In a perfect world, all digital sensors, lenses, shutters and light meters would be exactly on the “same page”, and light metering would be an exact science. Unfortunately the performance of any of these devices cannot be assumed, plus light meters are set to slightly different calibration constants from their respective manufacturers.
There is also the human factor, everyone will have their own opinion on what is a “correct” exposure based their eyesight. Since perception of light and color vary from person to person, judgment of exposure will vary also.
To calibrate your flash meter.
The following steps are provided by Sekonic so you may need to check with the manufacturer of your own device
- Choose a subject that contains highlights, shadows and mid tones.
- Position the meter so it faces the camera from the subject’s position. Mount it on a light stand or tripod so it remains in a fixed location.
- Set the camera to its highest possible synch speed so any ambient light will not affect the meter’s reading. (Sekonic meters will “see” any ambient light that is present and able to affect exposure-even in flash mode).
- Test-fire and adjust your lights so the meter displays an even-numbered aperture (no tenths)
- Take your first test image by setting the camera to agree with what the meter shows.
- Take additional test images by adjusting your camera’s aperture over and under by using the smallest aperture possible (usually 1/3 or ? stops). Do not move or adjust the lights.
- With a digital camera, you can evaluate the exposures immediately. If you are lucky, the “perfect” exposure will be the one that exactly matched the meter. No adjustment will be needed. If you feel that an over or under adjustment is needed, use the meter’s calibration* or compensation function (see below for the L358) to make your changes. Shoot another test with the adjusted meter.
- With a film camera, shoot transparencies, and make sure that when the roll is processed it is not cut and mounted. (It is easier to view a strip of film than it is a box of slides) Evaluate the images on a light box, and adjust the meter if necessary. A second roll will confirm your adjustments.
It is advisable to perform this test every six months. Repeat this test if you are going to use a different camera. You may also check the ambient mode the same way with a constant light source. (Modeling lights work well).
If you wish match the incident readings to a particular gray card, mount the meter in a fixed location and take your readings, taking care not to permit any stray light through the back of the meter.
Shoot some test images and evaluate them as before.
*To enter the calibration setting first turn off your Sekonic L 358.
Press the power button on while holding down the ISO 1 and IS0 2 buttons simultaneously; the screenwill dosplay CAL 0.0 (for calibration).
The calibration setting can be changed by rotating the jog wheel while pressing and holding down the ISO 1 and IS0 2 buttons simultaneously.
A range of +/- 1.0 EV in 1/10 stop increments is possible for calibration. The calibration setting is not displayed on the main screen once.