Share your Incident Meter Technique and Calibration

**ic-racer** · 30-Dec-2010, 00:17

A few threads recently indicate there is a group that like using incident meters. I thought it would be nice to have a thread for sharing info on incident technique and calibration.

To start off the thread, here is a list of the methods I have used in the past (in no particular order). I'm sure everyone has their favorites, so please post yours.

1) Gray card and spot meter calibrated alrady to 0.1 above film base. Meter gray card with spot meter. Set to zone v. Place Incident meter on gray card and match settings.
2) "4 stops below" method. Take incident reading and subtract 4 stops. Put a gray card in the same light and photograph it. If the negative density is 0.1 the meter was set correctly, repeat and adjust as needed.
3) "Modified Jones, 'panel of observers'" method. Take an incident reading using your favorite method (shadow, highlight, combined, direct toward lens, etc) and bracket at various ISO/ASA settings on the meter.
Print the pictures, show them to your friends with them sleecting the best ones. Work backward to find the fastest meter ISO/ASA settngs that gave excellent prints.
4) "Sunny 16" Face dome at sun. Adjust meter to read f16 with shutter speed denominator same as ISO/ASA
5) "Transparancy method" Bracket some transparency film. Select the best exposed slide and work back to the ISO/ASA

These methods are somewhat based on the following "incident theory" for B&W NEGATIVE exposure readings that I follow. Your methods may be different, so lets hear it.

a) usuall objects reflect light over a 5 or 6 stop range represented by white paint and black paint (5 stops)
b) outdoor representaional photography can involve objects lit by the sun, the sky or both.
c) a flat object lit by the sun or sky alone can be properly exposed when the indicent meter is set to provide shadow detail 2 to 3 stops below middle gray (some meters may come factroy set like this)(copywork)
d) an outdoor representaional scene with objects lit by the sun and the sky can be exposed correctly by placing the meter (calibrated like "c" above) in the shadow
e) a meter calibrated to give shadow detail 4 or 5 stops below middle gray can be used in 2 ways. Average a shadow and sunlight reading or place the dome so that sun hits part of it, similar to the sun hitting the subject matter and take a single reading.

**Vaughn** · 30-Dec-2010, 01:46

I have a bullentin board outside my office. At 400 ASA it always reads 1/4 sec at f16. I check all our meters (handheld and in cameras) and those of our students, with that board -- and it matched up well with the three Luna Pro SBC's I sent in for repair and calibration.

If the meter is within 1/2 stop, I don't worry about it. A stop off and I suggest the student keep an eye on their negs, and compensate if they aren't getting the shadow detail they want. Mostly B&W negs., some C-41 developed eleswhere.

Not very sophisticated. Our Luna Pro SBC's are both incident and reflective meters, so I assume that if the reflective metering is accurate, so will be the incident metering.

**Chuck P.** · 30-Dec-2010, 07:15

I have a Gossen Luna Pro F. I don't use my incident meter for critical work, but when I do use it, and I'm in sun and shade mix, I average a shadow reading and a sunlight reading. For non-critical work, I can get by with this easily.

**BetterSense** · 30-Dec-2010, 08:11

I use homemade digital light meters, which have to calibrated from scratch.

To obtain the relationship between my lumisphere-covered photosensors' electrical output for a given light level, I multi-point calibrated the incident meters against an expensive luxmeter and a point source of light at various distances. This gave me the raw lux vs. meter output curve I needed to get started. I was surprised to find that despite the claims of the datasheet and what I learned about photodiodes, the photo sensor does not double output when lux is doubled, meaning there is either significant dark-current effects from sensor itself, leakage currents in the frequency divider, or some kind of significant nonlinear absorption in the lumisphere.

To get from lux to photo units I used the mathematical fact that one stop change of exposure corresponds to a factor of 2 change in lux, and adjusted my incident-meter calibration constant C so that the meter gives 15.0 EV at EI 100 for Texas sunlight.

I in turn calibrate my spot meters off my incident meters by spot-metering a gray card held in light metered with the incident meter. I adjust the spot meter to give the same reading as the incident meter. I admit that I rarely use spot meters (and rarely use meters at all except for things like copy work).