I've read many puzzling articles on the subject of "spot metering" and the "zone system metering", and then have followed up by reading just as many articles how that with modern cameras equipped with "matrix metering", this subject is passée, and doesn't matter any more. Well, I've finally seen the light - I think I now understand what spot metering is useful for, I've successfully experienced using it, and will now attempt to explain it to you.
Spot Metering, and a method devised decades ago called the "Zone System" actually go hand-in-hand. Among the first cameras to use spot metering were the Pentax Spotmatics, and hence the name. In spite of being a Spotmatic owner and user for many decades, I honestly did not know that it's name was derived from it's metering method! Pretty dumb, eh? But it's true - the Spotmtic was designed primarily to use spot metering, and required knowledge of the zone system to do so. Spot metering, as it's name implies, is a method of measuring light entering a camera lens that is concerned only with a very small spot within the overall viewfinder frame - by definition, a spot of only six degrees or less. The first question that comes to my mind is
1) how could that be useful?
Typical camera manuals do very little to explain this. The Spotmatic manual, in fact does nothing - it just instructs you to adjust your shutter and aperture to make the meter read "0", with no explanation of the light actually being measured. More modern camera manuals, like those for later Canon SLR's and DSLR's explain a mode called "Partial Metering", which is almost the same thing, but in fact is a little different, which I'll explain later. Their explanation is that when a subject is back-lit, such as when a person is sitting with a window behind them, it's better to use "Partial Metering" and set your light-measuring point on the person herself. This makes sure that the reflected light from the subject is being measured, and the strong light from the window behind is being avoided, and therefore, the subject won't end up looking like a silhouette.
This is the most basic answer possible of how spot metering is useful - when you encounter strongly contrasting lighting conditions, you need to know which "spot" on which to measure the light, AND CONFINE YOUR CAMERA'S METERING SYSTEM TO IT, to get the desired result.
2) is this the only example of when to use spot... or partial metering?
No, but this is a very good example of a camera manual helping us to understand what it's all about, without getting into all the technicalities of the "zone system". Think of it this way - when you have a dark subject sitting in front of a very bright window, and you want to take that person's picture, you have two "zones" of light - a dark subject that you want to look normal, and a very bright back-light that you don't care about. It is important that you confine your camera's metering to a narrow "beam" as it were, to capture the detail of your subject, and not the light outdoors. So, although your camera manual doesn't say so, this is actually a beginning of an understanding of the zone system of light metering.
The "Zone System" proper actually uses 11 Zones, based on shades of grey from white to black. Therefore, it was intended to help us deal with lighting situations that are as complex, and also more complex than a person sitting in the strong light of a window. In order to actually use the zone system, you need two things:
- knowledge of the metering zones in terms of tones of grey
- a light meter that is capable of measuring from a very narrow angle (a "spot"); this light meter can be either off your camera or built into it.
Sometimes, the "Zone System" is spoken of as having less than 11 zones, like 7, or 5. For the purpose of this discussion, five is enough, and in fact, digital camera sensors are built to respond to five zones anyway. Some cameras fake more extended zones, such as Canon's "Auto-Lighting Optimizer", but we won't get into that here.
3) why five or seven zones, but not 6?
Ahhh - that is a "gestalt question" - this is the question that actually made the light come on for me. The reason is that 5 and 7, or any other odd number have a "middle" or "median" value - the median of 5 is 3, and the median of 7 is 4. The Zone System does something very important with this "median" value - it calls it "medium grey" (you'll see it expressed as 18% grey but ignore that) and on an actual scale, it can become the number "0". Therefore, the five value zone scale can actually be simplified to look like this (in my mind):
All I've done is translate the expression of the Zone System into the numbers of a typical battery powered camera light meter. However, THE ZONE SYSTEM HAS NOTHING TO DO WITH YOUR CAMERA'S LIGHT METER - this was simply how I came to understand it in my own mind, and how I'm sharing my understanding of it with you.
4) OK, now what?
Put it into practice. Here's a black and white version of the picture at the top:
This is the real thing folks - I was actually using Partial Metering and thinking Zone System when I took this picture. It actually has five zones or more, and it is a complex lighting situation - that is to say, if the trucks were sitting in fog instead of bright noon sunlight, it would not have been a complex lighting situation, and would not qualify as a shot where you'd need to use the Zone System. Keep this in mind - it's important to know when, and when not to use it.
The five zones in the picture are:
1) Black - The shadows under the trucks
2) Dark Grey - The grilles of the trucks and the trees in the background
3) Medium Grey - The bumpers of the two trucks on the left
4) Light Grey - The gravel foreground, the bumpers of the two middle trucks
5) White - the building at the far left, and technically, the sky
I underlined "Medium Grey" because using the Zone System, that's what you're supposed to meter on, unless you make a conscious decision to meter on something else. THAT IS THE ESSENCE OF THE ZONE SYSTEM! So, knowing these trucks weren't going anywhere, I knew I had time to figure this out.
In this case, I wanted perfect exposure, although the Zone System allows you to consciously make a "deviant exposure" if you want to - I'll explain shortly. So, to get a perfect exposure with the Zone System, you need to "spot meter" on something that's medium grey. So I set my Elan-7 to "Partial Metering" (it doesn't have true spot-metering, but some more expensive and believe it or not, some cheaper Canon cameras do), and then looked for something in the picture that would be "medium grey" - I decided that would be the bumpers of the two trucks on the left, because they were not as bright and shiny as the trucks in the middle.
So, the first step is to "meter the picture". Looking through the viewfinder and making sure the camera was set for Partial Metering, I aimed the centre of my lens at the medium grey bumpers, and pressed my Exposure Lock Button (on Canon SLR's, that's the "*" button on the back - upper right). When you do this, the "*" lights up in the Viewfinder, confirming your exposure is now locked for the shot you're about to take.
The next step is to compose and focus the picture. To do this, I simply moved the camera to take in all the trucks, pressed my shutter release halfway to focus, and then all the way to snap the picture.
Note that UNLESS YOUR MEDIUM GREY HAPPENS TO BE IN THE EXACT MIDDLE OF THE PICTURE, "spot, or partial metering" is always a two step process, requiring the action of locking your focus, using the "*" button before actually composing and taking the picture. First you meter, then you shoot.
5) what if you want a "deviant exposure"?
What I'm calling a "deviant exposure" is one that deviates from the use of the Zone System standard of "18% Grey". Now, this was what was really throwing me off... this seems to be the part that is not well explained in any articles I read about the Zone System. As it turns out, I was over-complicating it. If you want to deviate from the standard "medium grey", all you do is simply aim your spot meter (the centre of the viewfinder) at some other value, and lock the exposure. THAT'S IT!!! You don't turn any dials to adjust your exposure compensation, - you don't re-adjust your aperture or shutter speed - all you do is aim your spot-meter at a lighter or darker value than medium grey and lock the exposure, and then take the picture! It's funny how I wasn't getting that!
So, if I had pointed my lens at the white building on the left, the whole picture would've been much darker, or if I had metered on the shadows under the trucks, the whole picture would've been much lighter. If I had wanted more of the truck's interiors to be visible, I would've metered on the wind-shields instead of the bumpers! It would've made the whole picture brighter - the grilles and bumpers might've lost detail to "washout" (especially with a digital camera), but the seats and steering wheels would've been more visible. It really is that simple!
6) isn't the newer camera's "Matrix / Evaluative Metering" supposed to figure this all out?
Theoretically, yes. "Matrix" is Nikon, "Evaluative" is Canon. These systems are supposed to use complex computer algorithms to calculate complex lighting situations so that you won't have to think about the Twilight Zone of the "zone system". Ken Rockwell even says so! But have you ever noticed on Flickr, and other AMATEUR photo sharing sites how so many pictures just look dull, compressed and lifeless? Or how some black and white shots don't really have any black or white - just a lot of different greys? Maybe what we're seeing here is "The Matrix (metering)" at work! A lot of photographers aren't really learning anything about exposure if all they do is "set and forget" their computerized metering system, and end up with pictures that are void of highlights, because Matrix Metering tends to compute toward the safe side of not "blowing out the highlights".
7) is it possible that Evaluative Metering could've done wrong on this particular shot?
Probably not - it likely would've come out the same, especially considering my distance from the subject. It is possible that the nice "glint" around the chrome might have been supressed by Evaluative Metering. One way you can determine if Partial, or Spot Metering will make a difference or not is to switch it in and out, and observe what this does to your aperture and shutter speed settings. The way you do this will naturally vary from camera to camera... I just picked up a nice EOS 500n (Rebel G) for $20.00, which was kind of the VW Bug of Canon EOS film cameras - very cheap and basic, but one of it's great features is that pressing the "*" button AUTOMATICALLY switches the camera to Partial Metering, and locks exposure at the same time - how cool is that? Last evening, I went out with it and tried various scenes, to see if simply pushing this button made a difference in my aperture and shutter settings - I found that sometimes it did and sometimes not. I think as a rule, if the settings change, use Partial Metering, if they don't change, stay with the camera's conventional metering.
8) how can I do spot metering on an antique camera?
Well, if that antique happens to be a Pentax Spotmatic, you're in! But in other cases, if a camera has a built-in meter, it's either "Incidental" or "Center-Weighted Average" in design. Older cameras never have Matrix (Evaluative) Metering. So, if you choose to use an external light meter, buy one that does spot metering, although that will probably cost you more than the camera.
Here's a tip - the older Canon Powershot G-Series (the G2 through G5) are equipped with genuine spot metering! I just bought a G3 for $20.00 with a bad sensor - it will take a good picture in 1 out of every 4 tries, unless it's switched to "small-normal-jpeg" in which case every shot turns out, but it's only 640x480 pixels. However, the light meter still works, including the spot metering mode, so for $20 I got myself a spot meter, complete with batteries and charger, that'll also take a small B&W picture for verification. I'm now keeping this in the same bag with my Yashica Lynx and FED-5 Rangefinders, which have no built-in metering (well, OK, the Lynx does, but the battery chamber is corroded.) All you need to do is meter on the medium grey of your scene using the Powershot, observe the aperture and shutter speed readings on the little screen (make sure the ISO is set the same as the film in your antique film camera), and then set the same aperture and shutter values on your film camera.
I hope this has helped to de-mystify the topic of spot-metering. It's almost as if there's still an old "Photography Guild" out there that still likes to preserve it's secrets, and when they write up an explanation for things like this, they make it sound so complicated, you don't even want to try it.