Light Meter


TPF Noob!
Dec 8, 2020
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Hi all,

I just bought a Sekonic L-308X light meter.

When I metered an item (a box) in my house during a sunny day outside and plenty of ambient light through the windows (i.e. 3ft away from a window), I'm surprised that the meter shows that to get a correct exposure of the item the shutter speed should be 1/30 (i.e. slow and requires a tripod) on aperture priority mode F:4.0 and ISO:250.

If I use my camera sensor (5DM2), I could get away with 1/80.

Am I using the meter incorrectly?

Were you measuring incident or reflected light with the Sekonic? Your camera metering system measures reflected light.
Were you measuring incident or reflected light with the Sekonic? Your camera metering system measures reflected light.

I was measuring its incident light. I would think that with the reflected light the camera would have slower shutter speed, right?
I believe the issue was measuring incident light, which would have worked if your box was 18% gray. Say you have 3 boxes in the exact same situation, one white, one 18% gray, and another black. Using incident light readings, you would get the same exposure values for all 3 boxes. The white box will be overexposed, the gray box will be properly exposed and the black box will be under exposed. Do the same experiment, but measure the reflective light off of the 3 boxes to set exposure, which is what your camera does in spot metering mode. All 3 boxes would be exposed as if they were 18% gray. Your Sekonic light meter, or your camera's metering system can warn you of these types of situations, but it is up to the photographer to know whether the subject is in highlight and should be underexposed by a couple of stops and likewise know whether the subject is in shadow and needs to be overexposed by a couple of stops.

This is just slightly off your topic, but relevant. There are different ways of handling scenes with High Dynamic Range (HDR). A good idea is to shoot raw. It's a good idea to look at the image, histograms and blinkies after taking the shot and make appropriate adjustments. Another good idea is to use the camera's base ISO where the sensor's dynamic range is the greatest. Your camera's dynamic range drops significantly as ISO goes up. If you can't use base ISO, then it's a good idea to bracket +/-2 or even +/-3 stops and combine the images in Post Processing.
The difference between incident and reflective, like your camera meter, reflective measures the light after it hits the subject, and some is absorbed based on how dark or bright the object is and the light bouncing off is measured. You have to GUESS how much is absorbed and compensate in your camera. An incident reading measures light FALLING ON the subject, uneffected by subject tone. So it is accurate... but, you can get different readings. With the window or sun as the only source, pointing the dome in the direction of the window or light will give you a reading that will protect the highlights, important in digital where clipping results in no information in the file where it is blown out. Film is the opposite, most negative films have huge range into the highlights but the shadows block up. So there, if you point the dome from the shadow side into the shadow area, it will give you a brighter reading and protect the shadows. In studio, after setting the ratio, the main and fill overlap and add so those areas of overlap will be brigher than the reading from the main light without fill. So a meter reading in the area of the face receiving overlapping main/fill with dome pointed at camera gives as Dean Collins used to say, F good. When there is just one main source like a window, taking a reading towards the window should work. Take a few minutes and try these approaches and see how they look to you. Once you find what works for you, go with it. Once you get comfortable with your meter, mine lives in my R vest pocket so I can quickly meter my subject, set camera and then concentrate on composition, pose and expression. It's just done. But if you are going for a particular look, low/high key, you know where you are starting and can then adjust camera or lights accordingly.

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