dark images @1/250

[kirbster12]

When I shoot with my D5 MkII with the 580EX, everything looks good as long as I shoot at 1/200 or slower. It will shoot at higher shutter speeds but above 1/200, the images are 100% darker, dramatically darker. I tried this with a D5 MkIII and got the same result. The 580EX is set to ETTL on and HIgh Speed Sync on. I'm guessing it is a setting on the 580 but I don't know what needs to be changed

[Skirball]

Once you go above the max sync speed the flash will go into HSS, where it essentially pulses really fast instead of one big flash when below the sync speed.  This has a significant impact on the flash power.  The output will continue to lessen as the shutter speed increases (as it’s now a time based function), but the initial step from normal flash to HSS is a very distinct jump.

Edit: I'm assuming that by 100% darker you still mean that the flash is doing something.  But you should first confirm that the HSS is working at all.  For purposes of troubleshooting, I'd turn off eTTL and use manual.

Also, there's a known quirk with the previous models of the 5D where it was darker at the max sync speed, not just past it.  Good info here:  http://neilvn.com/tangents/high-speed-flash-sync/

[kirbster12]

I read that link with great interest and it explained the problem very well. What I didn't read was the solution to my problem. My subjects are reptiles and fish which often move so fast, a shutter speed of 1/400 is needed to freeze the action. Often the animal's tank is dark so I want the flash to shoot at maximum power. So I switched the 580EX to manual and turned off the HSS and set the power to 1/1 and the zoom to M 105. This appears to be the solution that I'm looking for, my test shots are just what I needed. Thank you, I just didn't understand what was going on.

[Skirball]

Ok, I see a couple issues here.  First, your flash shouldn’t be able to work with HSS off if it’s above your max sync speed (1/200) and on the camera.  Are you using radio triggers?  Either your flash is still in HSS, or maybe it isn’t firmly attached to your camera and it’s allowing it to fire (but you should get a dark bar across the lower portion of the photo).  Manual wireless triggers can also override the protection and allow the flash to fire without HSS above the max sync speed.

Now that said, lets discuss how flash works for a minute.  Flashes fire very quickly relative to shutter speeds.  At low power your flash can fire at 1/10,000 of a second or less.  However, in order to increase power it simply increases the amount of time it fires for.  I find the 580exII at full power to be somewhere in the ballpark of 1/200 (I can still get motion blur of falling objects).  But, move it down to 1/4 power and you’re somewhere around 1/1000 of a second, pretty fast right?  You can use this to your advantage to freeze motion with your flash.  Your 580 is more than powerful enough to light up reptiles and fish in tanks, full power should be overkill, even with diffusers in place.  Try setting your camera on manual, ISO 400, Aperture F5.6, Speed 1/160.  A nice middle of the road setting.  Now put your flash on eTTL and go take some shots.  It shouldn’t require anywhere close to full power, and those settings are way too dark for indoor shooting, so your subject is going to be completely lit by the flash.  This means that even though your shutter was open for 1/160 of a second, your scene was lit by a flash of very short duration, say 1/5000, which is effectively your shutter speed – motion should be frozen. 

There’s a ton of good information on this all over the internet.  I’m a big fan of Strobist.com.  Check it out if you want to learn more about flash.

[kirbster12]

Thank you for following up on my earlier posts. I took 2 sets of photos, both times the 5D MkII was set to 1/160, f9, ISO 200  but the first set had the 580EX set to manual mode and the second set it was set to ETTL, HSS was off and the power was set well below maximum (+0) to cut the exposure time. Both of these modes worked, the fish were frozen in motion but it seems that ETTL was better. Your explanation of how the 580EX actually works was a big help. 

I modified your settings because I wanted to test closer to what I actually use to shoot.

I will be checking Strobist.com, trial and error only takes you so far without knowledge.

[Skirball]

Glad it worked for you.  The settings I suggested were just 'middle of the road' settings, not too extreme in any direction.  Once you got some working shots my next suggestion was to modify them to fit your needs.   You should be able to shoot at ISO100 and still get plenty of light out of the 580 while still freezing motion. 

Remember, so long as your exposure isn't long enough to pick up ambient light, changing the shutter speed will have no effect so long as you stay under your max sync speed.  It's actually kind of a fun experiment.  Turn out the lights and use a slow shutter speed, like 1/2 second.  Double check you still don't pick up ambient, then set up the flash.  Voila!  Still frozen.  Kinda cool, right?  Welcome to flash.  It's a fantastic and addicting style of photography.

[cicopo]

What mode are you using & have you also added some Exposure Compensation or Flash Exposure Compensation? They might be messing with the settings you think you're using.