Light meter discs... What are they for?

mctb · Jun 21, 2016

I have three attachments plus the spot meter. My question is about the two others pictured. One is flat with a hole in it while the other is flat and white. The dome is what I have been using. What are the others for?

table1349 · Jun 21, 2016

Generally the flat diffuser is for reflective metering and the dome is for incident metering as well as flash metering. Don't know about the other attachment. Would help if we knew what meter you are using.

mctb · Jun 21, 2016

It's a Sekonic Digi Lite 318b.

480sparky · Jun 21, 2016

REF. = REFlected. It measures the light reflected off the subject. The white disc and dome are for incident readings.

dxqcanada · Jun 21, 2016

Yup, Sparky is correct.
The one with the hole will read reflected light ... light reflecting from the subject ... meter is pointed at the subject.
The dome one will get a reading of incident light falling in almost all directions ... the flat one gives a more is more directional reading ... meter is pointing at the light source, between the subject.

mctb · Jun 22, 2016

Thanks. All makes sense now

TCampbell · Jun 22, 2016

As Sparky says - the one marked "REF" is for reflected meter reading. That means it works just like a built-in meter and measure the light "reflected" off your subject (you point the meter toward your subject to meter the light). The meter should be pointed TOWARD your subject when using this attachment.

The "dome" is the normal incident metering use. For this you hold the meter sensor facing AWAY from the subject and point it in the direction of where you'll be with the camera. It is metering the amount of light which is landing on that dome (which presumably is the SAME amount of light landing on your subject if the meter is directly in front of the subject (not more than a few inches away).

Now for the flat disk... this is the tricky one. It turns out that if light is hitting a subject from an angle it is NOT as efficient as light hitting the subject face-on. If you try a mind experiment... imaging we have a piece of cardboard and we've cut a square hole in the center of the cardboard which happens to be exactly 1 square inch. If you hold that cardboard between a light source and subject, the amount of light passing through the hole should be exactly 1 square inch worth of light... right? Well it turns out it is IF the hole is exactly perpendicular to the light source. But if you tilt that cardboard at, say, a 45º angle to the light... and then measure the spot of light that lands on your subject (the light that was able to pass through the hole) you'll find it's actually LESS than the 1 square inch size of the hole.

So the idea behind the "flat" disk is to provide an even more accurate light meter reading for "flat" subjects. E.g. suppose you were trying to photograph a painting (any flat surface subject)... a light positioned at an angle will be less efficient than a light that can shine directly on the surface (not from an angle). But you usually don't want the light to be directly straight on to a flat surface because if that surface has any shiny properties (reflective light surface ... like a mirror, glass, or even a gloss polished surface) then you'll reflections... so you tend to position the light off to the sides. The flat disk should be over the meter and pointed so that the flat disk of the meter is parallel to the flat surface of your subject (facing away from the subject).

Since the "dome" is a hemisphere... it catches a lot of light even if the light source is off to one side. The disk, on the other hand, being flat, does not catch as much light if the light source is off to one side.

The flat disk is for those special situations when you need especially accurate metering for "flat" subjects (you probably wouldn't use one of these when photographing a human subject.)