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If you shoot slower film you of course have to use more exposure (EV) for the same
Light Value (LV), and vice-versa.
The EV is easy to calculate even if you forgot your meter, since each unit is one stop
different than the next. For instance, with film a stop slower than ISO/ASA 100 (like
50 speed Velvia) you just subtract one from the LV to get the EV. This adds one stop
of exposure.
For instance, if your subject is at LV14, expose at EV13 with ASA 50 film. EV13 gives
one stop more exposure than EV14.
With ISO/ASA 400 speed film you add two to the LV to get the EV, which is the same
as subtracting two stops of exposure. Therefore with an LV14 subject you expose at
EV16.
Remember that the higher numbers refer to higher light values, and therefore less
exposure. This is because the exposure values that correspond to those higher
numbers give less exposure.
EVs are a great idea: by talking about an EV you are talking about any one of many
different combinations of aperture and shutter speed that give the SAME exposure.
Cameras started to use these numbers in the 1950s, but today only the Hasselblad
retains them. With every other camera one needs to use the scales on light meters to
determine the EV values. Some cameras can be adapted, as I did to my Plaubel
Makina 67, which simplifies their use with spot meters.
Zero EV is defined as f/1.0 at one second. Therefore, EV0 is a pretty long exposure.
This is the same exposure as f/1.4 at 2 seconds, f/2.0 at 4 seconds, f/2.8 at 8 seconds
and so on. EV1 is one stop less: f/1.4 at 1 second. EV 2 is two stops less: f/2.0 at 1
second or equivalent EV is a camera setting. It was popular in the 1950s to couple
camera controls together so that once one set an EV one could rotate locked f/stop
and shutter speed rings to choose between different equivalent settings. Today only
Hasselblad continues the tradition. It is much easier to remember typical light
conditions as a single EV number than combinations of camera shutter and aperture
settings.
So what's the correct exposure for Velvia (ASA/ISO 50) in side-lit daylight? That
light is LV14. Since Velvia is one stop slower than 100 we need to give it one more
stop exposure, or SUBTRACT one EV from the LV to get the EV. Therefore, LV14 -
1EV= EV13. EV13 is 1/125 at f/8 or 1/15 at f/22. Light meters that read in EV have
scales on the side that show you all the equivalent camera settings for any EV.
What tricks does this tell us? Well, in nature nothing gets brighter than something lit
by full sun, which is LV15. If you see LV17 in your meter you know that that must be
a white object in daylight. Guess what: that's how evaluative and matrix meters know
that, too!
© 2007 KenRockwell.com 77 converted by Sándor Nagy