20D and 580EX - underexposures are the rule

I find my 20D and the 580EX always underexpose by 1-2 stops when I shoot on the green rectangle. The ONLY solution to get properly exposed shots is to use P and set the Flash Exposure Compensation to 1 -1/2 stops. This gives fine exposures in general. My question is: Is this what others are finding tp be the case? My little DIgital Elph SD10 gives terrific exposures on point and shoot, so why doesn't the green rectangle on the 20D / 580 EX do as well? THX in advance.

Can you post a couple of sample pictures? I can think of a couple of things but I want to see the pics first. Please post them resized from the camera but without any levels adjustments, and if you're using photoshop use "save as" rather than "save for web" so that exif information's embedded in the pics.

OK Here are the examples. THe darker one is a point and shoot with the "Green Rectangle" I get a great exposure with the baby elph SD10 but not with the $ 2500 20D and 580 EX ! The brighter one uses the P setting with a FEC of + 1 1/3. Is the camera working correctly? Is there a better way? THX

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The first shot in green box with no FEC is correctly exposed, more or less, the one with P with FEC 1 1/3 is massively overexposed. The camera is exposing the foreground correctly (the flowers), which is what it's meant to do, it doesn't worry so much about the background. If you want the background lit up as well you might try a diffuser like the omnibounce, with some light bounced off the roof.

What is it you think it's doing wrong?

Tim - If you think the darker image is the correct exposure, how would it show up as a print without adjustments in PS. The histogram of this darker image stops about halfway towrd white ! I agree about the overexposure in the lighter one. I guess less FEC would be about correct. I'm mostly interested in your belief that the green rectangle shot is proerly exposed. <aybe I'm not correctly calibrated as the observer. Pat

I don't have photoshop here, so I can't look at the histogram, but in the 2nd shot the flower is clearly overexposed. In the first the flowers are well exposed, but the background is dark. What are you actually trying to take a photo of here? It's an artificial scene that doesn't represent real world photos very well. Like I said, the camera assumes the main subject is in the foreground and exposes it well, and doesn't worry about the background so much.

I don't think the shooting mode is relevant here, the FEC setting is more important. I shoot M when i'm using a flash, or Av if I just want fill flash.

What color is that thing in the background in real life, dark brown or light brown?

I thought the shot would be a challenge since it has the flowers in the foreground and the fireplace in the background [about 7 feet away] The wood is bleached oak, so it's pretty light. I haven't figured out M for flash yet, but I'll give it a try. How does it differ from P? There also is an M setting on the 580 EX. I don't know what that's about! THX for your help. Pat

I don't think you can do this shot with direct flash, and have both the flowers and the background exposed well. SLR cameras assume you know what you're doing, and how they work. I'd like to see the shot from the Elph, to see what it does.

Try pointing the 580EX towards the roof, angled forwards a little, and pull out the diffuser panel so it stands straight up - don't let it cover the flash head.

Shooting M is easy - just set it to M mode, whatever F stop you want, and 1/250th of a second. The exposure time determines how bright the backgrond is, nothing else, because the foreground's illuminated by the flash. You have to be a bit careful, if there's some ambient light on the foreground subject and you choose 1/10th of a second, the foreground object can appear blurry, because it's made from light from the flash and from ambient. I don't think i've described that very well, maybe someone else can help out.

Maybe this will help (maybe not). The green rectangle uses ALL of the AF points and generally assumes that the closest thing it sees is your subject. So it focused on the flowers in the foreground and metered the flash according to that distance. By switching to a mode that allows you to choose the AF point (such as the center one), the camera would focus on the fireplace and the flash metering system would use that distance in its calculations, correctly exposing the fireplace (but probably overexposing the flowers). A P&S camera with only center AF would do the same thing.

The other element at work here is flash fall-off with distance. It's hard to tell from the photo, but if the fireplace is twice as far from the flash as the flowers, it will receive only 1/4 as much light from the flash (the inverse square rule). Proper flash exposure is very distance-sensitive.

Both of Tim's suggestions were good: Either bounce the flash off the ceiling or use Av mode so the camera will properly expose the background with ambient light.

Hope this helps.

Curtis, my understanding is that with ETTL-2 flash exposure is evaluative, not linked to the active focus point. This means it will put out enough power to just avoid blowing the highlights in any part of the picture, in this case, the flower. If you replace the flowers in the foreground with a dark object i'd bet that the whole scene would have a more even exposure.

That can't possibly be true, because it would blow my whole theory!

Honestly, I forgot the 20D has a different TTL flash metering system than my DRebel, so I'll shutup and listen now.:o

In case you didn't know, flash photography is HARD! Like Bloo said, practice, I think the basics can be taught, but in the end it comes down to experience. I'm starting to get a feel for flash photography, after 3-4 months with my 550EX. I strongly recommend people read this whole thing, even though it's long, then experiment, then ask questions, then read it again, then experiment, etc.

Alternately, shoot on manual flash, which is something I haven't tried yet, but is on my list for one day when i'm bored.

i find mine pretty darn accurate...but it still requires understanding of the flash and what it can do, how to bounce it ect..

I too am battling with flash exposure, or rather underexposure. I took heart in this thread: https://photography-on-the.net/forum/showthre​ad.php?t=80273

It can be done. I just hope it's not pure happenstance...It must be something that can be repeated..So far I like the LSII but haven't really played with it enough in all the different modes, most noteably M. But that is coming up this weekend.

Those are great pics. The scene's good too, reasonably flat, more dark than light, and not too close to the camera. The 550 or 420 should do that in ETTL mode with no FEC, and it'd be even nicer with a lightsphere 2.