CALIBRATING YOUR LIGHT METER
Okay, so you bit the bullet and splashed out on a fancy Sekonic light meter. Now you're feeling a bit narked because despite this, your image exposures still look off when you get them back onto the computer. Did you profile it with your camera yet, dummy? No? Well that's why!
Now before you go thinking "what a crock", there is nothing wrong with your meter. It's accurate. Very accurate. In fact down to 1/10th of a stop. What isn't so accurate is our camera and lens combinations and so essentially you have to adjust the meter so that it reads f/8 as your camera sees f/8.
There are a number of ways to do this.
1. Sekonic Exposure Profile Target
At almost £110, these cost almost as much as an L-308 meter! If you just shelled out £400+ on an L-758 meter a further £100 to calibrate it is a bit of a kick in the err - coin purse?
2. Xrite Colorchecker Passport
More reasonably priced at £70, Sekonic allows you to calibrate your meter using the CCP. It wont end up being white elephant either. Use this before every shoot to get accurate colour representation under any lighting condition. A worthwhile bit of kit!
3. Lastolite EzBalance Card
For under £20 you can purchase a Lastolite EzBalance 18% Grey Card. This is a great addition to your kit bag. It collapses down from 30cm to barely 10cm across. One side is 18% grey, the other a white balance card. You can get accurate white balance and exposures using this and calibrate your meter too!
4. QP Card
The cheapest option comes in the form of a QP Card. It has three patches, white (LAB 95*0*0), mid gray (48*0*0) and dark gray (35*0*0). The grey is 18% so ideal for profiling your meter with your camera. The colours are very fade resistant and the card, although thin is quite durable. Still Its best to keep it flat inside its protective wallet. For about £5 you get 3 of these targets.
So how do they work? Well which ever option you go for, it involves taking a series of shots of the target. For the EPT and CCP, take one at the exposure the meter is reading and a couple under and overexposed. The Data Transfer Software will analyze the resulting jpegs and automatically create a profile based on them. This profile is then transferred to the light meter. You can adjust the profile to extend the clipping points for shadows and highlights if you so desire.
There is no such automatic option inside the DTS software, with the Lastolite EzBalance Card or QP Card. To profile the meter with your camera and lens, simply place either in the studio with your strobe illuminating it evenly. Use as large a light modifier as you can to ensure the most even spread of light. Take a meter reading. Lets say it reads F/8. So dial f/8 into the camera and take an exposure. For best results, tripod mount the camera. Now up your light 1/10th of a stop and take another picture. Repeat this until you have gone 1/2 a stop above and below taking a picture for each one.
Once done you will need to take the photographs into Photoshop and using the histogram, sample the same area on each image. The one closest to 128, 128, 128 is how much you need to dial in adjustment into the meter. Lets say that the photo that we reduced the light by 2/10 from f/8 came the closest. So we dial in two-tenths negative exposure into our meter. This process will need to be done with both the incident and reflective modes of the meter. If done correctly, your exposures should now be extremely accurate. If you can shoot tethered to a laptop then you'll find the whole process a lot quicker. For this you can use Adobe Lightroom which will import the images directly into the computer.