question wrt YS-D2 on manual
- Thread starter M DeM
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Chris Ross
Contributor
I think around it differently. You have two exposures - flash and ambient and you want flash to be dominant in the foreground. When you're setting flash exposure I could care less about the shutter speed - as long as it's fast enough that you get the little input from ambient light. Set up your exposure in a darkened room subject about 30cm away +/- set your aperture f8 for m43, f11 for APS_C, f16 for full frame, and base ISO that's about what you need to deal with dome ports and having a chance to get the corners as sharp as they will be. Adjust flash until your subject is exposed correctly. Stop there and use that as your first setting underwater. You are only trying to get the first half of the exposure dialed in.
When UW, take an reading on the BG, and take some test exposures and dial in your shutter speed. Take your first shot and adjust flash output as required.
Doing it that way you start with enough flash power, you can get a good histogram that with the exposure weighted too much towards ambient light and that looks OK at first glance UW. If you start out with a flash setting that gives the right exposure you avoid falling into that trap. Your first iteration after the first test shot is to try it with more flash .
When UW, take an reading on the BG, and take some test exposures and dial in your shutter speed. Take your first shot and adjust flash output as required.
Doing it that way you start with enough flash power, you can get a good histogram that with the exposure weighted too much towards ambient light and that looks OK at first glance UW. If you start out with a flash setting that gives the right exposure you avoid falling into that trap. Your first iteration after the first test shot is to try it with more flash .
Ok whatever.
You clearly don't agree with Chris! Too bad, he is spot on. He, and I, and perhaps soon you, have learned that many of the exposure rules and methods on land do not work so well underwater.
My friend, I have no idea what you mean. The fact that "many of the exposure rules and methods on land do not work so well underwater" was PRECISELY my point. So why one would spend time trying to get a "rough idea" of how much strobe power one needs at SEA LEVEL out of water when one will be shooting 100 feet UNDER sea level is a complete mystery to me (and, may I add, many others).
I mean really, did you even read my post??
Ok, whatever.
Touche, bien joue. 
Chris Ross
Contributor
OK back to the original question, yes on land works to dial you in to approximately where you need to be. You want enough flash that the flash exposure is correct whether you are at the surface 100 feet deep or in the dark, without experience that can be hard to judge looking at the image and histogram in combination with ambient light. That's what testing on land does, if you are really close and the water is clear the exposure should be quite close. Take that setting underwater and start at maybe 1/125 if you're between ISO100 and 200 and take shots varying shutter speed until you get a nice deep blue. Only change ISO if your shutter speed gets too low or too high. If you change ISO vary flash power by the same amount - plus one stop of ISO.....change flash output down by one stop and vice versa. Limit your task loading on your first try or two - just stay at your selected aperture dial in the shutter speed and ISO if needed and start shooting - vary the shutter speed as required as ambient lighting changes as you get deeper, the sun starts getting low or a cloud comes over - etc. If you want to test your flash exposure dial up you shutter speed to max sync speed and see if your subject illumination still looks good from the flash, you may need to dial up power a bit if the visibility is low or if you are a bit further away.
The thing to remember at the same aperture and ISO and flash power the exposure you get on your subject remains the same IF the flash is the same distance from your subject, if you vary your distance you will need to change power. Shoot in RAW for flexibility if you blow highlights or under expose a little. The ambient light should have little impact on this as long as the ambient exposure for your subject is not to too high for your choice of ISO and shutter speed., which is unlikely unless you are snorkelling.
Of course if you are shooting macro with no ambient background to consider, just dial up the shutter speed to flash sync speed and adjust your exposure purely with flash at 1/250 f16 ISO100, ambient light won't be a problem, though you may need to stop down more especially if you are shallow to make a black background depending on how bright things are.
The thing to remember at the same aperture and ISO and flash power the exposure you get on your subject remains the same IF the flash is the same distance from your subject, if you vary your distance you will need to change power. Shoot in RAW for flexibility if you blow highlights or under expose a little. The ambient light should have little impact on this as long as the ambient exposure for your subject is not to too high for your choice of ISO and shutter speed., which is unlikely unless you are snorkelling.
Of course if you are shooting macro with no ambient background to consider, just dial up the shutter speed to flash sync speed and adjust your exposure purely with flash at 1/250 f16 ISO100, ambient light won't be a problem, though you may need to stop down more especially if you are shallow to make a black background depending on how bright things are.
The guide numbers on the Sea and Sea YS-D2 are on land at 1 meters with ISO 100. In terms of reading them in absolute terms
32 is full power other values are half of the previous. There is no relationship that can be established to your underwater shots because
1. You are not on land
2. Probably you are not at ISO 100
3. You are not shooting at one meter with the strobe aligned to the lens
Nowhere in the manual I recall a suggestion you could actually use this number to set exposure manually so it is just misleading
32 is full power other values are half of the previous. There is no relationship that can be established to your underwater shots because
1. You are not on land
2. Probably you are not at ISO 100
3. You are not shooting at one meter with the strobe aligned to the lens
Nowhere in the manual I recall a suggestion you could actually use this number to set exposure manually so it is just misleading
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