EV units: Luna Pro-S vs Pentax spot

[Chris Gittins]

I bought a Luna Pro-S a few months but didn't get around to working with it until today. Something I don't understand when comparing it with my Pentax spot... I tak e an EV reading off a gray card with the Pentax and calculate appropriate aperture*exposure time combos. For comparison, I took sun and shadow "scale" readings with the Luna Pro, averaged them, and came up with the same aperture*exposure times I calculated with the Pentax. Good. The thing is, the scale reading on the Luna Pro supposedly translates to an EV reading and that EV reading is very different that what I got with the Pentax - don't recall precisely what the difference was but something like three or four units. Seems pretty clear EV means something different on each meter, but what? I couldn't tell from reading the Luna Pro manual. (Not a big deal, as it all seems to sort itself in the end, but I'm curious.)

Any insights appreciated.

Thanks,
Chris

[Gary Frost]

The EV scale on the Pentax is 'hardwired' for ISO 100. If you set the Luna Pro to ISO 100, the

EV should match. The EV scale on the Luna Pro will adjust according to the film speed when

taking a light reading.

The Luna Pro gives the actual EV value while the Pentax reading is 'transposed' from ISO 100

to whatever ISO on the dial. (This is the way the Luna Pro 'Null' meter works. I'm not sure

on the older versions)

[Conrad Hoffman]

I don't know the meters, but Gary's explanation sounds right. There are two ways the EV system is used. When tied to ISO100, the EV number represents a specific amount of light. As an example, the light sources used to calibrate light meters are adjustable in increments of EV units, and it's understood that ISO100 is the reference point. Then there were the cameras that could be set using EV numbers, like the old Retina IIIc. Obviously, the exposure would only be correct for a given EV setting if the EV number took into account the film speed. Confusing, never caught on, but makes a reasonable amount of sense when you work with it.

[Chris Gittins]

This makes sense. I had the Luna Pro set for EI 400. I just did a back-of-the-envelope calc:

EV = log2[ (f-number)^2/shutter speed ]

based on some info I found on the web. Based on my memory of my typical aperture-exposure combinations determined from Pentax readings, everything seems to hold together.

Thanks,
Chris

[Jim Glynn]

I have two Luna Pros and the actual EV being indicated is fairly easy to interpret. There are two ways to go about this. The first, as mentioned earlier, is to set the ISO to 100 and then transfer the indicated needle reading to the corresponding black numbers on the yellow scale in the bottom window. Line up the numbers to the yellown triangle index mark ( the other two index marks are only for use with the vari-angle 15/7.5 degree spot attachment). At that point the EV window will accurately indicate actual EV. This is true only at ISO 100.

The second method simply involves reading the needle indication and relating that reading directly to actual EV, ignoring the yellow scale and ISO setting. The way the Luna Pro is set up, just subtract 5 from the indicated needle reading to get actual EV. So if your needle reads say 15, then your actual EV is 10. This is the same for the upper and lower scales. Also, this does not change regardless of what setting you have in the ISO window. So whether you have ISO set for 400 or 600 or whatever, you can simply look at your needle indication and know what the actual light conditions are in EV.

Hope this helps.