Many lighthouse photographs that you see are taken in the daytime and there's nothing at all wrong with that. Architecturally, lighthouses are a beautiful part of our maritime heritage. Personally, however, I find photographing them in low light, the conditions that a mariner would find their beacons most hospitable, to be a rewarding challenge.
For wackos and others who don't have any good feel for how to go about doing this, I thought I'd put together several threads on this forum to give you some tips and ideas with photos to illustrate what I'm talking about. Unfortunately, most of what I'll talk about below in this thread can't be applied if you're using a point and shoot camera.
THE RULE OF RECIPROCITY
Exposure is based on what is known as the Rule of Reciprocity. Basically, that rule states that if you halve the amount of light getting to the film but double the exposure time, you get the same result. Conversely, if you double the amount of light and halve the exposure time, again, you get the same result. What does this translate to with an adjustable camera? Let's assume that you've got a correct exposure with 1/125th sec and an aperature of f5.6. OK, if you were to set the camera to 1/250th sec and f4 or 1/60th sec at f8, you would get identical results for all three exposures. That is a working illustration of the Rule of Reciprocity.
In the example that I just described, each of the exposure times that I mentioned differ by what is referred to a 1 "stop." Likewise, the aperature changes each represent 1 "stop" (f-stop). The full stops on your lens (you may not have all of these) typically might include f2, 2.8, 4, 5.6, 8, 11, 16, 22, 32. Your lens may also have intermediate values between those I just listed, in steps of 1/2 or 1/3 stops. If you've got one intermediate choice, your lens operates in 1/2 stops; two intermediate values and you're working with 1/3 stops.
YOU vs. YOUR CAMERA'S LIGHT METER
When you aim your camera at some object with the intent of taking a photograph of it, a metering system in your camera interprets the light levels for you. Great, you think! Well, maybe, maybe not.
Your light meter assigns "normal" as the equivalent of 18% grey, which is a medium grey color. What does that mean, you sit there asking yourself about now? Well, if your camera is aimed at pure white snow, the meter thinks that is average and it's going to make it look sort of dull greyish if you trip the shutter. Ever had that happen? How about if you're aimed at something that is coal black? Yep! Medium grey again here I come! OK, what's going on? The meter is doing EXACTLY what it is supposed to do for you. If you're not aiming it at something that is a medium color, the meter doesn't know that. If you're aimed at clear blue sky, that's about a medium blue and it turns out correct. To make snow look pure white the way it is supposed to, you have to adjust the settings of your camera to intentionally overexpose by 2 to 2 1/2 stops of light. In other words if your aperature was f5.6 when you took that reading of the snow at some shutter speed, you need to open the lens by two full stops, i.e. to f2.8 or some combination of aperature and shutter speed that gives you the equivalent of about two full stops to get the snow to look right. How about that lump of coal? Same thing, but in the opposite direction, you need to underexpose it by 2 to 2 1/2 stops relative to what your meter is telling you to make it look coal black. That's the way things are supposed to work with your light meter. The range of light is about 5 full stops from pure white to pure black. That's all film can "see." In contrast, the human eye has a sensitivity to light that spans about 14 stops, nearly three times the range!
WHAT'S THE CORRECT EXPOSURE?
The "correct" exposure isn't necessarily what your camera's metering system dictates. However, if you're shooting in an automatic mode, what it thinks is right is what you'll have to accept when you trip the shutter. Rather, the correct exposure is whatever segment of the image in your viewfinder that you, the photographer, decide to assign as medium whatever color it is. The sky, as I mentioned above, is usually a good value for medium blue on a clear, blue sky day.
If your camera allows you to adjust the metering area, you'll usually have three choices: matrix metering covering the entire viewfinder; center weighted metering, which usually center's the meter "attention" on a circular region about 12 mm in diameter in the center of the 24x36 mm are of what will be your image on the slide or negative; and a spot meter that is about 4 mm or so in diameter.
For puposes of this discussion on low light photography, let's say that you're photographing a lighthouse at sunset. OK, set your camera to meter the entire scene first. Take your light reading, and the camera will give you settings that will make your sunset sky medium red/orange/yellow or whatever it is. Your lighthouse is going to end up silhouetted in this scenario, which is fine if that's what you want.
The image below, shot a few weeks ago at South Haven, Michigan was done exactly this way.
This photograph was taken with my camera mounted on a tripod - I happen to use a Bogen 3021Pro tripod (legs) with a Bogen 3039 pan/tilt head. I was shooting with an 80-200 mm f2.8 Nikkor lens on my camera and the film that I was using was Fuji Velvia slide film (iso 50). There are any number of possible aperature/shutter speed combinations that I could have chosen, but it so happens that I used 0.625 sec and an aperature of f22 with the lens set at 200 mm to capture this image. When I composed this image, I matrix metered the entire scene, knowing that the lighthouse would be silhouetted against the colorful sunset, which is what I wanted in the image. OK, that's where the exposure settings ended up before I tripped the shutter. That ISN'T how I metered the scene.
With the lens stopped down that far (f22 is a very small aperature) everything in the viewfinder at sunset is going to be pretty dark. Rather than trying to see where things were in a dark viewfinder, instead, I composed the image with my f2.8 lens wide open at f2.8 so that things were as bright as possible to allow me see what I was doing with my composition. In most cases, unless you're using a professional level lens, your lens won't open that wide, but open it to whatever it's maximum aperature is at the focal length you're using. If, for example, I had been using a Tamron 28-200 mm lens, which is a good, general purpose travel lens, and I have one of them and a Tamron 28-300 mm as well, it would open to about f5.6 at 200mm so that's where I'd start. Take your meter reading. At f2.8 with my camera and lens, I would have been metering at 1/100th sec at f2.8 (this is 1/3 stop slower than 1/125th sec). For each full stop you close the aperature down, double your exposure time. So, for f4, you'd use 1/60th if you began at 1/125th and f2.8. For f5.6 you'd be at 1/30th sec, which might be where you'd start with a 28-200 mm lens open to f5.6 at 200 mm. Continue doubling the exposure time for each stop as you close down the lens until you get to whatever aperature you wish to use. In this case, I had my lens stopped down as far as possible to the minimum aperature to give me the longest possible exposure time under the conditions, which happened to be 0.625 sec at f22. Why the smallest aperature? Quite simply, with Velvia, I've found that with long exposures using very small aperatures I can get the most saturated colors in my images, which translates to the result in this post above, which was what I wanted in the photograph.
SOME FINAL THOUGHTS FOR THE MOMENT...
A good tripod is essential if you're going to do good, low light photography. The heavier the better! My Bogen 3021 with the 3039 head weighs about 12 pounds without the camera on it, which adds about another 5 pounds between the Nikon F5 that I use and the 80-200mm f2.8 Nikkor lens. If Terry reads this, she might want to comment on the differences in her own photos that she's seen between the lightweight tripod that she used on her first visit to Michigan vs. my older, heavy Bogen 3221 tripod with a 3047 head that I let her borrow on her second visit here.
The other thing that you'll need is a cable release. If the exposure time that you'll use is one that you can dial in on your camera (mine happens to allow me to dial in exposures of up to 30 minutes before I have to resort to the "B" (bulb) setting on it, but that is certainly not typical) dial it in and if you don't have a cable release, use your camera's self-timer to trip the shutter for you so that you don't touch the camera itself. If the exposure you want to use can't be dialed in, you'll have to get yourself a cable release to trip your shutter.
I'll add more on this topic a little later, probably in a separate thread, but I'll quit here to let anyone who's curious or didn't understand what I'm talking about to ask questions.
Gary
[This message has been edited by lighthouse_photo (edited 02-16-2002).]
