............
or a faster shutter speed to decrease the exsposure. You can also use a lower ISO number.
Make sure the ISO is fixed (ISO 50 or 100 should be fine) and not set to auto. When discussing strobe influence, it is the foreground exposure that is the topic. Shutter speed should not be a consideration in the foreground exposure equation.
You should almost always shoot with your strobe on full power to get the most light and color as possible.
imho you should always set your strobe intensity based on need. At very close macro distances a DS 51 (guide number 28, iso 100, feet) may blow out an image when set to "F", possibly even with the diffuser attached (gn 20, iso 100, ft).
With the SD 550 aperture stopped down to f/8, ISO set to 100, manual controller set to 4 (-4 stops from full) and the DS 51, diffuser installed, according to the guide number formula, your minimum strobe to subject distance should be about:
Guide Number = Aperture times Distance
Guide Number of DS 51, w/diffuser minus 4 stops at ISO 100 = @ 5
Aperture set to f/8
GN=A times D
5 = 8 times .62
.62 feet or @7.5 inches
At ISO 50, you could get even closer..........in theory..............ditto for man con set to -4.5
That said your camera has a maxium aperture of only 4.9 so the strobe may be to much.
f/4.9 is the widest aperture available when lens is set at telephoto distance.........not the smallest aperture available..............(f/stop is a relationship between focal distance and size of iris opening)
If your using the highest aperture and shutter with the lowest ISO and still washing everything out you will need to dial down the strobe controler.
Shutter speed has little to do with the foreground exposure.......as long as the shutter speed chosen is not quicker than the sync speed of the camera. Shutter speed has more impact on background exposure where it controls by time (along with aperture and ISO) the amount of ambient light allowed to pass through the lens.
Brian,
May I suggest you first set the ISO to either 50 or 100. Otherwise when set to auto, the camera will be constantly changing the ISO. Set the camera to manual so you are in control of both aperture and shutter speed. For typical fish pics or reef scenes I would next suggest a starting point of possibly f/4 and 1/100 sec. As mentioned, if you're capturing macro images, reduce the size of the aperture.
Make sure the DS 51 (I recommend diffuser installed) is set properly to the TTL/Auto setting when combined with the Manual Controller. Confirm the Manual Controller is also set according to Ikelite guidelines:
http://www.ikelite.com/web_pages/mansenswitch.html
Next I would recommend a simple approach (Jim Watt inspired) .......set the manual controller midway....around the -2.5 area............then follow Jim's digital mantra:
Shoot, Review, Adjust, Shoot Again!
If the exposure is too bright, move the man con closer to -4.5. If the exposure is too dark, move closer to "F".
If you prefer a darker background, as Alison has suggested, increase shutter speed.
Here's another guideline that Ike Brigham would frequently contribute when discussing the DS 125 ( the DS 51 should be very close since it is almost equal in intensity.......but not in width of beam). Set the aperture to f/8 and the manual controller to "F" (ISO 100, no diffuser). You are now prepared to capture an image with the strobe to subject distance of 4 feet! Now for every foot closer your strobe is to the subject, all you have to do is reduce the manual controller by two clicks (minus one full f/stop).
hth,
b
