another flash question. - DPChallenge Forums

Canon EF 24-70mm f/2.8L USM

02/27/2008 07:28:29 PM · #1

Still trying to get a grip on using flashes...I guess I don't have a brain for this!!!

My Q: When using a flash for my canon 20D ( either a 580EX or a qflash) if I understand correctly, if I shoot in EttL mode the flash will automatically read the camera's shutter speed, and aperture and make the necessary adjustment on the amount of flash to fire to have proper exposure? Maybe someone can clarrify b/c I keep reading that this always doesn't work, and that it's better to shoot the flash manually.

I just ask b/c i wnt to start shooting some portraits of family and friends...maybe even weddings one day, but I am just trying to learn the basics before I buy a specific flash unit/units. At this point in my learning curve it seems easier to shoot in ettl and let the camera do all the work. But it seems like most pros do everything manually which seems very confusing to me.. maybe it's not as hard as I think it is?

Any thoughts would be much appreciated.

dwterry

Canon EOS-1D Mark III

02/27/2008 07:56:49 PM · #2

Here's the sequence of events:

1) Half-press shutter, camera reads ambient light exposure

2) Full-press shutter, flash fires a "pre-flash", camera reads "thru the lens" (thus TTL) the light bouncing off the subject

3) And a split second later, the shutter opens and the flash fires a second time, this time with an amount of light calculated to properly expose the subject in your image.

In Av or Tv modes, the camera is going to set the exposure to what was determined in step #1. So in low light, in Av mode, you're going to have a very slow shutter in order to let the proper amount of ambient light in. This *may* not be sufficient light to avoid blurring your picture. So keep that in mind if you try to shoot Av mode in low light.

In manual mode, ambient light doesn't matter to the camera. You've told the camera "you're in charge" and you've set the shutter and aperture manually. The job of the camera+flash, then, is it properly expose the subject based on the measurements in step #2.

Canon EF 17-40mm f/4.0L USM

02/27/2008 08:10:30 PM · #3

ok that makes sense.

Now if I am in Av mode and it's low light what mode should I change to?

Also, lets say I am in manual mode...how do i know what shutter speed and aperture to set? Also, I read about needing to make sure I am the right distance so the flash can reach, and i also read about having to change the power of the flash...but it sounds like in ETTL i dont need to worry about that, the flash should calculate the proper amont for the right exposure?

Tanks again.

Nullix

Canon EOS-5D Mark III

02/27/2008 08:18:30 PM · #4

Originally posted by brimac:

Now if I am in Av mode and it's low light what mode should I change to?

If it's low light and you want to freeze the action with hand held shots, I would shoot in either P or M mode. If you need to shoot in Av mode, use a tripod.

Originally posted by brimac:

Also, lets say I am in manual mode...how do i know what shutter speed and aperture to set? Also, I read about needing to make sure I am the right distance so the flash can reach, and i also read about having to change the power of the flash...but it sounds like in ETTL i dont need to worry about that, the flash should calculate the proper amont for the right exposure?

In manual mode(M), you set these values and the flash will blast away with enough light to get a good exposure. The settings all depend. If you're hand-holding, you want some hand holdable speeds (1/focal distance or 1/100 to be safe). With the aperature, you set to what ever you need to get a good exposure (check the exposure meter and guess-ta-mate).

For starters, try 1/100 and a wide open aperature and check the meter that your not over exposing the shoot. If you're getting pictures that look like deers in the headlights, try lowering the speed and/or upping the ISO.

Hope this helps. I'm trying to get a flash tutorial, but I'm trying to get time to shoot the pictures needed.

Canon EF 24-70mm f/2.8L USM

02/27/2008 08:23:49 PM · #5

thanks Nullix... i get that too, but one question. you said that "If you're getting pictures that look like deers in the headlights, try lowering the speed and/or upping the ISO." maybe this is where i have trouble/ confused...wouldnt the flash fire just enough to aviod this? I guess I get confused b/c I just assume that the flash will make the correct exposure. Does this makes sense????

dwterry

Canon EOS-1D Mark III

02/27/2008 08:36:48 PM · #6

I think what he means by "deer in the headlights" look is that ... the more the flash becomes your primary or even your 'sole' source of light, the harsher the light will become and the darker the background will be.

For example, let's say you turn off all the lights and blast away. The subject might look okay, but the subject will probably be surrounded by darkness.

If you turn the lights on ... but your shutter is too fast or aperture too small to let in ambient light, you essentially have the same situation as the dark room. Right? But if you slow down the shutter, open up the aperture or increase the ISO, then you'll let in more ambient light and the subject won't appear to have been blasted by your flash.

With all of that said ... here's my preference for when I'm shooting candids at a reception: ISO 1600, Shutter = 1/125, Aperture f/2.8 to f/3.5 depending on what I'm shooting. For me and my shooting style, that combination generally gives me "some" ambient light, while letting the flash isolate and expose the subject properly. Of course, different ambient lighting conditions may require different settings... your mileage may vary. :-)

02/27/2008 08:42:12 PM · #7

OK that makes sense...so you are saying that the flash is still firing for the proper exposure for the settings, but that the aperture, or shutter speed need to be tweeked to give the the right look that i might be looking for. ( in this case some lighter background, but still allowing the subject to pop). Right?