How to Calibrate a Light Meter

A Step by Step Guide to Calibrating Any Light Meter

How to Calibrate a Light Meter

How to Calibrate a Light Meter

In this guide I’m going to show you exactly how to calibrate your light meter. Before we start, let’s look at why you need to calibrate your light meter.

Why You Need to Calibrate Your Light Meter

Calibrating a light meter involves calibrating the light meter itself, and your camera. There are variances in light meters and in cameras, and by calibrating the light meter you are actually calibrating the combination of a specific light meter and a camera.  By calibrating your light meter you are eliminating the variances that exist in your light meter and your camera and lens, while ensuring more accurate exposure readings.

Calibrating the Light Meter

A light meter can never be 100% accurate, there will always be slight variances between light meters – even if they are identical models. Light meters can also drift over time, so it’s a good idea to calibrate your light meter every year or so to ensure the most accurate readings possible.

Calibrating your Camera to your Light Meter

Each camera has a different set of variances when it comes to ISO. For instance, ISO 100 on a Canon EOS 5D Mark III is not the same as ISO 100 on a Nikon D810. ISO 100 on any given camera is never really ISO 100 – it’s usually close, but could be as much as a 1/3rd of a stop or more off. This is usually due to the fact that different manufacturers use different methods to calibrate cameras, and even different methods from camera model to camera model.

Some camera manufacturers also down rate their ISO reading in order to get an unfair noise advantage – you might think that your camera shows remarkably little noise at ISO 100, when in fact the true ISO could be ISO 125 or even ISO 160 or more.

Calibrating your Camera and Lens combination to your Light Meter

As if the variances in cameras and light meters are not enough, we also have to deal with variances in camera lenses. There's 2 reasons why you would want to calibrate your camera - lens - light meter combination :

1) Your lens's aperture reading at f2.8 is not f2.8. It's usually close, but more likely to be something like f3.2 or f3.5
2) Large zoom lenses with a lot of glass elements will naturally let less light in than a small 50mm prime with only a few glass elements.

That being said, if you’re a studio photographer that wants the most accurate calibration that you can get, I would suggest that you calibrate your light meter – camera – lens combination, but for most people including myself the light meter – camera combination is more than accurate enough.

How to Calibrate a Light Meter: An overview of the method and what you’ll need

A simple, fool proof method that works on any light meter and any camera is to take an incident reading in front of an 18% gray card, and taking a picture of the gray card. By opening the photo in Photoshop, we can measure if the gray card is properly exposed or not, and adjust the meter accordingly.

Calibrate a light meter : the gray card method - How to Calibrate a Light Meter
Calibrate a light meter : the gray card method
Calibrating a light meter using the gray card method is a simple but effective method. To demonstrate this, open a new window in Adobe Photoshop, create a new layer, and fill it with 50% gray. Make a rectangular selection using the marquee tool, then select window -> histogram.

The median value in the histogram window will now show 128 which is the expected value for an 18% gray card. Using this principle, you can determine if a photo you took after measuring an 18% gray card with a light meter is over exposed or under exposed, and adjust your light meter until the gray card is exposing properly.

To calibrate your light meter, you’ll need the following:

An 18% gray card
To complicate matters further, there is a variance in the method used to calibrate a light meter to a camera : not all gray cards are exactly 18% gray. I tested the X-Rite ColorChecker passport, the QPcard 101, the X-Rite ColorChecker Grayscale and the expensive Sekonic Exposure Profile Target II.
Every gray card I used gave different readings, but the two X-Rite ColorChecker cards I used were very close to each other.  Since I use the X-Rite ColorChecker passport for color management, I used the X-Rite ColorChecker passport as my 18% gray scale for this test.

Since X-Rite is the standard in color management for photographers, and most professional photographers use the X-Rite ColorChecker for color management, I strongly suggest you use your X-Rite ColorChecker passport for calibrating your light meter.

How much difference is there between the different gray cards? Using my calibrated Sekonic L758-DR, I get a gray value of 131 on the X-Rite ColorChecker passport and X-Rite ColorChecker Grayscale, while the QPcard produces a gray value of 118 – so there is quite a big difference.

If you’re going to be using a QPcard to calibrate your light meter, and use and X-Rite ColorChecker passport for color calibration and setting your white balance, the variance introduced through the different gray values between the QPcard and the X-Rite ColorChecker could actually cause more harm than good and you might as well leave your light meter uncalibrated.

If you want to calibrate your light meter, and absolutely want to use a QPcard, I don’t have any tests or data to prove which gray card is the most accurate, but what I encourage you to do is to eliminate variances as far as possible. If you’re going to use a QPcard to calibrate your light meter, then use the QPcard to set your white balance for color management too, but don’t mix the QPcard with the X-Rite ColorChecker or with another brand of gray card.

A Light meter, using the incident mode

Incident mode measures the actual amount of light that falls in front of your subject, whilst reflected mode (using a spot meter on your light meter) measures the amount of light reflecting off the subject.

A Studio Strobe

Use a studio strobe with consistent power output. Most studio strobes give the least consistent output from shot to shot on their lowest and highest settings. To ensure the most consistent results, set your strobe to a setting somewhere near the middle power setting.

An even light modifier

Use a medium to large modifier like a 3ft OctaBox or a similar sized softbox. I tested the difference between a 3ft and a 5ft OctaBox to see if the larger OctaBox would deliver more even light and results – and while the 5ft OctaBox casts a more even light over a wider area, the 3ft OctaBox was just as even as the 5ft OctaBox in the smaller area in which my light meter and gray card was place.

Sofware

You’ll need Raw Conversion software that can set the white balance and an image editing application to check the grayscale values. I used Adobe Camera Raw and Adobe Photoshop, but you can use equivalent software as the principles will remain the same.

How to Calibrate a Light Meter : A step by step walkthrough

Step 1 :

Place an 18% gray card on a table, and measure the light falling on the card using your light meter in incident mode. To keep things simple, adjust the light output so that you get a full f stop reading such as f8, and not a reading such as f8 ³ which is f8 and 3 /10ths of a stop.

Take a light meter reading off the gray card using incident mode with your light meter. - How to Calibrate a Light Meter
Take a light meter reading off the gray card using incident mode with your light meter.
When metering, be sure to point the light meter towards your light and not the camera to ensure an accurate light meter reading.

Step 2 :

Set your camera to manual mode and use the shutter speed and fstop as measured on your light meter, and take a picture.

Step 3 :

Open the file in Camera Raw, and set the white balance using the white balance tool in Adobe Camera Raw. I used a X-Rite ColorChecker passport, so I clicked on the 18% gray patch as shown below to set my white balance.

Set the White Balance in Camera Raw - How to Calibrate a Light Meter
Set the White Balance in Camera Raw
Set the white balance in Adobe Camera Raw using the white balance tool. If you're using an X-Rite ColorChecker, the 18% gray patch is on the bottom row, 4th from left as shown above.

Step 4 :

Click open Image to open the file in Photoshop.
Select the rectangular marquee tool and make a selection on your 18% gray card.

I used the X-Rite ColorChecker, so I made a selection on the 18% gray patch. See the image in Step 5 for details on how to do this.

Step 5 :

With your selection still active, select Window and then Histogram, and look at the median value.
The median value is the average value of your selection, and an 18% gray card should show a median value of 128 when it is 100% calibrated.

In my instance, I got a reading of 138 using my Sekonic L-758-DR, and a reading of 150 using my Sekonic L-308S light meter. Both meters produced a reading that was over exposed.

Determine if your gray card is properly exposed - How to Calibrate a Light Meter
Determine if your gray card is properly exposed
Using the histogram window in Adobe Photoshop, you can measure the exact value of your gray card. A properly exposed 18% gray card should have a value of 128. Anything above that indicates that your light meter is over exposed, and anything below 128 indicates that your light meter is under exposing. Using this information, you can adjust your light meter accordingly.

Step 6 :

Since both the Sekonic L-758-DR and Sekonic L-308S produced over exposed readings, we know we need to bring down the values by adjusting the light meters.

The Sekonic L-758-DR meter supports calibration adjustment on the meter itself, while the Sekonic L-308S does not. However, you can calibrate the Sekonic L-308S by using a different ISO. 

Since the Sekonic L-758-DR  light meter supports calibration, I dialled in a value of -0.4 by pressing down on the ISO1 and ISO2 buttons and thern turning the wheel on the meter.

On the Sekonic L-308S, I set the ISO to 160 in order to bring the over exposed value down. If the median value I took indicated that my meter was producing an under exposed reading, I would have lowered the ISO to less than 100.

Step 7 :

After adjusting your light meter, you need to take another reading and adjust your strobe output level again to get to the original f-stop setting you used in step 1.

In my instance, I took another reading and got a reading such as f11 and 4/10ths on the L-758-DR (I dialled in a negative value on my meter), so I adjusted my strobe power level with a few clicks up to the original power setting I used in step 1 (I used f16).
After making the adjustments, I took another reading to ensure that my light meter is measuring f16, took another picture, and opened it in Photoshop.

For demonstrative purposes, I also performed the same procedure above for my Sekonic L-308s (I took a separate reading, adjusted the lights separately, and then took a picture).

Step 8 :

Open the file you took in Step 7 in Adobe Camera Raw, set your white balance, make a selection on your gray card, and view the median value.

In my case, I now got a median value of 130 for the Sekonic L-758-DR light meter which is close enough to 128, and a perfectly acceptable value

I got a value of 128 for the Sekonic L-308s light meter, which is perfectly calibrated. On the Sekonic L-308s, I now know that whenever I want to shoot at ISO 100, I set my camera at ISO 100, and I set my meter at ISO 160.

If you find that your values are still not close enough, you can repeat this process until you are satisfied with the numbers, but bare in mind that you might not always get it calibrated 100%. Instead, aim for acceptable gray values in the range between 123 and 133.

Calibrate any Light Meter - How to Calibrate a Light Meter
Calibrate any Light Meter
I calibrated the Sekonic L-308s which doesn't have a calibration function by adjusting the ISO on the light meter to 160 and keeping the ISO on my camera to 100. The calibration was perfect as I achieved a median gray value of 128.

Conclusion

Calibrating your light meter is just as important as color calibration, if not more important. If you attempt to color calibrate an under or over exposed image, you are not starting from the best possible base that you can start from.

Most professional photographers will not attempt a shoot without color calibration, yet most people happily use light meters that are not calibrated or even worse : they rely on the histogram or their built in light meters and don’t use a hand held light meter at all.

If you’re a professional photographer and want consistent results there’s simply no excuse : use a calibrated light meter for accurate exposures just like you use an X-Rite ColorChecker for accurate color calibration.