- Use a spot light meter (reflective metering)
- Set the ISO and shutter speed on the light meter to match the camera’s current ISO or gain and shutter speed or angle also keep filtration compensation in mind as well
- The reading across the entire blue or green screen should stay only .5 stops above or below the aperture on the camera. This translates to a 65-87 IRE or .65-.87 values in previzion
Lighting a blue or green screen for Previzion
Lighting for a good key in Previzion is a little less forgiving than lighting for a post key mainly because you do not have the time or ability to set multiple keys withing a single shot like in post. That being said Previzion’s keyer is very good as long as you follow a few guide lines.
The green or blue screen should be measured by a reflective (spot) light meter, and should give you an average reading the same as the current F or T stop on the lens. On previzion this will translate to around a .75 in the G or B channel, depending on if it is a green screen or blue screen.
If the average reading is at the current stop then the brightest and darkest point on the screen should only be 1/2 a stop above or below that. This will give you an IRE of around 87-65.
Basics of light meters for Previzion operators
On the left is an incident meter (notice the white ball) the right is a reflective (spot) meter, which the user looks through like a telescope to get a measurement. Some meters have both functions combined into one device.
There are mainly two types of light meters that a Previzion operator will encounter on a production; Incident and Reflective meters also known as spot meters. When set up properly they both measure brightness levels relative to the cameras current settings in “stops1”. The main difference is that an Incident meter measures the amount of light on the set hitting the light meter at its current position. A reflective meter on the other hand measures the amount of light reflecting off of a specific object. That difference is subtle but important. If you were to use an incident meter to measure the amount of light hitting a green screen and a blue screen sitting next to each other in the same lighting conditions you would get the same number, however if you were to use a reflective (spot) meter you would get quite different numbers. The spot meter would show that a digital blue screen is about a stop and a half darker than the digital green screen. For our purposes in Previzion what we care about is the reflective number, this tells us how bright the camera sees the screen, rather than how much light is hitting the blue or green screen.
1The difference from one stop to the next is double the amount of light, so if you need something 2 stops brighter you need 4 times as much light.
Setting up your lightmeter
Previzion operators don’t really need to carry both a reflective and an incident meter, they can get away with a spot meter and an 18% gray card. Spot metering an 18% gray card will give very similar results to an incident meter.
There are 3 main things to keep in mind when setting up a light meter for a shoot.
- ISO or ASA or gain – This determines the sensitivity to light of the sensor on the camera
- Shutter speed or angle – This determines how long the sensor is exposed to light
- Filtration – Filters like ND or 80a or 85a can reduce the amount of light getting to the sensor
ISO also known as ASA or gain is a setting on a camera which has the ability to make the picture brighter but with the trade off of making the image more grainy. However keep in mind more grain is bad for Previzion to get a clean key. A light meter will want the sensitivity fed into it as an ASA ISO type number. Cine style cameras will give you an ASA or ISO number, (those two terms are mostly used interchangeably) while ENG or broadcast type cameras will give you a gain number. This gain number will have to be converted to ISO/ASA in order for it to be a useful number on your light meter. Typically most cameras have a native sensitivity equivalent to 320 ISO and each 3db of gain added doubles that, so a camera that natively has 320 ISO with +6 gain will now have an ISO of 1280 and a fairly grainy picture.
More info can be found here:
Shutter speed or angle is a camera setting which determines how long the cameras sensor is exposed to light. The longer the sensor sees light the brighter the image, but also the side affect of having more motion blur. Cine style shows and broadcast shows may give you this number in different forms, but it is pretty easy to convert between the two of them. To further complicate things photo style light meters will only want shutter speed, cine style meters will take either angle or speed. Shutter speed is given as fractions of a second, so if you are working on a soap opera that shoots 30fps they may want the sensor exposed for 1/30 of a second (more motion blur), or if you are working on a grittier drama they may want it exposed for 1/60 of a second (less motion blur). With shutter angle people usually shoot at 180° shutter angle, which equates to exposing for ½ the frame rate. This means that a show shot at 24fps exposes each frame for 1/48 of a second, or a show shot at 30fps exposes each frame for 1/60 of a second. Many photographic lightmeters will not have a setting for 1/48 of a second, but they will usually have 1/50 which is close enough. The formula to figure out the exposure if you are shooting at anything other than 180° shutter at 24 is a fairly simple ratio. Shutter angle\360° = frame rate\shutter speed’s denominator (lower number in the fraction, the upper number is always 1 for normal shutter speeds less than 1 second). So for example 90°/360° = 60/240 would be a show with a 90° shutter angle shot at 60fps and would have an exposure time of 1/240 for each frame.
More info can be found here:
Filtration is the last thing one needs to know about when setting up a light meter. Filtration is not commonly used on blue and green screen shoots because it has the potential to ruin the key and make visual effects significantly more difficult if the DP is incorrect in filter usage. That being said it is important to know how to deal with it if it is used. What you need to find out is how many stops is the filter going to darken the image. Once you have determined how much darker the image will be you need to compensate on your meter readings for that. Some meters have the ability to subtract full or partial stops from their reading, others do not. If yours can do that for you do it it will save you thinking. If your meter is unable to do that it isn’t too hard, you just need to get to know the F/T stop scale. 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22 are the whole stop numbers that are on most lenses, some lenses may not go down to 1.4 or 2, or up to 22, but you will have at least a few in the middle. The way it works is that the darker the light meter or camera sees the room the lower the number you will get, and if you have a filter adding an offset making the image darker you will always be moving to a lower number. A .3 or 1 stop ND (neutral density) filter will make it so if your meter is reading a 4 pointing at the screen the camera with the filter will actually see it as a 2.8. Which means as long as that filter is in, you need to remember to take out 1 stop your reading putting you at a 2.8 on your meter.
More info can be found here: