Review of the Zone VI modified Pentax Digital Spot and the Sekonic L-508 light meters
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I own both a Zone VI modified Pentax Digital Spot and a Sekonic L-508.
The Pentax is your basic, stripped down, minimal functionality spot meter. It reads in EV (with the numbers normalized to ISO 100) in increments of 1/3. The display is easy to read, being nice bright LED digits with two LED dots to indicate the 1/3 increments. The Pentax viewfinder is fairly large and offers reasonable eye relief and a large exit pupil.
As you can see in the photo above, the Sekonic L-508 is more or less the same size as the Pentax. Advertised by Sekonic as an all-singing, all dancing wundermeter, the Sekonic L-508 does both spot metering and incident metering, and includes the ability to do flash metering. The spot meter capability has a zoom capability which can adjust the spot size from 5 degrees to 1 degree. (Too bad it isn't 1 degree to 0.2 degree). The spotmeter viewfinder is bright, but has a smaller exit pupil and slightly poorer eye relief than the Pentax.
The Pentax Digital Spot is small, compact, and robust. Mine has taken some fair abuse and continues to function perfectly. The Sekonic seems equally robust; mine has taken a 3 foot dive onto rock without suffering even cosmetic damage.
The Sekonic has a battery life indicator, which will warn you when the battery is starting to get low. The Pentax will warn you that the battery has just run too low to produce good readings by behaving in a random erratic fashion designed to confuse you. It runs on one px28 battery, commonly available in camera shops (but not available in places like 7-11, grocery stores, the wilds of Nepal, and so on). In contrast, the Sekonic L-508 runs on a single AA battery, which is available virtually any place on the surface of this planet.
One highly prized feature of the Pentax is that on the barrel of the lens, there's a simple analog computer that allows you to calculate exposure given your film speed and the meter reading in EV. By happy design, this calculator also allows easy zone system use, because you can apply a simple label that will mark spots on the dial that correspond to zones 2 through 8. This is considerably harder to describe than it is to use. Trust me - thousands of Zone system practitioners live day by day placing and reading values on this little calculator.
The Sekonic L-508 is, in addition to a spot meter, an incident meter. The incident mode allows use with the dome/integrator in either hemispheric or flat mode. Both spot and incident mode can be used in flash meter mode. The flash mode is both cordless and corded. It can accumulate multiple flashes as well, including counting how many flashes it's added up. I rarely use flash and so I've no earthly idea how useful such features are for those who use flash.
The Sekonic does not have the handy, familiar analog calculator that the Pentax has. Instead, it has a fairly well thought out display. The exposure calculation can be done based on the desired shutter speed, the desired aperture, or in a fairly useful mode which displays shutter speed, aperture on a semi-analog scale, and EV. Readings are in increments of 1/10 stop. The meter reading cannot be read through the spot meter viewfinder. This nearly fatal flaw is somewhat mitigated by a fairly simple memory system that allows you to make multiple measurements (of, say, shadow and highlight) and display them all on the semi-analog aperture scale. In practice this is a fairly simple system to use, but it is still not quite as simple as using the analog calculator on the Pentax. The memory feature of the Sekonic L-508can also be used to produce an 'averaged' reading. I can't for the life of me figure out what the hell you would use that for, so I've never used it.
The Sekonic display is a LCD panel with good contrast. In low light levels the display is backlit with a nice blue-green fluorescent backlight that is easy on the eyes and just the right brightness. Now, you're probably thinking "If it's so dark I can't read the display, won't it be too dark to photograph?" First, that's not the case. I regularly do theater production photos, and I'm standing in the dark and photographing a brightly lighted stage. Second, the makers of the Sekonic were clever enough to use the INCIDENT light level to determine the backlight mode, even when you're using the meter in spot mode. That is, the backlighting is turned on when you're standing in the dark, not when the light reading you've just taken is low.
The Sekonic has two film speed setting memories, which you can toggle between when displaying the exposure calculation. This would be useful if you were, say, using Polaroid material to proof shots, and then making final exposures on some other material. I never do this and thus I use this feature only for amusement. ("Say, what WOULD the exposure be if I were using film with a speed of 8000?")
The Sekonic is advertised as 'weatherproof'. The Pentax is not. Both my Pentax and my Sekonic have been rained on without any apparent damage.
In careful comparisons of readings in various situations, I've observed that the readings of my Pentax and my Sekonic agree to within 1/3rd stop with a single exception. In general I've found that my Pentax reads generally about 1/3 stop lower than the Sekonic. If you offset this systemic difference, the readings appear to generally be in complete agreement, regardless of the color of the metered surface and regardless of the intensity of the light. (I'm discussing spot readings, here, of course). The single exception I found was reading a very dark blue object where the Pentax read one stop lower than the Sekonic. I suspect that this difference may be caused by differences in UV or IR sensitivity but don't really know. In all other cases, I find that the two meters are in complete agreements (with an offset of about 1/3 stop and a standard deviation of perhaps 1/6 stop or less) even when metering various colored objects through variously colored filters. By using the exposure compensation feature of the Sekonic, I'm able to bring the two meters into complete agreement in virtually all cases. Whether this means that the much vaunted 'adjusted to match Tri-X Pan Professional' spectral sensitivity of the Pentax is just so much bunk or that the Sekonic has the same carefully adjusted spectral sensitivity I don't know. I've always gotten good results with the Pentax and get the same good results with the Sekonic.
Although initially I felt that the Sekonic is a little less flare prone, I've since concluded that flare with both meters is essentially the same. If someone ever makes a 1/2 degree spot meter that is completely free of flare I will buy one in a heartbeat.
The Sekonic has a lens cap that is coldly calculated and designed to get lost. The front optics are not recessed and are likely to get scratched if you don't cap the lens. The Pentax also has an easily lost lens cap but the optics are recessed. The lens on the Sekonic is carefully placed so that you are maximally likely to get fingerprints on it, and you will need to clean it regularly. The lens on the Pentax is so recessed that it can be difficult to clean.
I suppose my Pentax came with a manual (I bought it new) but to be honest, I've never looked at it because there's no need. The Sekonic came with a handy "Quick Guide to the Sekonic L-508" which folds to fit in your pocket and will tantalize you with almost (but not quite) enough information to enable you to use the meter productively. It also comes with a 28 page manual that will tell you how to use the meter in exhaustive detail and a mail-in certificate that you send in to get your "Ph.D. in Sekonic L-508" certificate, suitable for framing.
The Sekonic has some cleverly hidden DIP switches under the battery door, which can be used to customize the behavior of the meter to your preferences (and thus ensure that when you pick up your friend's L-508 it will behave in ways unfathomable to you).
They weigh about the same.
I like them both. For theater work, I use the Sekonic; it's incident metering capabilities are excellent for checking evenness of lighting on stage, and the pace is too fast for anything resembling zone system practices. Instead, I generally set the exposure compensation on the meter to offset by one stop, then meter faces, which essentially amounts to placing skin on zone VI. The backlit LCD display on the Sekonic is easy to read in the dark and can be used as a low intensity light to check camera settings.
Although at one point I was using the Sekonic about 1/3 of the time for large format zone work, I now find that I'm always using the Pentax. The simplicity of the Pentax for zone system use, along with the better low light sensitivity mean that the Pentax generally wins for large format use. Some of this sentiment may well be the fact that even after more than a year, the Sekonic L-508 still feels like a new gadget in my bag, and the Pentax seems like a comfortable old friend.