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C-19 topic. Incident and reflected light meter, basics 101

LRList001

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Joined
Dec 23, 2012
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Having suggested some topics to fill the long days ahead (in the UK we have all just been told to self-isolate for the next three weeks at least) here is a starter for ten on the basics of the two types of light meters.

This is intended to promote discussion. It is not an advanced treatise on light metering!

I'm starting off with the assumption that it is a good plan to get the exposure right in camera (what ever 'right' means). LR tweaks come later, the assumption is it is easier to work on a close to correct image than trying to fix a broken one.

For general purpose photography, there are two type of light metering:

* Incident

* Reflected

Each has advantages and disadvantages.

Incident light metering needs a separate light meter. Mine is an old Minolta. It has that white dome that you see on meters. The great thing about incident light metering is that you don’t have to take into account what you are photographing at all. Lets assume you are outside photographing a wedding type scene. You take the meter, stand in front of the main subject (it isn’t that important, you just need to make sure that your scene is reasonably uniformly lit), point the dome towards where the camera will be and take a reading. As long as light levels don’t change, you have the exposure settings necessary for the scene. If you have dark or bright subjects, no problem. You have measured the amount of light falling on the scene, no compensation is necessary. The light meter I have is reasonably sophisticated and allows me to slide the results up and down, so it does the maths if I want to change aperture, shutter speed or ISO. The camera is put into a manual exposure mode, dialling in the readings off the meter. (There are some tricks to cope with back-lighting, but I’m keeping with this basic configuration.)

Reflected light measures the amount reflected off the scene, into the camera. Your camera meter uses reflected light. Reflected light has one immediate advantage, it is the light right at the instant of taking the image and in one of the automatic metering modes this is what the camera will use. Another advantage is if say, you are taking a landscape scene, you don’t need to travel a mile or two away to take a reading. If you wanted an incident light reading, well good luck, the scene is too far away and probably far too big to have uniform lighting. One snag with reflected light is it makes a massive assumption, that the world is 18% grey. It turns out that this assumption that the world is 18% grey is astonishingly effective, it works a lot of the time, but certainly not all the time, and compensation and other techniques are required, for example using spot metering within the scene. Going back to that wedding scene, stopping the dress blowing out can become a major hassle.

Comments welcome! A couple of additional topics to get started (I'm hoping to learn from you experts out there):

* Using the histogram, exposing to the right (ETTR)

* Dynamic range of human vision, scene, camera sensor and display medium; HDR.

I haven’t talked about flash metering, my Minolta meter can be used for that too, it has a bunch of modes.
 

Zenon

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Jan 13, 2017
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Yes I have a lot of time on my hands these days. I have a Sekonic L-258 and it is nice to use incident metering but not always practical. This is an interesting topic for me and I often use metering as an example to explain how Canon's flash system works. Hope you don't mind.

Without a flash if you are metering 80% snow or 80% tar with reflected light you will either need to over or underexpose.

In ETTL -2 the flash system works the same way but has nothing to do with the cameras ambient light meter. It has its own metering system. When you press the shutter half way it takes an ambient reading. When you press the shutter all the way it fires a pre-flash. Using multiple metering zones it compares the ambient reading, the light reflected back from the pre-flash and isolates the closest object which is typically your subject. It also determines the correct subject exposure. A bride in a white dress, a groom in a black tux and both together reflect light back differently. This is what FEC or Flash Exposure Composition is for.

ETTL-2 can be a big a pain as the cameras ambient meter. Manual flash is better but I found in a fast moving environment ETTL-2 was just easier to work with. Normally you would get a somewhat decent exposure. I relied on whites a lot. I just made sure the histogram was to the right but not all the way to the right wall to maintain detail in the whites. It was not perfect but it was a tool. I used manual flash for important shots when I had the time.

I'm calling it ETTL-2 because there is big difference between the first generation ETTL. ETTL was focus point based but Canon went to ETTL-2 and dropped AF as part of the equation so you could focus and recompose. These days with multiple AF points that is not as necessary and focus/recompose is not that great either. ETTL-2 has been out for a long time now.

I had some good videos, etc links on this stuff but Canon started pulling lots of them down. Don't know why.
 
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