I practically never go out to shoot without a light meter on my belt. Actually it's a flash meter, incidence type.
For bigger location jobs I've got two backups in my lighting kit: one is a second incidence flash meter, the other a simple incidence meter.
If you get a flash meter, you can use it more ways. But, if you never have any reason to meter flash or strobes, and have no plans to do so in the future, you could save a bit of money by getting a simpler meter, perhaps even one that doesn't rely on batteries at all.
The digital Sekonic you referenced at the link looks like a good choice, and it's an incidence meter. These are easy to use and are what I'd always recommend. With an incidence meter you just need to be sure you are measuring the same light that's falling onto your subject. You don't want to be metering while standing in the shade, if your subject is in full sun, for example. Step out into the same sun as your subject, then take your reading.
90-95% of the time there's no need for any sort of compensation with an incidence meter that's being used properly. The only reason you might dial in a little + or - is if a particular subject calls for it... e.g. it's a dark colored animal and you want to lighten up 1/3 stop or so to show more fur detail. This is essentially a case where a scene is exceeding the dynamic range of your camera, and you bias the exposure a little one way or the other to increase detail in what you determine is the more important end of the spectrum. In other words, either to preserve a little more highlight detail, or a little more shadow detail. Even then, it's usually not much compensation.
With an incidence meter you don't need a gray card to make accurate readings (but one still might be handy to set a custom white balance) and you do not have to compensate for meter error (due to subject tonality, like you do with a reflective meter.
Your camera has a reflective meter in it, and this is used to determine all your auto exposures. A reflective meter is constantly fooled by variable subject reflectivity. A light colored subject needs + compensation because it reflects more light than "average", which the camera's reflective meter tries to make "average gray" in your image. A dark colored subject reflecting less than "average" light needs - compensation because the meter is, once again, trying to make it "average gray". With a reflective meter, the most accurate way to meter is... you guessed it... by carefully taking a reading off an "average gray" card. That's not needed with an incidence meter.
Want to see a reflective meter in action? Try this... half press the shutter release on your camera and watch through the viewfinder as you pan around a scene. Keep an eye on the exposure settings. See the readings changing wildy and constantly? It's up to you to try to decide, based upon your experience, whether or not the meter is correct, and whether or not any + or - needs to be dialed in.
Or, just use an incidence meter. You take a reading by holding it with that sensor dome pointed toward the light source, instead of the subject. So long as the light source is steady (and they usually are), you will get the same read out over and over again. Set your camera to M, dial in the recommended settings and you can shoot accurately as long as the light source remains steady and consistent.
During a day's shoot, I re-meter every half hour to hour, more often in the morning or evening when light changes faster. I also use the histogram feedback to "tweak" my settings, as light values change during the day.
Obviously, if the sun is occasionally hidden by clouds, and you want to keep shooting, you need to re-meter.
Now, a few things I like about the Sekonic meter you pointed out.
1. It's got a flash sync range up to 1/500 second. Be sure this specification works with your camera, some of which have flash sync speeds higher than some flash meters. One of my flash meters reads out only to 1/200, but four of my five cameras can sync up to 1/250.
A few Canon can sync up to 1/300. And most anything using a leaf shutter (large format and some medium format) can sync at all their available shutter speeds, often 1/500 and in more rare cases even faster. Of course, you can mentally extrapolate from a reading at a lower sync speed... But that introduces an opportunity for error.
On the other hand, if you are working with studio strobes, your max sync is usually lower than the camera's rated capability with portable, electronic flash, anyway. I use 1/160 max with all my Norman monolights and cameras, well within both my meters' capabilities.
2. It's got a wide ISO range: 3 to 8000. A little higher would be nice, because some cameras are starting to have higher ISO capability. However, this covers all existing Canon, and many others. Once again, you can always extrapolate from a reading at a lower ISO setting that's within the meter's range.
3. It reads out in your choice of 1/2, 1/3 or full stops. This is good, since it matches your camera. My two digital flash meters read out in 1/10 stops, which I have to mentally convert to the 1/3 stop settings I use on my camera. So, if the meter tells me to set 2.8 point 9, I need to set f4. Sometimes in a rush I make mistakes. If it read the same as my camera settings, it would be better.
Now 1/10 is nice for extreme precision, when time allows. Plus many studio strobes can be set in very fine increments, often 1/10 stop. But, I wish my older meters had the capability to display in the same increments as my camera, for all other uses. Still, I'm used to it and I manage. The meter you are looking at doesn't appear to have 1/10 stop capability, but that may not matter to you at all.
Fine increments are less of a problem for a meter that reads out with an analog dial, like my old Sekonic 398 Studio Deluxe. It's pretty easy to tell from the pointers, exactly where to set the aperture. Of course, it can't read from flash or studio strobe.
4. All digital meters are going to require a battery. The one you are looking at uses one plain old AA alkaline. That's great. I've had meters that required special, harder to find batteries. The one I use most now also uses a single AA, which I always have plenty of to power my flashes, anyway.
Oh, the old Sekonic 398 is even better... no battery required. It uses a selenium cell that generates it's own power. Now, many selenium cells degrade gradually over time and became inaccurate over time, requiring recalibration and eventually replacement. However, I gotta say the old 398 is pretty amazing. I've had it at least 25 years and it's still deadly accurate. But the same can be said of my dad's old Weston Master meter, which dates from the 1950s. It's also still accurate (but it's a reflective meter).
As to how meters operate.... You are close, but not quite there yet. The photographer has to enter two of the three parameters: ISO, or shutter speed, or aperture. The meter then indicates the third setting. In all cases I can think of, you have to enter ISO (but may be able to change the setting to see how it effects the other factors). Then, depending upon the meter, choose either shutter speed or aperture, and the meter will tell you the third and final setting that is correct, based upon it's reading of the light. Read the instructions that come with the meter (some can average several readings, etc.)
It quickly becomes second nature to use a separate, hand held meter. The fact that you know what one is and are trying to figure out how the works puts you well ahead of probably 75 or 80% of people who are out happily shooting with their SLRs.
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