Flash exposure

[jhaden]

I have a couple Mark II Ns, and use a 550 and a 580 flash. In almost every case, using shutter priority AE, my photos are woefully under-exposed. I try bumping up the power on the flashes and bumping up the exposure on the camera... still usually very underexposed. Any thoughts?

[Matthew Gilley]

higher iso is the last variable to change besides a faster lens.





Maybe post examples with exif data for more input.





Good luck!

[Jon Ruyle]

Strange... even without the flash, it should expose okay, right? *Do* you get better results without the flash?


What kinds of shutter speeds are you using?

[Oren]

From another thread:


If you use aperture priority the camera will meter based on ambient
light and the flash will just add "fill". This may result in extremely
slow shutter speeds if you're in low light


Here is a link to the whole thread: http://community.the-digital-picture.com/forums/p/2045/15679.aspx ("/forums/p/2045/15679.aspx)

[peety3]

As I understand it, shutter-priority E-TTL flash works like this:


ISO is set by you. Shutter speed is set by you. Exposure compensation is set by you.


When button is pressed halfway, scene is metered and aperture value is calculated.


When button is pressed fully, aperture value is finalized, a test flash is fired, flash power is calculated, mirror flips up, shutter opens, flash fires, shutter timer completes, shutter closes, mirror drops.


It's possible that your shutter setting is driving a small aperture, making the flash incapable of lighting as designated. Does the flash then take 3-5 seconds to recharge (an indication that it fired a full-power pop)? Are you getting a green flash confirmation, suggesting that the flash thinks it exposed properly (if not, you're probably hitting max power and coming up short).


Are your pictures coming up with the shutter speed you selected, or are they coming up with a slower (i.e. 1/250th) shutter speed? If 1/250th or so, you may need to enable high-speed sync, and your camera is punting back to the sync speed automatically, making your pictures longer than expected, therefore aperture smaller than expected, and therefore hard for the flash to keep up.

[Alan]

This is what I've experienced, too, and have not been able to figure it out as well. So, I'm interested in what everyone has to say.


But, what I've done to get around this, is to set the camera to manual, use a shutter speed of 1/160th or 1/200, and set the aperture to 6.3 or 7.1.


This gives a nearly properly exposed image. A bit of PS touch up (fill the background some) and the image looks good.

[Chuck Lee]

I have a couple Mark II Ns, and use a 550 and a 580 flash. In almost every case, using shutter priority AE, my photos are woefully under-exposed. I try bumping up the power on the flashes and bumping up the exposure on the camera... still usually very underexposed. Any thoughts?



I really like peety3's insights. But.............


Why are you using shutter priority AE? What apertures is the camera selecting? My guess is the aperture is wide open.


If you are underexposing, then increase the ISO. That's the only adjustment other than flash compensation that's left.

[jhaden]

Thanks for the feedback. A couple responses to questions: I tend to
use shutter priority just because I tend to photograph events with lots
going on... don't want low light to cause a slow shutter speed and
therefore blur. Typically use 1/125. I almost always get the green "proper exposure" light... but turns out not to be the case. Biggest issue is with group photos; if I'm relatively close to an object and it fills the frame, exposure is better. If I'm back 15 or more feet photographing six or eight people full length... not so much.


Oh - typically the camera does select a wide open aperture.

[Chuck Lee]

Oh - typically the camera does select a wide open aperture


Then peety3 is right.


I was going to post this next bit but at the last second decided to pull it. I never want tooffend or come off as a know-it-all but nowI feel like I can humbly submit this to help with your situation. I hope you find this useful.


Change to aperture priority. Set your "ISO" (which has not been mentioned yet) to 400. Set your aperature to f4.0. Go to the the camera custom menu (it's 03 on my 5D) and set Flash sync to 1/200 fixed. Set exposure compensation to "0" and flash compensation to "0". Fire away my friend. Anything within 40ft will be lit. If it's not, your cameras or flashes are broken.....not likely. Also, check the flash compensation on the flash units to make sure it is set to "0".


Next, go read lighting 101 by Dave Hobby over at strobist.com. He'll help you make more sense of it all. Don't worry about the off-camera part as much as the fundamentals of flash.


E-TTL can ruin your ability to understand how flash really works. Set your flash units in manual mode and experiment using 1/4, 1/2, and full power flash output. After learning how it works you may never go back to E-TTL again. Again, not likely. But you'll have a better understanding of how to work with E-TTL.


What you need to know:


1) The aperture sets the brightness of what is illuminated by the flash independent of the shutter speed. (this is the most important, so please read this line 3 or more times). With your 580 on manual 1/4 power, put your camera in manual mode, set shutter to 1/125th, aperture to f4.0, ISO 400. then shoot a subject around 10 ft away. If the subject is blown out, stop the aperture down. Try 5.6 then f8, etc. until the subject is illuminated correctly. Go the oposite if the subject is dark. If you run out of aperture, increase ISO. Now, with that done, change the shutter speed to 1/60th and shoot again. What, the subject is still illuminated correctly? Yeah, but look at the background. Try 1/30 and 1/15. Then 1/160 and 1/200. Say what? The subject is still illuminated correcty? Man, that just blows my mind!! [:D]


2) The shutter speed sets the level of ambient light independent of the flash output. (Read 3X)


3) The ISO controls the overall exposure and balance between the two. (Read 4X)


4) E-TTL or thyristor or manual switch and/or flash zoom head and flash to subject distance controls the output intensity of the flash. (Read 8X)


The bottom line is that there are only two functions on the camera that control how brightly lit the subjecct is. ISO and aperture. When the aperture is "wide open" and the flash is firing at full power and the photo is underexposed, the only thing that can bring it up is ISO.


Theflashduration isat 1/10,000 sec. Do you think shutter speed has anything to do with it? Do you think that's air you're breathing now? (Matrix quote LOL)


It's knowing how to use these four things correctly thats the trick........[:)]

[BCalkins]

I toofind auto-exposure with the 580EX often underexposes. It should depend on a few factors including the subject brightness, etc., but even then it seems inconsistent. I often get cases where the flash seems to be using full power (long recharge time after the shot) yet my histogram is far short of correct exposure. Switching to manual totally blows out everything to white so I know there is lots of available power... I frequently shoot with +1/3 or +2/3 for neutral subjects. See my blog for an example:


http://lessonsineos.blogspot.com/2007/11/ettl-autoexposure.html ("http://lessonsineos.blogspot.com/2007/11/ettl-autoexposure.html)

[Mark Elberson]

Do you think that's air you're breathing now? (Matrix quote LOL)
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Nice!

[peety3]

Thanks for the feedback. A couple responses to questions: I tend to
use shutter priority just because I tend to photograph events with lots
going on... don't want low light to cause a slow shutter speed and
therefore blur. Typically use 1/125. I almost always get the green "proper exposure" light... but turns out not to be the case. Biggest issue is with group photos; if I'm relatively close to an object and it fills the frame, exposure is better. If I'm back 15 or more feet photographing six or eight people full length... not so much.


Oh - typically the camera does select a wide open aperture.
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I was afraid of that. "I want good shutter speeds!" sometimes translates into "why are my pictures dark?", and if the camera typically selects a wide-open aperture you're likely on the edge or into the danger zone with your settings. I don't know the IIn featureset, so I don't know if it offers "Safety Shift" or what ways it can help. If you don't have Safety Shift enabled, and there isn't enough light for your shot, you've locked the camera to 1/125th and a particular ISO, so the only thing it can do is open the lens wide and take a shot at 1/125th, underexposing everything if it's dark. If your aperture/ISO combination is insufficient for ambient light shooting at 1/125th, flash may not be able to help much unless you're close enough.


Try aperture-priority at wide-open sometime, with or without flash. If you find that your shutter speeds are real slow, then you should be boosting the ISO to a point that you can still avoid the blur. Then, you might find that your flash has enough oomph to get things done. If it doesn't, you may need to dial in some flash exposure compensation and just put a little human magic into the computer you're holding. All of this assumes that the background is relevant to your shot; otherwise, manual exposure may be a better choice.