reefrat:
-hh
I'm afraid I don't understand the following bit:
"The trick is to meter your water and set up your f/stop to get ~1/45sec, then set your strobe power accordingly"
What do you mean ~1/45sec ?
I would imagine that I would use the meter to get a reading from the water and then use that f stop/ shutter speed with a fill in from the strobe (perhaps TTL using the film speed to bracket- eg correct exposure is at ASA 100 so take three shots with the TTL at ASA 50, 100 and 200)!!
You're on the right track, but I need to
seriously clarify what I said.
First, I shoot a Nikonos and my normal shutter speed setting that I use is almost always 1/60sec. This is not the "A" setting, as I'm not using TTL, but shooting purely manual.
The way to meter a scene is to push the shutter button halfway, and then compare what the camera's meter says versus your settings. If they match, in theory you'll have a properly exposed picture. More on the strobe in a sec.
The problem is that cameras are stupid, or more accurately, they are color blind: they don't know if the thing you're pointing them at is red, blue, green black, white or anything in between. So the basic way that metering systems work is that they assume everything is "average", which is known as "18% Gray".
What this means is that if the subject is darker than 18% Gray, its going to try to overexpose it to _make_ it into 18% Gray. Similarly, a pure white scene is going to be underexposed to try to make it into 18% Gray.
The most modern metering systems work by having a matrix of a whole bunch of metering spots - - the idea is that they can hopefully avoid this problem by looking "everywhere" and the lightest thing it finds is assumed to be white, and the darkest found is assumed to be is black...and hopefully you can see how a low-contrast scene trashes this assumption. Note that this applies to both film & digital equally.
Okay, back to what I said about trying to get 1/45 sec.
What I really mean by this is that I don't like how the exposure turns out on a photo of bluewater if I take it at 1/60sec when the meter says that the proper exposure is 1/60sec, so I'm using the meter, but I'm not strictly following its advice: by shooting a scene at 1/60sec when the meter says 1/45 is the proper exposure, I'm purposefully underexposing the water by a 1/2 stop...in other words, my water will be darker than 18% Gray.
Since I generally don't want to alter shutter speed, I get the camera to give me my 1/45sec by changing f/stop. Let's say that for a given situation, this results in 1/60sec at f/8.
So that's how I set up my basic background exposure. What I do next is set up my strobes. I gage the distance to my subject and use the above f/stop to look up what my strobe's chart says the proper power output is. For my system (dual SS-200's) that's usually 1/4 power on both strobes.
So for this example (which happens to be my default settings), I'd shoot 1/60sec @ f/8 with both strobes at 1/4 power.
Normally with land photography I would bracket by adjusting the aperature one f stop each way.
With a system with a strobe, you actually have two basic systems that you can bracket with - - the f/stop and the strobe power.
I've been shooting long enough that I normally change my strobe power settings before f/stop...its what I've learned with my equipment is what gets me the right total exposure shot.
So if I'm going to bracket, they'll be:
#1: 1/60sec @ f/8 with both strobes at 1/4 power.
#2: 1/60sec @ f/8 with both strobes at 1/2 power.
#3: 1/60sec @ f/8 with both strobes at 1/8 power.
If its a fabulous subject and I really want to make sure, then here's the rest of my bracketing sequence:
#4: 1/60sec @ f/5.6 with both strobes at 1/4 power.
#5: 1/60sec @ f/5.6 with both strobes at 1/2 power.
#6: 1/60sec @ f/5.6 with both strobes at 1/8 power.
#7: 1/60sec @ f/11 with both strobes at 1/4 power.
#8: 1/60sec @ f/11 with both strobes at 1/2 power.
#9: 1/60sec @ f/11 with both strobes at 1/8 power.
If you've followed all of the above, it should also be obvious to you that in the above sequence, #5 should be very overexposed and #9 very underexposed. If you're trying to conserve film, these are the first two that should be dropped.
BTW, the sequence in which you shoot bracketed images isn't too important, but there are some things that can make for a difference. For example, you'll see that I modulate strobe power first, then f/stop - this is because I can change strobe power a lot faster than f/stops. Similarly, if you have a system with a very slow-to-recycle strobe, it probably pays for you to shoot lower-power strobe settings (which will recycle faster) before the "full dumps", because the more time you spend on a subject, the greater your odds of kicking up silt, etc.
- - -
Finally, back up to the top, where you said:
"...(perhaps TTL using the film speed to bracket- eg correct exposure is at ASA 100 so take three shots with the TTL at ASA 50, 100 and 200)!!..."
Yes, you can also 'cheat' the TTL system by changing what the camera thinks the film's ISO speed is. This is a very effective way when using an automatic camera (or a Nikonos on "A") to get the system to shoot better images of any "Non-18% Gray" fish.
As you can tell I'm pretty new to u/w photography!
Have you spent more than the cost of your dive equipment yet?
-hh
