From time to time I will have 3-4 prints out of several rolls come out very underexposed. Dark and very grainy, somewhat of a vinetting effet as well - very subtle though. The only thing I can think of is that the settings during these shots are as follows: AV with the 550EX set to High Speed Sync. At times I could have set the mode to Program and forgot to turn the HS Sync off on the flash. Not sure though if this would effect exposure.

Any thoughts would be appreciated,

David Pascolla

-- David Pascolla (dpascolla@yahoo.com), April 24, 2001

Answers

I'll try to come up with an answer for you.

• If you use a hood, it may have become rotated.
• You may be metering a white subject without +1.5 or +2 FEC.
• Your flash may not be zooming correctly and is stuck on a setting more telephotoish than the lens you're using. Like the flash is stuck at 50mm while your lens is at 35mm.
• I think that the switch set to HSS and shooting below 1/125th would create a multiple flash effect across the image. This is because of the output of the 550EX in HSS mode. In HSS mode, the flash has to simulate a strobe effect to follow the pattern of the vertical travel focal plane shutter blades.
• Try switching to a slower film or add a ND filter to get out of HSS. Maybe the HSS is the problem. Then shoot in M mode.
• Also shoot a roll of slide film and make that the determianting factor of camera equipment trouble. Maybe the mini lab's machine is messed up. A roll of slide will show you exactly what your camera and flash are doing.
• I'm pretty sure you can use Program mode along with HSS. I mean it's P mode, but it's on the other side of the lock setting from the idiot modes. You can;t do anything creative over there.
• I leave my HSS switch set to "on" when I'm going back and forth betweeen light and not so light. I've never had a funny looking print. I shoot in Manual mode and adjust my shutter speed by using a handheld light meter in outdoor conditions, though.

Let us know what lens you're using and if you use a hood.

-- Colin Miller (ckmiller@pond.net), April 24, 2001.

David, are the underexposure problems evident on the actual negatives (extremely clear negs). Or is it only the prints that appear dark and grainy?

The reason I'm asking is that it is not uncommon for labs to over- or underexposure the prints from negs that are perfectly fine. Moreover, even if the negs are off several stops, a caring printer can compensate while printing and make reasonably good prints.

-- Puppy Face (doggieface@aol.com), April 25, 2001.

When you turn it on, FP (high speed sync) becomes an automatic function. In other words, it only works when it needs to. It won't effect exposure as long as you are in range for your flash. Having 3 or 4 prints underexposed out of several rolls is fairly common, and while I hate to say it, usually caused by operator error. There are more possible causes than I could list, but a few are:

Out of range (very common, even with a 550EX).

Flash not fully seated in the hotshoe, or hotshoe contacts dirty. Metering on a white subject, or with extreme backlight (metering on the light).

Picking the wrong metering mode for the subject (Is you camera set to evaluative or partial metering? Is linked to the focus point via custom function? Is E-TTL activated on the flash?).

What film are you using? Some pro films like Portra 160 don't do well with flash at the rated ISO.

In other words, there is no real way to diagnose the problem here, just give possiblities. Anyway, unless you are carfully controling the conditions, a few underexposed shots will happen here and there.

-- Brad Hutcheson (bhutcheson@iname.com), April 25, 2001.

I have an EOS-3 and a couple of 550EX. I have been experiencing the same problem in a totally random fashion for some time. Sometimes I will get 2 bad shots out of a sequence of 4. I always wait for the capacity charge light to come on and check for the proper exposure light afterward. I dillegently check everything and eliminate all possibilities but haven't been able to sort out the cause. Yes, I've tried test rolls (prints and slides), same film, same batteries, same subjects. I'm ready to send the units in the shop but there is no guarantee that they will find the "random" problem. I had an idea today that perhaps the "fill-in" automatic exposure reduction is activating randomly when it shouldn't (underexposing by 1.5 stops). Thanks everyone for all the input, and I hope to find someone with a similar problem who has found the solution.

Geoff

-- Geoff Mann (geoff-mann@look.ca), April 28, 2001.

Geoff wrote: I always wait for the capacity charge light to come on and check for the proper exposure light afterward.

I always got a green "you did a good job!!!" light too. I still was always using -1.5 FEC and focusing on whatever I wanted to. Even white stuff. I trust the confirmation light about as far as I can throw it. Wait, bad example. :) I don't trust the light. That's better.

I'm not sure we can eliminate this problem. I've had much better luck using FEL and appropriate FEC or focusing on caucasion faces with 0 FEC. This whole thing sure makes good manual flash seem nice, eh?

Seriously, I'm curious why they can't or won't make a flash system based on distance. Come on, the distance info is already there, right? How hard would it be to transfer it to a flash unit. That would be best in my opinion. Then we could use true fill. Camera set @ f/8; distance requires 1/4 output, operator desiers -1 stop; flash emits 1/8 power. Jeez, am I oversimplifying it?

-- Colin Miller (ckmiller@pond.net), April 28, 2001.

Do the Canon EOS lenses transmit distance information to the camera though? At any rate I use a 550EX on a Rebel G and Elan 7 and haven't experienced this problem. Having a mode of flash that's based on distance would be nice though.

-- Steven Fisher (srf@srf.com), April 30, 2001.