An In-Depth Tutorial for Using a Light Meter to Find Your Camera's Exposure Latitude
This is a guest post by cinematographer Angelo Lorenzo.
Maybe you’re an army-of-one indie director. Maybe you’re a cinematographer who has decided to step up your game or reinforce your fundamentals. Whatever the case may be, the cornerstone of video capture is the control of the light that hits your lens and knowing the limitations of your recording system: the camera.
Your eyes don’t see light the way a camera does. You need a measuring tool -- a light meter -- to analyze a scene objectively. If you’ve never used a light meter or you’ve slid back into the comfort of judging an image through the video monitor, Able Cine has produced a video on basic light meter usage that you can take a peek at.
The other half of the battle, after you know how to measure light, is knowing the camera’s limitations in recording that light. Exposure latitude is the range of light that a camera records and reproduces on the screen, from pure black to pure white. It can guide a number of practical and creative decisions for a production: from shaping the rental list of grip and lighting equipment to setting realistic expectations on color grading and final output.
Don’t know your camera’s limitations? Experimenting with a new in-camera gamma curve or picture style? Renting a camera you haven’t used before? You’ll want to run this test to see where you stand with exposure latitude.
What you’ll need to test exposure latitude:
- A lens capable of f/2.8 to f/22. This is purely for convenience as you can test 6 full stops of latitude at a time.
- A spot meter.
- An 18% gray card.
- Adequate lighting of any kind; A 650w fixture should do the trick if you have it hooked up to a Variac or have some scrims handy.
- An editing program that has an IRE scope or a field monitor that has an IRE scope built-in. Adobe Premiere Pro is used in these screenshots.
Set your camera to its base sensitivity and your preferred shutter speed. This usually means a setting of 100 ISO or 0 db for sensitivity, and 1/50th of a second or 180 degrees for the shutter speed. Make sure to set your spot meter to aperture priority and dial in matching ISO and shutter settings. If your camera uses db for sensitivity and you don’t know the base ISO rating, this article will cover a way to reverse engineer that information with a meter reading further down.
Set your gray card on a stand and position your lighting so that it evenly lights the gray card. For the next step, your spot meter should read as close to f/22 as possible.
Set your camera to f/22. If you have a field monitor with a built-in IRE scope, you should notice that your gray card creates a line on the scope at 50 IRE. Begin to open your lens up until your gray card creates a line at 100 IRE. The number of stops between f/22 and the f-stop that reads 100 IRE is your upper latitude limit – the amount of detail you can capture before the highlights blow. Note down the number of stops before you continue.
Adjust or dim the light until your spot meter reading on the gray card reaches f/2.8. Set your lens to f/2.8 and repeat the process by closing your lens until the IRE scope reads 0 IRE. The number of stops between f/2.8 and the f-stop that reads 0 IRE is your lower latitude limit – the amount of detail you can capture before detail is crushed to black.
If you're not lucky enough to have a field monitor with the built-in IRE scope, or to be doing these tests at a rental house that has one available, the next best solution is to use the IRE scope in your NLE (seen above). Set up the latitude test as described above but record it, pausing for a second or two after each 1/3 stop lens adjustment. This will allow you to count how many stops you've opened or closed when you watch playback. You should record one video opening your lens and a second video closing your lens.
In Adobe Premiere Pro ((Ed. Note: as of this post, you have six days left to get 50% off Premiere Pro if you're an Avid or Apple editor.)) for example, place your footage into a timeline. As you play back your footage place a marker on the timeline every time the lens is adjusted 1/3 stop. Open your reference monitor and set video output from “Composite Video” to “YC Waveform”. As you shuttle between each marker, the line on the IRE scope will also rise or fall accordingly. As the line reaches 100 IRE and 0 IRE, you can count the number of markers to figure out your upper and lower latitude limits; this is why you should record two separate videos for your upper and lower limit testing.
That’s it, you’re done! Testing my Canon 5D Mark II with Technicolor’s Cinestyle, for example, gave me the results of 3 1/3 stops of upper latitude and 5 1/3 stops lower latitude. This test also revealed a particular quirk of Technicolor’s Cinestyle: it crushes black detail at around 8 IRE rather than 0 IRE.
Going back to cameras that use db for sensitivity, finding their base ISO with the above setup is fairly easy. Set your camera to 0 db, whatever shutter speed you prefer and a modest aperture of f/4. Adjust your lighting until you create a line at 50 IRE on your IRE scope. Take your spot meter, set you shutter speed to match and take a reading from your gray card. Adjust the ISO on the spot meter until it reaches the equivalent aperture of f/4 – this ISO setting is your native ISO. Testing a Panasonic HPX500 gave me the correct base ISO of 640.
Knowing your camera’s exposure latitude and, to a further extent, trusting your light meter along with it will result in more finessed exposure settings that are more consistent setup-to-setup.
Angelo Lorenzo is a Los Angeles based cinematographer and camera operator that has worked on a number of commercial, music video, and film sets. When he’s not on set, he’s readying the launch of Films For Us, a platform that allows filmmakers to sell their films and shorts while blogging and connecting with their audience.