I don't understand this...Auto vs. Manual exposure

[jnick]

This has happened to me before and I'm simply baffled. Here's the scenario...

I'm using my 30D, my 50mm, and my 480ex flash. I put the camera on AUTO and the settings show 1/60 @ F/4. I take the shot, and in turns out pretty well!

I then compose the same shot, of the same subject, literally SECONDS later. However, this time, I shoot in manual mode. In order to get the light meter to read "0" the settings are something like 1/10 @ f/1.8. If I dare increase the aperature, the shutter drops below 1 second. Both scenarios create a blurry image, as I'm not using a tripod.

However(!), if I stay in Manual mode, and dial in 1/60 @ f/4, while the light meter "predicts" that the image will be greater than 2 stops underexposed, the picture turns out great...just like it was shot in auto.

Huh?!

I simply don't grasp what's going on here. The only thing I can think of is that AUTO was adjusting the ISO. While, in manual mode, I left it at ISO 100.

HOWEVER, even at ISO100, if I used the settings 1/60 @ f/4 - the pictures exposed very well! They were not under exposed as the light meter said they would be.

Can anyone explain to me what's going on? I know I'm not understanding something, but I'm not sure what exactly that something is.

Thanks!

[cosworth]

Your camera knows it's dark and knows it has a flash. ETTL is doing it's job.

Low light with M is great with a lfash beacuse you can expose and bring out the background yet freeze the scene with the flash. There should be no blur of the flashed subject. A little shutter drag ghosting can occur sure but that's fine.

Your camera is doing what it's supposed to do.

[ssim]

In manual exposure the camera is not taking into account the light that the flash will be adding to the scene. When working in manual with a flash do not pay attention to what the light meter is telling you. It is giving you a reading based on ambient light. When you are on auto mode, you are asking the camera to do everything for you and it does recognize that you have a flash on the camera. While on auto turn the flash off and then see what your reading is, it will be a substantially longer exposure. When working in manual you do not need to adjust your settings you get your meter to "0".

I have never been a fan of using auto mode but to each their own. I always use manual exposure when shooting with a flash. You set the shutter and aperture and then the flash will output the correct amount of light to match those settings in order to get a properly exposed image.

I would suggest that you spend some time in the flash forum as there are some excellent links to online resources explaining this in great detail.

[StewartR]

cosworth is right; your camera is doing what it's supposed to. But it can seem a little counter-intuitive.

Let's start from the meter reading you got in Manual mode. It was 1/10th at f/1.8. That's what you need to get a properly-exposed picture without flash.

In Auto mode, the camera selected 1/60th at f/4. A moment's thought will tell you that that would be w-a-a-a-y under-exposed: about 5 stops under. But what's happening is that in Auto mode the camera knows you're using flash, so it picks a middle-of-the-road shutter speed and aperture and relies on the flash for lighting. It fires the flash briefly and meters it, then adjusts the flash power output and fires it again to take the picture. With 5 stops of under-exposure compared to what you need for the ambient light, something like 97% of the light is from the flash.

In M mode, the camera doesn't know that you're going to use the flash. It's metering based on ambient light only, so it recommends 1/10th at f/1.8 and it tries to complain that 1/60th at f/4 would be much too dark. However, when you do use the flash, the camera follows the same process as before: fire the flash once to meter it, and fire again to expose. That's why you get the same results as you did in Auto mode.

Does that make sense?

[jnick]

Thanks for the reply guys. I think I get it now. However, I do have a few more questions. I mainly shoot in M mode, as well. The only reason I jumped to AUTO is because I noticed the pictures didn't look good at all when I set the light meter to 0.

So if I understand what you're saying, technically, when using a flash, by setting the light meter to read 0, wouldn't I be over exposing the image? Mainly because I'm getting the "perfect" exposure for ambient light, however I will now be firing off a burst of light form the flash. Wouldn't this cause an over exposed image?

With that said, if I don't/shouldn't listen to the light meter when using a flash in M mode, how would I know what shutter & aperture to use to get a properly exposed image? Is it that when the camera's in M, the flash automatically adjusts it's output based on the shutter and aperture I dial in?

Thank you...again!

[jnick]

Well I just stopped by the Flash forum, and read their first chapter to flash photography. I think I understand it a bit more now :).

So when in M mode, the flash does automatically adjust it's output based on the aperture. So technically speaking, if the aperture is set to where the shutter is equal to or below flash sync, I should pretty much get a properly exposed image correct?

[StewartR]

So when in M mode, the flash does automatically adjust it's output based on the aperture. So technically speaking, if the aperture is set to where the shutter is equal to or below flash sync, I should pretty much get a properly exposed image correct?Well, in M mode you set both the aperture and the shutter speed yourself. So as long as you set the shutter speed no faster than flash sync, and the aperture no wider than is necessary to get a proper exposure from ambient light, then yes you should get a properly exposed picture.

Of course there are lots of possibilities. With a fast shutter and small aperture your picture will be predominantly lit by the flash, and with a slow shutter and wide aperture it will be predominantly lit by the ambient lighting. In either case it will be properly exposed according to the ETTL algorithms, which is not necessarily what you might want. I think ETTL tries to expose the picture so that the highlights will be on the verge of blowing out.

[Curtis N]

When working in manual with a flash do not pay attention to what the light meter is telling you.Well ok, unless the meter is at +2. ;) The flash can add light but it can't take it away.So when in M mode, the flash does automatically adjust it's output based on the aperture. So technically speaking, if the aperture is set to where the shutter is equal to or below flash sync, I should pretty much get a properly exposed image correct?That's the beauty of a dedicated flash unit. The camera will calculate the required amount of flash power, accounting for aperture and ISO. It will even do this in Tv mode, where the aperture isn't determined until you lock exposure.

[John_B]

jnick,
You could always go full M by setting the flash manually. ;)

[PhotosGuy]

You could always go full M by setting the flash manually. That's my solution, too. See post #4 here:
Settings?? Help Needed! (http://photography-on-the.net/forum/showthread.php?p=2953374)

[CyberDyneSystems]

In simple terms (one sentence)

When shooting in manual with flash, Set your aperture and shutter to where you want them, and the ETTL flash will compensate by providing the required amount of light to make a good exposure.

[In2Photos]

In simple terms (one sentence)

When shooting in manual with flash, Set your aperture and shutter to where you want them, and the ETTL flash will compensate by providing the required amount of light to make a good exposure.
And then use FEC ( flash exposure compensation) to adjust your flash exposure up or down if needed.

[Hangerhead]

is it reasonable to assume then, that wherever possible, get a mter readng fr the background, and if your chosen aperture/shutter/iso are going to be fine, then all you next is switch to M mode, enable flash and copmose the shot using the setting you got earlier...
so...
1. meter the scene as if flash won't be used (so that the ambient lght is recorded)...

2. go to M mode, switch on flash and focus/pre-fire flash for foreground flash metering.

3. take the picture using the shutter/aperture/iso gained in step 1.

if foregound is a little blown out, then just reduce FEC.

???

[In2Photos]

is it reasonable to assume then, that wherever possible, get a mter readng fr the background, and if your chosen aperture/shutter/iso are going to be fine, then all you next is switch to M mode, enable flash and copmose the shot using the setting you got earlier...
so...
1. meter the scene as if flash won't be used (so that the ambient lght is recorded)...

2. go to M mode, switch on flash and focus/pre-fire flash for foreground flash metering.

3. take the picture using the shutter/aperture/iso gained in step 1.

if foregound is a little blown out, then just reduce FEC.

???
Yes, that would work. That would essentially be fill flash and to me it is easier to accomplish this in Av or Tv rather than M mode. I set my aperture and ISO, let the camera dictate shutter speed and based on the pre-flash the camera sets the flash exposure.

[Hangerhead]

In2Photos,

cheers - the reason why i was going more for manual mode is that it will FORCE me each time to ensure shutter speed is fast enough ( i mean, make me evaluate each shot as i feel i am still learning, long way round for me until it finally sinks in)... although if time is short/shots are passing me by, then i'd switch to AV mode and set to f8 for sharpness or f4 (all i have...) for DOF.