Is a Spot Meter
A hand held 1-degree spot meter such as the Sekonic L-558 or L-758DR is a valuable tool, but it isn't a silver bullet for exposure unless we understand what it is telling us about what we are reading.
Meters, by design, return a combination of ISO, shutter, and aperture which will render whatever they measure MIDDLE GRAY. Let's not quibble about whether its 12% or 18% because it really doesn't matter.
Meter a white towel, filling the entire frame of the camera or hand held meter with it and then and use the reading the meter (or camera) gives us and it will not be white in the photo but MIDDLE GRAY. It does not matter if we have used full frame, center weighted, partial, or spot in the camera; or a 1-degree spot meter it will still be MIDDLE GRAY. Let's not quibble about what shade of gray it is because it really doesn't matter.
So how do we make the white towel look like a white towel? We interpolate from the meter reading.
By doing a simple bracket test starting with the meter reading and opening the aperture +1/3, +2/3 ..... +3 (full stops not "clicks" of the dial) and looking at the files in Photoshop we will see the towel get lighter and lighter -- just like in the Tide commercials - until it finally gets to the point of being overexposed with the detail in it blown out.
Seeing texture disappear as the indicator of over-exposure is why we use the towel - we don't need no stink'n eyedropper with a towel, just our eye balls and brains to see when it disappears. We can even see it in the camera playback its so obvious!
So we do that test and discover that when we read the towel with our $500 spot meter we must mentally add X stops to turn the gray reading into a white reproduction of the towel. Let's say, just for example, the test shows it takes 2-2/3 more exposure than the meter indicates to make the down white.
So knowing that we can now meter the towel with our 1-degree meter, mentally add 2-2/3 stops, and BAM - perfect exposure every time !!!!!! Is this great or what !!!
But wait ... It's even easier!
The meter has a compensation function. If we simply dial-in that 2-2/3 stops of "Kentucky Windage" into the meter, we can eliminate the mental math.
Now when we point the meter at the white towel or thing in the scene we want reproduced at the brightest textured highlight in the photo the number on the meter LCD when dialed into the camera will result in a perfect exposure every time !!!!!! Is this great or what !!!
But wait ... It's even easier!
We just look at the camera playback.... It will also tell us when the detail in the towel is getting blown. When the towel begins to get overexposed the brightest parts of it will begin to black out on the play back. That is the camera warning based on what was ACTUALLY RECORDED.
By doing the same bracket test we can learn to judge exactly how the towel SHOULD look on the playback when perfectly exposed.
Once we know that do we really even need that $500 spot meter? Nope, just a 50 cent white towel and the excellent meter in the camera which tells us what is actually in the file.
So why are spot meters so popular?
Well back in the days of B&W film and the zone system there was a thing called a negative. By altering is development it was possible to record any range of brightness in a scene and make it fix the fixed 10-11 stop range of density a B&W print could record. To fit a a high contrast scene on the paper we zone system practitioners (like me in the 1970s and 1980s) first needed to know the range of illumination in the scene. We would measure the darkest area in the scene where we wanted shadow detail (not always the darkest thing) and then measure the brightest highlight where we wanted to retain texture (not the brightest thing) and compute the range. If it wasn't 10 stops we knew we would need to adjust our "normal" negative development time. If the scene had 12 stops (not uncommon) we would develop the negative a bit less so it fit the #2 printing paper. If it was an overcast day and the same scene only measured a 9 stop range we would develop the negative a bit more o it fit the #2 printing paper.
Are They Practical for Digital?
Spot meters were invaluable when lugging a 4x5 view camera and using sheet film, but with the sophisticated metering in a camera today they are just excess baggage. Sure they will produce very accurate exposures if used as I've explained above, but there are easier and faster ways to the same end: exposure for the textured highlights using the diagnostic aids in the camera, the histogram and over-exposure warning.
With digital cameras we can't alter the range of the capture - it is limited by the dynamic range of the sensor. So all we really can do is get the highlights right. If more shadow detail is need because the scene exceeds the range of the sensor we have only two options with digital: add fill flash, or bracket and blend multiple exposures in Photoshop.
We are not using slow and cumbersome view cameras and sheet film. In most cases there is the 30 seconds or so needed to take a test shot to zero in on the best exposure. Lets also not lose sight of the fact that even Ansel Adams didn't climb up a mountain with one film holder - he bracketed exposures. The more critical the shot the more exposures he would make! We can bracket automatically with most DSLRs but few photographers use this feature.
Holistic Concepts for Lighting