One way I’ve become accustomed to setting my exposure when lighting a scene or studio set is by using a light meter. It is simple, accurate, can be used to pre-light and check lighting ratios, and also allow you to setup your lighting days or hours before a camera is brought on set. It can also be very useful during tech scouts and developing a list of what lighting equipment needs to be rented. But best of all for me it allows me to recreate my lighting setups at any point in the future with great accuracy. (providing I wrote down my measurements)
Before you can faithfully use a light meter and start trusting it implicitly, first you need to go through the process of rating the ISO of the camera. This calibrates your light meter to the camera in combination with specific camera settings and also the lenses you are using.
A light meter once properly calibrated to your camera tells you exactly what f-stop to set your lens iris to in order to be exposed for middle grey. From there once you have a point of reference established it becomes easy to work with your lighting.
I always get asked what kind of light meter is good to get and honestly this is not one of those areas you need to spend a fortune on and buy the latest and greatest. I’ve been using the Sekonic L-508 Zoom Master for the past 10 years and had no reason to change it for a newer one. The Sekonic L-308DC is another one similarly priced to the L-508 Zoom Master at around $300 USD on ebay.
Some light meters have “cine” functions which means they include a fps mode where you dial in the frame rate you have set your camera to. This is handy to look for in a meter, but even without this mode you can still use a traditional light meter by setting the shutter speed on it to 1 over 2 times the frame rate set on your camera. So for example if you are shooting 24fps you would set the meter to 1/48th of a second or 1/50th if the meter won’t go to 1/48th. If shooting 25fps you set it to 1/50th. 30fps would be 1/60th. This is assuming a 180 degree shutter setting. If you specifically set a shutter speed on your video camera, you should set the same shutter speed on your light meter.
The method is simple and you can do this with any camera. I’ve done this with the Sony F3, F55, FS5 and other camera’s that I own and shoot with. To do it most effectively you will need a 18% grey card, a monitor with a waveform function, a light meter, and a very even light source that is preferably soft and larger than your grey card and target area as this helps reduce reflection, glare, or hot spots reflecting off the grey card which can cause inaccurate measurements.
1 With the FS5 I used the stock lens and placed the camera 1.5 meters away from the grey card, then I zoomed in until the complete frame was filled with the grey card. My light source was a 2 foot 4 bank kino flo placed beside the camera just off the lens axis. Even a professional grey card can have some reflectivity depending how the light is hitting it so tilt your grey card slightly off axis from your camera lens to reduce any light reflection on the card which can skew results.
2 Set your camera to the desired frame rate, for example I used 24fps in my test, next set your camera shutter speed to 1 over double the frame rate. So 1/48th for 24fps, or 1/50th for 25p. Be sure you are at the lowest ISO for the gamma curve you have selected. Ensure you have no gain enabled anywhere. Set the ND filter to clear.
3 Now adjust your lens iris until you read the appropriate middle grey value on the waveform monitor for the gamma curve you have selected. In my test I set the FS5 to the Cine 2 gamma profile which has a middle grey value of 30% IRE, so I adjusted my lens iris until the grey card reached 30% IRE on the waveform monitor.
4 Now set your light meter to the same ISO and frame rate as the camera. (ISO 2000 and 24fps in my case). With the light meter lumisphere raised and pointed towards the camera, hold the light meter right in front of the grey card as close to it as possible and take a meter read.
Look at the f-stop reading on the meter compared to f-stop set on your camera lens. They probably do not match, which is fine. Neither did mine. Next change the ISO value on your meter until the f-stop matches the value that your camera lens was set to. When they match, you will see a new ISO value on the light meter. This is your new ISO rating of the camera, and going forward as long as you set your light meter to THIS ISO value for the specific gamma curve you selected you can be sure that when you take a reading and match the f-stop on the meter to your camera lens that you are exposing perfectly for middle grey.
In my test I rated the ISO on the FS5 one stop lower then the ISO set on the camera. If it really bothers you not having the ISO values match between the light meter and camera, some light meters will allow you to set a specific exposure compensation so you could use this to get the meter ISO to match the camera but I typically don’t bother with that. I just use the NEW ISO value and leave it at that.
Some are particularly bothered that their light meter does not already match the camera ISO however and the most likely explanation for this is the fact that some light meters are actually calibrated to the ANSI standard of 12% grey versus 18% grey. In this case 1/2 a stop difference should be expected but the real issue is that this standard can vary from manufacturer to manufacturer and even from model to model from the same manufacturer. I will write a more in-depth article about this in the near future.
One thing to keep in mind is that light meters meter light that is falling on them, not what is reflected off objects like what our camera exposure tools show us. This is important to keep in mind as you tweak your lights falling on subjects which have different levels of reflectivity, or are darker or lighter. Darker colors absorb light and do not reflect them as much as a white object so you might want 1/2 to 1 stop of more light on darker colors when compared to grey or white objects.
It might sound rather silly but if you use different lenses on your camera you should repeat the entire process of rating the camera ISO. Everything small can affect light transmission so even though lenses use the same f-stop rating, there is a chance that from lens set to lens set or brand to brand that they transmit slightly more or less light. Remember the goal here is to know the exact amount of light is required to expose properly for middle grey so if you start to ignore changes you introduce into your camera system then your results suddenly can become less accurate.
If your light meter has a foot candle mode, using your existing setup used to rate the camera ISO you could also take another meter read in foot candles. This value in foot candles is worth writing down, because in my test with the FS5 I determined that I need around 54 foot candles to have middle grey exposed properly with my camera lens iris set to f4.
Knowing this can be extremely useful when looking at the various output levels provided by lighting manufacturers
Kino, ARRI and several other manufacturers publish their foot candle values (FC) taken at various distances from the fixture. Below are the lighting photometrics provided by Kino for a Kino 4 bank DMX lighting fixture:
Looking at this chart, I can tell without any effort at all that if I use a 4 foot 4 bank placed at around 8 feet away from my subject I will get 49 foot candles, and I know that at around the same foot candle level I would middle grey exposed properly at f4. Now if I want more depth of field in my shot, I can just add a one stop ND filter to my lens and set my iris to f2.8, or I can move the light fixture 12 feet from my subject and open up my iris to f2.8!
All this valuable information and I didn’t even have to set anything up.
Where using a light meter gets really handy is if you intentionally want to overexpose your footage in a very accurate way. The FS5 camera does not have very good exposure tools in my opinion, zebras are ok in the absence of other tools but for more professional work where you want accuracy and consistency in exposure I would always defer to an external monitor with a waveform monitor and a light meter.
Check out my Ultimate Exposure Guides for the Sony F series cameras including the FS5 with over 10,000 words and topics covering everything from exposure fundamentals (Fundamentals, Dynamic Range, The Crux of S-Log, S-Log vs RAW, Standard and Cine Gammas, Hybrid Log-Gamma), exposure methods (Grey Card, White Card, Light Meter, Zebras, Monitor WYSIWYG, Auto ND), exposing with LUTs and much much more!
Visit www.ultimateexposureguides.com for more information!
About the author
Dennis Hingsberg is an award winning cinematographer and professional colorist based out of Toronto Canada, and founder of StarCentral Inc. – a video and film production company specializing in 35mm film production and film related post production services. Dennis also works as a paid consultant managing post production workflows for TV and film related projects.