At the risk of bringing out more comments from photographers who have a single preferred way to determine exposure, I will offer one final look at the subject.

On an earlier thread someone asked, "WHY would I want to burden myself with this?"

Well, if learning is a burden, stop here. Don't read any further and please do not comment. For everyone else, please keep an open mind.

A response on one of the threads was, "A rigid system won't be able to accommodate unpredictable variations such as how much incident light is reflected at the camera."

That comment certainly contained the central issue of the question. However, incident light measuring is not a rigid system and the amount of light reflected from a surface is not all that mysterious.

Let's start with the latter - how much incident light is reflected back to the camera. We are not talking about specular reflections like the image of the sun in a mirror or bouncing off of water or glass. That can be almost as bright the sun itself and equally impossible to render without a strong neutral density filter.

What we are concerned with are diffuse reflections. They can range from around 90% of the incident light down to almost 0%. They come from leaves, skin, earth, rocks, clouds, clothing, paint, etc. If we can see tonality in them then their reflectance is probably well below 90% and well above 0%. If we can see their color, the range is even narrower.

Suppose we start at 96%. What happens each time you cut the incident light intensity in half - one stop? You end up with 48%, 24% 12%, 6%, etc. Middle gray is commonly defined as 18% reflectance or about about 2½ stops below 96%. If you take a reflected meter reading of a middle gray card in direct sunlight, the recommended exposure will do exactly what you would expect - render a JPEG where that card shows up as middle gray.

An incident light meter will come up with the same recommendation as a reflected meter reading plus a gray card. The gray card needs to be in the same light as the subject and so does the incident light meter. But the incident meter means that you do not not need to carry a gray card with you.

If you don't want to carry a separate light meter you can get an app for you smartphone that is almost as precise. Even the Pro version is very inexpensive. Here are a couple of measurements taken by pointing the front camera directly at the sun at noon.


At ISO 100 Exposure Value (EV) 15 is the same as Light Value (LV)


At ISO 400 the EV is increased by two stops to 17 but the LV is still 15

The light value (LV) in full sunlight will never be higher than 15. But if you are taking images of snowy scenes you might want to raise the EV by one (one stop darker) to accommodate some of the specular reflections coming from the snow and ice.

Since we all have a lot of time on our hands right now, why not try it? You might be pleasantly surprised how simple it is.

At the risk of bringing out more comments from photographers who have a single preferred way to determine exposure, I will offer one final look at the subject.

On an earlier thread someone asked, "WHY would I want to burden myself with this?"

Well, if learning is a burden, stop here. Don't read any further and please do not comment. For everyone else, please keep an open mind.

A response on one of the threads was, "A rigid system won't be able to accommodate unpredictable variations such as how much incident light is reflected at the camera."

That comment certainly contained the central issue of the question. However, incident light measuring is not a rigid system and the amount of light reflected from a surface is not all that mysterious.

Let's start with the latter - how much incident light is reflected back to the camera. We are not talking about specular reflections like the image of the sun in a mirror or bouncing off of water or glass. That can be almost as bright the sun itself and equally impossible to render without a strong neutral density filter.

What we are concerned with are diffuse reflections. They can range from around 90% of the incident light down to almost 0%. They come from leaves, skin, earth, rocks, clouds, clothing, paint, etc. If we can see tonality in them then their reflectance is probably well below 90% and well above 0%. If we can see their color, the range is even narrower.

Suppose we start at 96%. What happens each time you cut the incident light intensity in half - one stop? You end up with 48%, 24% 12%, 6%, etc. Middle gray is commonly defined as 18% reflectance or about about 2½ stops below 96%. If you take a reflected meter reading of a middle gray card in direct sunlight, the recommended exposure will do exactly what you would expect - render a JPEG where that card shows up as middle gray.

An incident light meter will come up with the same recommendation as a reflected meter reading plus a gray card. The gray card needs to be in the same light as the subject and so does the incident light meter. But the incident meter means that you do not not need to carry a gray card with you.

If you don't want to carry a separate light meter you can get an app for you smartphone that is almost as precise. Even the Pro version is very inexpensive. Here are a couple of measurements taken by pointing the front camera directly at the sun at noon.


At ISO 100 Exposure Value (EV) 15 is the same as Light Value (LV)


At ISO 400 the EV is increased by two stops to 17 but the LV is still 15

The light value (LV) in full sunlight will never be higher than 15. But if you are taking images of snowy scenes you might want to raise the EV by one (one stop darker) to accommodate some of the specular reflections coming from the snow and ice.

Since we all have a lot of time on our hands right now, why not try it? You might be pleasantly surprised how simple it is.