Effect of Focal Length on Macro Perspective

by Roger Stockton

The purpose of this informal test is to demonstrate the effect on perspective that the choice of focal length has when doing closeup shots. The effect is exactly the same as we have all probably seen illustrated in a basic photography text for more distant subjects, but the near universal mantra of "must have 100 mm or 200 mm lens for macro" suggests that it might be useful to illustrate that these rules continue to hold at closer focus distances as well.

The test setup is illustrated below, taken with a Tamron 28/2.5 at f/32 from a distance of about 14 cm, measured from the plane of focus to where the LX film plane mark would be if it had one (I'd never noticed it's absence before).

The Test Subject

The far bolt is nearly parallel to the fins on the black heatsink which supports it, while the other bolt is approximately perpendicular to both. FWIW, the heatsink is 95 mm wide and the near bolt is 60 mm long (about 55 mm from the face of its head to the point where it meets the heatsink); both bolts are 6 mm in diameter. I can't locate the far bolt to measure its length at this time, but it is about 30-35 mm.

In all pictures, the plane of focus was placed as close to the line defining the interface of the black O-ring and the near bolt. The bolt/O-ring interface was maintained as close to the same size in all pictures to maintain constant magnification despite changes in focal length. The lens axis was kept as close to the same angle to the subject (i.e., about 30° to the left of the near bolt) in all pictures. Tripod height was adjusted as required to compensate for slight uneveness of the floor as it was repositioned for the different focal lengths.

Three lenses participated in the test: a Tamron 28/2.5, a Pentax SMC-M 50/4 Macro, and a Pentax SMC-M 100/2.8. Two sets of pictures were taken, one at f/8, and another at f/4. The intent was to ensure that differences in the distribution of depth of field on either side of the plane of focus would be visible, but probably the more successful result is the illustration of foreground bokeh (i.e. on the near side of the plane of focus)...

[Test Results]

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