Although the Luxi came in decently close in the incident metering, it did fairly poorly with spot metering due to it’s wide field of view. All in all, the Luxi isn’t quite accurate enough to use for any professional light metering. In my opinion it’s worth the extra investment.
A Correctly Exposed Image is composed of three camera settings; ISO (or ASA), F-stop (or T-stop), and Shutter Speed (or Shutter Angle). More on that latter.
ISO which is also commonly called ASA is the electronic brightness from your camera. A high ISO typically creates a noisy image which is undesirable. Many cameras have a native ISO which creates the optimal image. You can find this by googling your specific camera’s native ISO. Doubling a ISO is a stop more of light. More on that latter.
F-stop is a setting on your lens. A small F-Stop like F/2 lets in a ton of light. While a large F-stop like f/16 lets in only a little light. This is why lenses with smaller apertures like f/2.8 are typically more expensive than larger F-stop lens like f/3.5-5.6. F-Stops also control the depth of field. Depth of Field is how much of your image is in focus. For example a lot of this would be in focus at F/16, while very few things would be in focus at F/2.8. A shallow depth of field such as from F/2.8 is fairly desirable as it pulls the subject off the background. The most common f-stops are 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, and 22. They are each a “stop” of light away from each other. It’s fairly important to remember these as they are often used on film sets to talk about lighting. This is because it is the only setting you typically change on a film set. An easy way to remember them is that every other f-stop doubles. For example 1.4 doubles to 2.8, 2.8 doubles to 5.6.
Some Lenses, often cinema lenses, are labeled in T-stops instead of F-stops. So what’s the difference? The difference is actually very small. F-stops measure the theoretical light going through the lens while T-stops measure the actual light going through the lens. Therefore, T-stops are slightly more accurate, this is why cinematographers trust them more often.
Shutter Speed is how long each frame is exposed for. For instance, 1/60 is 1/60 of a second. Typically in film we use double our Frame Rate or FPS for our shutter speed. The most common frame rate is 24fps leading us to 1/48 shutter speed. Shutter Speeds change a stop every time they are doubled. For example, 1/100 is one stop darker than 1/50 and 1/25 is one stop brighter than 1/50. You may think that you should always shoot at slower (lower) shutter speeds, however the lower the shutter speed, the more blurry your image gets. This is how those blurry night time street shots are made. In photography it’s good practice to use a shutter speed double your focal length to get consistent non-blurry images. For example, 1/100 for a 50mm lens. Some action movies use a higher shutter speed to create a choppy motion effect although it is best for normal use to stay at 1/48.
If that’s Shutter Speed then what is Shutter Angle? Well shutter angle is a little different. It’s measured in degrees based in the old days of films when there was a rotating wheel that blocked out light for part of the image to get the right shutter. It’s often times used only on digital cinema cameras. Shutter Angle, unlike shutter speed, changes with the frame rate so that the normal shutter angle is always 180 degrees. So when you change frame rates on a cinema camera take the stops of light difference into account. Additionally every time you half or double the shutter angle, it’s a stop of light difference. For example 360 degrees is one stop of light more than 180. 90 is one stop less than 360.