lmd91343 said:
I've seen that Calculight XP number of -7 before. That's tough to believe. I've metered nighttime scenes with my F1 and special prism, that I could not see! The dial may read -7, but the sensitivity is probably higher. Their other models list only -3 on their web site. My Luna Pro dial reads down to -8, even though their specs state the sensitivity is -4! Isn't -7 below human vision in most circumstances?
Meters need those lower numbers to accommodate film speeds lower than 100.
Remember, an EV number by itself does NOT have anything to do with light levels -- it's just a shorthand for a set of shutter speed/aperture combinations. For example, EV 0 refers to 1 sec. @ f/1 or any equivalent combination, such as 2 sec. @ f/1.4 or 4 sec. @ f/2. It's just a way of designating camera settings by a single number.
To get this number to specify a light level (to avoid confusion, let's call this an LV number) you also need to specify an ISO film-speed rating. Customarily the rating used is ISO 100. So for example, the amount of light we call LV 0 is the amount of light for which an exposure of 1 sec. @ f/1 gives you a correct exposure
on ISO 100 film.
If you're using a slower film under the same lighting conditions, you'll obviously need to give more exposure. For example, with ISO 25 film (two stops slower than 100) you'd need to give two stops more exposure -- 4 sec. @ f/1, for example. The EV number -- remember, this is just a way of specifying camera settings -- that corresponds to a setting of 4 sec. @ f/1 is EV -2.
So, even though the
light level is still LV 0, your meter needs to read down to EV -2 to show you the correct settings for ISO 25 film at that light level.
It may sound confusing, but if you get out a dial-type meter and play with the different dial settings, it should become clear quickly. Just set the pointer to a particular light level and leave it there. Set the calculator dial for ISO 100 and note the reading you get in the EV window. Now set different film speeds and notice that you'll see different EV readings -- even though the light level hasn't changed.
That's why you may see really low EV numbers on a meter that isn't particularly sensitive. Those low numbers simply let you read out exposure values for slower films. My Weston Ranger 9, for example, can be set for ISO numbers as low as 1.5! (yes, there used to be films that slow.) At that ISO setting, an LV of 0 (for ISO 100, remember) translates to an EV of about -6! That's because a film with a speed of ISO 1.5 would be about six stops slower than an ISO 100 film, so you'd need to give it six stops more exposure under the same lighting conditions.
Now, getting back to what human vision can do: According to the
chart I referenced in my last post, an LV of -7 corresponds roughly to the light of a quarter moon.
No, that's not much -- but if you're reasonably young and in good health, and give your eyes enough time to get adapted to the dark, you can see well enough by a quarter moon to at least avoid tripping over the landscape, although that's probably about all you can see. If you ever go camping, though, getting well away from city lights and letting your eyes get well-adapted to the dark, you'll be surprised at how well you can function under the light of even a crescent moon (LV -10.)
And all of us who have ever done darkroom work know that after you've sat in a dark darkroom for a while, you can start to perceive shapes and movement from even tiny light leaks; surely you've had the sensation while loading film reels of saying, "I can see my hands moving, my film is going to be fogged!"
There's no light meter that can match the eye's sensitivity to these really dim light conditions, although film itself can easily record an exposure if you leave the shutter open long enough.
For a couple of practical examples of photography on the darker fringes, I've attached two of my late-night drag racing pictures. These were taken outdoors, lit by distant street lamps, under light so dim I couldn't quite read my meter dial: I just set the slowest shutter speed I thought I had a prayer of hand-holding, which was 1/8 sec., and cranked my Canon 50/0.95 lens open to full aperture.
This combination translates to an EV of 3; allow for the fact that I was using Kodak T-Max P3200 film rated at 1600, and you can see that the corresponding LV was -1. Under these light levels I had no trouble seeing well enough to move around without running into things. On the other hand, it wasn't until I got home and developed the film that I was able to see that the woman sitting in the truck bed in picture 1 was talking on a cell phone, or that the girl in picture 2 was wearing a midriff-baring top. You can also see that my hand-holding ability at 1/8 was actually pretty crummy; for serious pictorial photography under these conditions, you'd definitely want to use a tripod!
These results suggest that a meter capable of reading down to LV -2.5 should get you right down to about the limit of the dimmest light under which you can photograph "casually." This would be a light level requiring an exposure of about 1/8 sec. @ f/1 on ISO 3200 film. Since 3200 is about the fastest practical film speed rating (if you care about shadow detail at all), f/1 is the fastest lens you can buy, and 1/8 sec. is the absolute dicey limit of hand-holding for most of us, this combination of settings is pretty much rock bottom for hand-held photography.
Anything significantly dimmer than that and you're going to be in the realm of tripod-mounted long exposures and reciprocity-law-failure calculations -- conditions under which you're probably going to want to make several bracketed exposures and pick the one that gives you the results you like, rather than assuming a single "correct" exposure based on a meter reading.
