1/4 power 4 ½ 5.6 ½ 8 ½ 11 ½ 16 ½
1/2 power 5.6 ½ 8 ½ 11 ½ 16 ½ 22 ½
full power 8 ½ 11 ½ 16 ½ 22 ½ 32 ½

Bellows Adjustments

A camera bellows is an accordion-pleated square-shaped spacer that is placed between a camera body and a lens. The bellows is a standard component of a view camera system and is part of its mechanism to control perspective as well as image magnification, but for 35mm cameras a bellows is simply an adjustable width extension tube.

Any extension of the lens causes a reduction in the brightness of the image that must be adjusted for. Corrections in exposure settings ("bellows adjustments") are necessary to compensate when the distance from lens to film plane is increased beyond the focal length of the lens--the light has to travel further so there needs to be more of it.

If closeup/macro means bring bellows factor into play (and any macro means other than plus diopters do), the exposure must be increased. Often the most satisfactory way to accomplish that is to calculate the meter reading for an ISO film speed slower/lower than the film's rated one. Time can be increased--but only if subject motion isn't a problem--and fstop can be reset to a larger aperture--but only if depth of field isn't an issue.

Measurements

The bellows extension length should be measured from the diaphragm of the lens to the film plane.

When doing bellows factor calculations and adjustments, it's important to use the same system of measurement throughout. In the 35mm camera world, everything tends to be measured in millimeters, but with large format cameras, lens focal length is often measured in inches. As rough equivalents between English and metric systems re camera lenses,

 25mm=  1 inch       150mm=  6 inches  
 50mm=  2 inches    175mm=  7 inches  
 75mm=  3 inches    200mm=  8 inches  
100mm=  4 inches    225mm=  9 inches  
125mm=  5 inches    250mm= 10 inches

Formulas

Once the bellows extension has been measured, to make the bellows adjustment:

  1. Divide lens focal length by bellows extension length.
  2. Square the result.
  3. Multiply the stated ISO by that result.
  4. Set exposure based on the adjusted ISO.

In closeup/macro, bellows adjustments control degree of magnification of image, and moving the camera position controls focus:

  1. Determine how much lens extension is desired
    and how much exposure compensation is required for that extra distance.
  2. Lock the camera controls.
  3. Bring the image into sharp focus not by turning the lens focusing ring
    but by moving the whole camera back nearer and farther away in relation to the subject.

If the magnification is known,

Conversely, if the bellows extension and focal length of lens are known,

And if the magnification desired and lens focal length are known,

Lynn Jones, head of the Photographic Technology program at Austin Community College, Austin, TX, provided this content in his Macro class in Spring 1998.


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Last modified on August 17, 2008 by Kay Keys by (kay@kaykeys.net)