Strobist: How to Calibrate a Sekonic 758DR or Any Other Digital Light Meter

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• published: 28 Dec 2011
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All DSLR (heck all cameras with a removable lens and digital sensor) cameras need to e calibrated to match your light meter. Each lens sensor combination has its own specific light transmission characteristics. So you cannot pick up a digital light meter and expect to nail exposure. Each camera / sensor combination and light meter see light differently. You need them to match. In order to do this you need to use a known variable, 18% Grey Card (not a white-balance tool). You will use the Grey Card as a target much like a color checker passport, but in a more primitive way. You will light it evenly (a large light source) and use a light meter to set your camera. Next you will take a shot with those settings (tethering is a preferred method) and analyze them using a histogram to determine if you camera/lens como is over or under exposing based on the input info from the light meter. If you dont know, 18% grey is the middle value for all digital exposures it represents 128 (127.5 actually) or the middle value between 0 and 255. When you see what the camera and lens combo are doing, you can adjust the light meter to compensate for the variance. You want the R,G,& B channels to be 128 (the average) in the histogram. If the camera is under exposing you need to dial down the light meter so you have to increase the light falling on the target to achieve what ever value you have before you dialed down. The Sekonic 758DR had a easy way to calibrate (press both ISO buttons) and use the job wheel. My Minolta IV-f has a little screw (potentiometer) to adjust the meter up or down. However if you do not have any way to adjust the compensation, just use the ISO adjustment. Example, lets say you are at ISO 100, Shutter speed 1/125th and the meter tells you F8 from the flash reading and you look at your grey card image via the histogram and it tell you you are under exposed. Dial down the ISO on the meter so that now f8 reading now reading f5.6 you just dialed down a stop of light. So in order to get back to F8 you need to increase the flash power. In fact you need to increase 1 stop. So when you are back at F8 reading on your meter, take another shot and insect it in the histogram. If you are at or near 128 (averaged) you are now calibrated. If you are still under exposed, dial down the ISO again and increase flash power until you are back at the initial meter reading, and check the histogram again. Side note** if you are over exposed you need to Increase the ISO and dial down the lights to get to the 128 Average. Also use a raw workflow, correct for WB before you try to read your histogram average,. You also want to make sure your camera is set as neutral as possible. Example zero out contrast, saturation,and sharpness. We want to see as close to what the sensor is seeing when the camera makes an image of the reference 18% target . Since it is known to be 18% grey it makes the perfect calibration tool for a digital light meter. You will hear people say a digital camera's exposure is based on 12% grey. They will prove it to you, here is the most important fact. This calibration works because you have a known value as a reference, if you have a 12% grey card, then use it. Dont try to get an average of 128 in the histogram with a 12% target, as 12% is not the middle between 0 and 255. I have never seen a 12% card, however 18% grey cards are everywhere and can be had for 5 to 10 bucks. If you now own the Sekonic 758DR or any spot meter for that matter, you will need a 18% grey card/target to establish the correct exposure. If you shoot film it is highly recommended to own one. Without one you are just guessing, that is unless you own a spectrometer and can measure things to establish a reference to work from. If you have any questions, I will try to help. If I dont know the answer I know people that do. Thanks for watching, I learned how to calibrate a digital Light meter from a great photographer named Frank Doorhof. He is from the Netherlands. He shoots Medium format and was how I originally found him. He is a great resource for all things photographic. I learned how to calibrate my 758DR without that $130 target. All you will need is a large light source and 18% greycard. This was my first attempt at making a video to teach anything related photographic via a video. I have to find a studio if I plan to do any serious video training. So from my living to you, I hope this long winded video can help someone to calibrate their light meter.