Camera Exposure Behavior with Flash in M Mode?

[WayneHixson]

Assuming I am shooting in Aperture priority with Evaluative Metering on 5D Mk III, and have a 580EX in M mode. Does the camera assume there is no flash and set a shutter speed based on ambient?

Thanks!

Wayne

[Skirball · Accepted solution]

Yes, camera set the shutter speed to expose the background, then fills in the subject with the flash.

For sake of discussion, any reason why you're using Av mode with flash?  I recommend trying out Manual, I think you'll find you have a bit more consistency over results.  Issues like ghosting and blurry photos can result if your camera has to drag the shutter too much to get exposure. 

[TCampbell · Accepted solution]

The camera basically ignores the flash when considering the exposure.  The only thing that it really does consider is that the shutter speed needs to be at or below the maximum "flash sync" speed (1/200th on your camera) unless the flash is in high-speed-sync mode.

There are numerous situations where you have to treat the lighting as a "flash + ambient" exposure.   The camera sets the exposure based on the "ambient" exposure.  In E-TTL, the camera and flash work together to calcuate the amount of power the flash needs to use to create a good "flash" exposure, but that part of the exposure calculation mostly ignores ambient light.  When the camera finally does open the shutter to take the exposure and fires the flash, the flash will fire at the pre-calculated level of power necessary for a "flash" exposure, and the camera will use settings necessary to get a good "ambient" exposure (at the same time.).  The only nuance is that the camera will keep the shutter speed at or below 1/200th (unless high-speed-sync is enabled on the flash.)  The notion of continuing to leave the shutter open long enough to collect ambient light (which you'd do because the flash has a "light fall off" behavior (lookup the "inverse square law")) is called "dragging the shutter".

Syl Arena has a book called the "Speedliter's Handbook" which you may enjoy.  While many of it's topics could be applied to general flash photography, Syl is a "Canonista" -- he pretty much shoots with Canon gear.  This book is fairly specific into the function of Canon bodies and Canon speedlites, so it's an excellent book considering that also happens to be your gear too.

[hsbn · Accepted solution]

Similar to Av mode, you'll set your desired Aperture, then set the shutter speed so that the metering indicator is at the middle then you'll have your Av mode in M mode.

There is also an easier and more accurate way: set your desire Aperture, set your Shutter speed according to the in-camera meter, take a photo with flash off. Take a look at the background, model may be dark at this time but don't worry about it. Increase/Decrease your shutter speed to brighten or darken the background until you like it. Now turn on the flash, retake the photo again, and everything should be good. You can decrease or increase the flash power to make it look more natural.

The good thing about manual mode is consistency. As long as the light is not changing, you don't have to worry about it for the entire shoot regardless of subject matter. With Av, if you move from lighter background to darker background even in the same light, you'll have to compensate your exposure to get the same result. With M mode, you only have to worry about the flash and less worry about the ambient.

[Skirball · Accepted solution]

Yes, camera set the shutter speed to expose the background, then fills in the subject with the flash.

For sake of discussion, any reason why you're using Av mode with flash?  I recommend trying out Manual, I think you'll find you have a bit more consistency over results.  Issues like ghosting and blurry photos can result if your camera has to drag the shutter too much to get exposure. 

[WayneHixson]

Thanks. I was using Av to control depth of field and let the camera help with the exposure. I just wasn't sure how the camera would do that with the flash in M. If camera is in M also, I'm not quite sure how to approach setting exposure. Can you give me a scenario, say for a portrait-type shot in which you want to darken the background?

Thanks again!

[hsbn · Accepted solution]

Similar to Av mode, you'll set your desired Aperture, then set the shutter speed so that the metering indicator is at the middle then you'll have your Av mode in M mode.

There is also an easier and more accurate way: set your desire Aperture, set your Shutter speed according to the in-camera meter, take a photo with flash off. Take a look at the background, model may be dark at this time but don't worry about it. Increase/Decrease your shutter speed to brighten or darken the background until you like it. Now turn on the flash, retake the photo again, and everything should be good. You can decrease or increase the flash power to make it look more natural.

The good thing about manual mode is consistency. As long as the light is not changing, you don't have to worry about it for the entire shoot regardless of subject matter. With Av, if you move from lighter background to darker background even in the same light, you'll have to compensate your exposure to get the same result. With M mode, you only have to worry about the flash and less worry about the ambient.

[Skirball · Accepted solution]

---

@WayneHixson wrote:

Thanks. I was using Av to control depth of field and let the camera help with the exposure. I just wasn't sure how the camera would do that with the flash in M. If camera is in M also, I'm not quite sure how to approach setting exposure. Can you give me a scenario, say for a portrait-type shot in which you want to darken the background?

Thanks again!

---

Me personally?  I almost always want to darken the background anytime, except shooting people outdoors in bright sun where I want the picture to scream: SUNNY!  Darkening the background (it doesn’t have to be dark, just darker than the subject) is a great way to make your subject stand out.  It’s all relative.  Another way of saying it, I want my subject brighter than the background.

Shooting with an eTTL flash in a dim (but not dark) setting is probably the best way to learn to use Manual mode.  All you really worry about is how the background is exposed, and the camera takes care of exposing the subject through eTTL.  Start by getting the exposure in the ballpark.  If you don’t have a rough idea of where the settings should be then take a shot in an auto or semi-auto mode and note where the camera chooses.  I set my ISO and aperture (based on DoF I want) and then the shutter speed basically sets itself (within reason of hand-held speeds and max sync speed).    I like to keep my background around a stop below “normal exposure”, but if it’s a dark/moody/intimate look  I’m going for I’ll drop the background to 2-3 stops below ambient.  Take a test shot (no flash).  The background should just look like an underexposed shot.   Now put the flash on, in eTTL.  Adjust FEC as needed.  Voila, normally exposed subjects with darker background. 

Make sense?

[hsbn]

Check out this site: http://neilvn.com/tangents/flash-photography-techniques/
It's really great site to learn about flash. He's a great guide. His tutorials are easy to understand and most of the time you don't have to buy more stuffs. 🙂

[TCampbell]

WayneHixson asks:  "Can you give me a scenario, say for a portrait-type shot in which you want to darken the background?"

YES!  Watch this video by Mark Wallace (Adorama TV):  http://www.youtube.com/watch?v=nk9cTa3UthM

It's a long video, but it is worth your investment in time to watch it all the way through.

Understanding the inverse square law is one of the most important concepts to understand when doing flash photography (that, and understanding what "diffuse" light means.)

[ebiggs1]

Agreed. Av would not be my first choice, unless there is some other extenuation circumstance.

BTW, 5D Mk III and 580 EX 

[TCampbell · Accepted solution]

The camera basically ignores the flash when considering the exposure.  The only thing that it really does consider is that the shutter speed needs to be at or below the maximum "flash sync" speed (1/200th on your camera) unless the flash is in high-speed-sync mode.

There are numerous situations where you have to treat the lighting as a "flash + ambient" exposure.   The camera sets the exposure based on the "ambient" exposure.  In E-TTL, the camera and flash work together to calcuate the amount of power the flash needs to use to create a good "flash" exposure, but that part of the exposure calculation mostly ignores ambient light.  When the camera finally does open the shutter to take the exposure and fires the flash, the flash will fire at the pre-calculated level of power necessary for a "flash" exposure, and the camera will use settings necessary to get a good "ambient" exposure (at the same time.).  The only nuance is that the camera will keep the shutter speed at or below 1/200th (unless high-speed-sync is enabled on the flash.)  The notion of continuing to leave the shutter open long enough to collect ambient light (which you'd do because the flash has a "light fall off" behavior (lookup the "inverse square law")) is called "dragging the shutter".

Syl Arena has a book called the "Speedliter's Handbook" which you may enjoy.  While many of it's topics could be applied to general flash photography, Syl is a "Canonista" -- he pretty much shoots with Canon gear.  This book is fairly specific into the function of Canon bodies and Canon speedlites, so it's an excellent book considering that also happens to be your gear too.