A Primer on Incident Metering.
Why do we meter?
Simply put, film that is accurately exposed makes makes printing easier and humans are not really that good at assigning accurate EV numbers to all the scenes we see. So the primary goal of metering is to "reliably" find a camera setting that will allow us to easily get our desired print. (It is not about making pretty negatives, it's about making pretty positives.)
Sure in a front lit sunny 16 situation we can do okay, but when the subject is backlit, when the sun is setting, when we move indoors, when the light source is a camp fire, or when the clouds roll in; we can normally benefit from a little help.
A second, and optional goal, is to get a contrast measurement so that we can make well considered film development choices that will make our printing even easier.
That's basically the whole enchilada.
A few definitions.
Incident light is "the light that is falling on the scene". This can be from the sun, house lights, open sky, reflections off the snow, or anything else that is throwing light at our subject.
Reflected light is simply "the light reflecting off our subjects that our eyes and cameras see".
E.I. is exposure index, it is a personal setting. This is often improperly referred to as film speed, it is not. It is just a number we use to tell our meter how we want things done given all the variables involved.
The ISO film speed rating, aka box speed, is found through standardized testing. Film manufacturers do this for us. If we get good at following the manufacturer's instructions for normal, we can come very close to getting the same results they do. I normally use this number as my E.I.
EV is exposure value. In common terms, it's how bright the scene is. More properly this is a measure of luminance.
Luminance is a photometric measure of luminous intensity, how much light.
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light.
Three concepts to help understand metering and meters and help dispel some myths:
1-"A given amount of incident light, falling on a subject of a given reflectance, produces a given amount of reflected light."
For any given E.I. incident light meters know how much incident light it takes to make a middle tone subject fall as a middle tone on the film.
For any given E.I. reflective light meters know how much reflected light it takes to make a middle tone subject fall as a middle tone on the film.
That is a fancy way of saying that both types of metering are fully capable of getting us to the exact same camera setting in any situation.
2-"Our meters have absolutely no clue about what they are being pointed at."
Here is where incident metering and reflective metering differ most in practical use.
Reflective metering is truly handy because many times the meter is built into the camera so there is nothing extra to carry and with spot meters we can pick specific subjects/targets to meter from quite a distance.
For incident and reflective meters to get us the same camera setting reliably though, the reflective meter needs a target in the scene with a known offset to the proper camera setting. We don't always have or put a known target in our scenes, more typically we just pick a random target and make judgements/guesses about the characteristics of what the meter is seeing. That introduces the distinct possibility of human error. With experience this issue is minimized, but still reflective metering is normally subjective.
Incident meters are stand alone tools, so it is an extra piece of kit that adds weight, costs money, needs a free hand, and an extra pocket or a strap to keep them handy. Their big advantage though is that they don't need a target in the scene and they provide an objective reading. They don't get fooled by the reflectivity of any given subject. Snow, lots of bright clouds in the sky, dark clothing, water, reflections, blah, blah, blah... none of these will fool an incident meter because incident meters don't see the scene, they see/measure the light falling on the scene.
3-"When we know the placement of one point in the scene, we essentially know where the rest will fall."
If we know from testing that there is a specific offset that is common in our shots, like a 2 stop difference between the proper camera setting and the shadows with good texture, or 1 stop difference between say Caucasian skin and the proper camera setting, then by measuring either of these, we can know the placement of the other. Technically it makes no difference which point we measure.
This is why if we are familiar with our tools, even when we use different techniques and methods, we can regularly end up with equivalent camera settings.
A few words about E.I.
Your own E.I. is simply a reference point. You can change that reference to any value you please for any reason you please. If it works for you, its fine for you; that doesn't mean other people's numbers will help you though, or that your numbers will help them.
If you think you need or want to use an E.I. different from the box rating, do some testing, make sure it solves your problems. Encourage others who want to mimic you to test too.
I say this because every E.I. is a subjective choice. Your preferences of subject matter, camera equipment, metering methods, lighting situations, lens filters, printing process, paper choices, and your artistic biases all effect the E.I. you choose.
Testing need not be formal. If in your normal shooting and printing you find that shadow detail is lacking adjust your E.I. to add a little more exposure. If you consistently have more shadow detail than you want you can adjust your E.I. to reduce exposure a bit. Just make sure you are solving real problems.
For clarity here I'm going to deal with and assume that contrast rate decisions are made before the camera settings are chosen. I also want you to understand why and when film contrast needs adjustment.
First, you need to answer this question before adjusting the film contrast. "Will it matter?"
If, unlike Ansel Adams, your plan is to use modern variable contrast papers to adjust print contrast or your lab will be printing for you digitally, then I strongly suggest that you stick to making normal contrast negatives and skip film contrast adjustments unless you find a real problem.
But, if like Ansel Adams, you are specifically targeting a grade 2 fixed contrast paper and you are testing your work all the way to that paper, then film development changes are important.
Be honest with yourself here, because if you don't test clear to the paper, you are just guessing. I played this guessing game before I had my enlarger, before I started printing. My negatives from my "guessing period" are very hard to print compared to the "normal" contrast negatives I make today.
Okay, so if you decide that it matters and you need to adjust film contrast, what is important in taking an exposure contrast measurement is finding out how far and which way contrast is different from your normal.
With an incident meter this is regularly found by getting one reading with the meter pointed at the light source and another pointed at the camera. Practice and testing are important here. You need to know what normal is and you need to see how changes effect your various subjects before these measurements become meaningful.
The basic reason for adjusting film contrast is to make printing easier. As with adjusting E.I., adjusting contrast is subjective.
It's also important to understand that changes in film development do not significantly change how much info the film captures nor the real speed of the film, your exposure choice is the biggest factor in how much and what range of detail the film captures, bar none. Plus or minus development, simply changes what "straight prints" onto a "specific grade of paper".
Being able to burn and dodge to print more detail is the proof that negatives normally have "extra" info available outside the "straight print" range.
Reliability and why normal matters.
I have found that without exception the manufacturer's of my photographic kit, including meters, cameras, chemicals, films, papers, and everything else have really done a wonderful job. Their products typically work exactly as described and even though it may seem like magic seeing the film come out off the reel with images or when watching a print come up in the developer, it really is a well proven industrial process.
If I want a given output, I simply need to apply the proper input.
In any industrial process knowing what normal is and understanding how to get there, is important. Yes, I really believe that film exposure, development, and printing are mature industrial processes. Most of us are using off-the-shelf products and or well proven recipes. Within these constraints; given inputs, beget given outputs, with no surprises. The real wild cards in the system are you and I; our failings or successes in the application and understanding of the various inputs determine the quality of our results.
For me choosing to use an incident meter and to follow the manufacturer's instructions for shooting and processing my film has proven incredibly reliable. Those two choices remove most of the human fallibility and unfounded mythos that surround exposure, film choice, development, and printing. Those choices have provided me a baseline that has allowed for huge improvements in my prints and my understanding of photography.
So, why do I want to be normal? Because normal means getting reliable high quality repeatable results. I like that.
So lets talk about what is your incident meter sees?
Incident readings are typically suggested to be "taken at the subjects nose with the dome pointed at the camera". In certain situations, like for a studio or window light portrait, metering at the subject's nose is important because the subject's relationship to the light source makes a big difference. Outside though we just need to be in the same general light as the subject, not at the subject.
Incident meters normally have two "modes", dome in and dome out. That choice controls it's angle of view.
Dome out mimics 3-D subject matter, it sees and measures all the light coming from all the directions that may have an effect on the subject being recorded on our film.
Dome in readings are meant to see the light that will effect flat subjects, like art, and or used isolate the effect of different light sources. For example with contrast readings and in duplexing which are meant to measure the effect of two different light sources. (More on duplexing in a moment.) The main light in the scene, which for a window portrait would be the window and the secondary light coming from behind the camera, which might be house lighting. Outside this could be the sun as main light and open sky or the reflection from a building behind the camera as the secondary light.
So, as I said above, the normal way to use an incident meter for 3D subjects is dome out, at the subjects nose, pointed at the camera. If the meter has been set with the proper E.I. taking a reading in this orientation for use with most negative films will provide an exposure setting that will give you a really nice general purpose negative that can make a very nice print where the subjects will look normal with a nice amount of shadow and highlight detail.
I honestly believe that most of us could do just fine using this metering technique alone for the rest of our lives. It is that good, that simple, and that reliable.
Adding artistic biases.
Typically when we decide to take a photo it is because something in that scene got our attention. We saw something we thought was important to remember or share. This is where the concepts of pegging exposure and place-and-fall become important, we need to get what we saw on the film.
Pegging comes in three basic types; shadow, mid-tone, and highlight. When we peg we are simply picking one of the three as the most important point for that shot.
Place-and-fall is the concept that the camera can only place or peg one luminance point, the rest of the scene "falls" relative to that peg.
For example, if there are faces in the scene I will peg my exposure to the mid-tones because for me faces are always the anchor subject for the print and they need to be placed where they look best, the rest of the subjects in the scene are allowed to fall where they may.
For landscape photographers the shadow point is the typical peg and the rest of the scene is allowed to fall.
It is easy to bias our meter readings to accommodate different pegging choices for each shot by simply changing the direction we point the head of the meter. The motivation can be technical or artistic, it makes no difference.
If you want to give shadow detail more importance and you are willing to compromise highlight detail just turn the head away from the light source some, if highlights are more important turn the head more toward the light source. This is like using salt, adjust to your taste, with experience you can find out how much works best for you.
Certain situations require very accurate exposure to fit a scene onto our film. This is very typical of slide film. It can also be a concern where we wish to minimize exposure or with very long scale scenes on negatives.
One technique that can help here is called duplexing. The motivation here is normally that you have competing priorities and a limited amount of room on film. In this type of situation we set the meter "dome in", measure two peg points and average the readings.
For slide film typically the readings would be taken, one pointed at the light source (to protect the highlights) and the other pointed at the camera (to protect the mid tones). This technique works in all situations but is most useful in high contrast situations where the subject is backlit significantly.
There's more than one way to skin a cat..
Meters of all types simply provide exposure suggestions. Just because incident meters are really good at giving you the camera setting as a direct reading doesn't mean that's the only way to use it.
If your subject is in sun and you are in open shade, or vice versa, and you can't get into the same light, you can simply apply an offset as you would with a spot meter reading. Sure this is a bit subjective but the world isn't perfect and its much better than just guessing.
Use your imagination and you can find simple ways to make any meter do what you need.
I could go on but I might never finish that way.
Please comment and provide your techniques and ideas. If there's something here that doesn't make sense, ask about it so we can figure it out.