Ratical : Analog Photography

How Focus Changes Exposure

One of the basics of photography that I missed for a long time is the simple fact that the aperature labels printed on a lens are for focus a Infinity. This was explained brilliantly in Nick Carver's Light Mtering Course for Film Photography. While the effects are most dramatic on large format, this applies to any camera. The following images of a clear sky were taken with the same settings using my Fuji 60mm Macro lens:

f/6.3 at Infinity

f/6.3 at 0.4m

f/11 at Infinity

Changing the focus dropped the exposure by 1⅔ stops! This because light falls off with distance, and the lens elements move out to focus at close range.

If you're used to TTL this fact is hidden from you, but when metering manually this is a critical insight. On digital cameras today the effective aperature is usually not displayed.

Auto-Extension Rings

The following table is from the M645 documenation when using the 80mm f/2.8 lens, plus measurements I took for the extension length:

Macro Ext Length Magnification Exposure Factor
No. 1 7mm 0.15 - 0.30 1.3 - 1.7
No. 2 23mm 0.29 - 0.45 1.7 - 2.1
No. 3 35mm 0.44 - 0.60 2.1 - 2.6

Measuring the Variation

On many old lenses lense assembly moves out of the body as you pull focus inward, so it's easy to measure—with a ruler. My M645 55mm, 80mm, and 110mm lenses all have a lock-to-lock centerline travel of about 13mm.

Based on the documentation for the extension tubes I can use a ruler to estimate the compensation required for each lens, since the 7mm extension ring incurs a ½-stop loss. This means all three of my go-to lenses have an aproximate near-focus loss of ⅔ stops.

55mm f/2.8N
Distance Exp Compensation
4' ½
80mm f/2.8N
Distance Compensation
5' ½
110mm f/2.8N
Distance Compensation
8' ½

Measuring with a Prism Finder

The meter in the FE401 finder confirms that exposures that I guessed above. If you are using a constant light source, the prism finder may be a supurb way to learn what settings to use.

Last updated on December 17, 2020