Difference between P Flash and Av flash

[Steveo31]

Reading the 500EX manual, and it says that on Av mode, the cam will choose the correct sync speed to match the surroundings. Now, I thought that the camera/flash would do this automatically, regardless of the mode.

I shot a test roll, but haven't gotten it back. Could someone explain this?

[Ken Fong]

I'm not positive, but I think this is how it works...

The settings on your camera will override the flash settings. With the camera on P mode, I think you get a standard 1/60 shutter...good for snapshots. On Av mode, your flash will fire, then the shutter will stay open longer to capture the area behind your subject. I've never set Av or P mode on the flash itself.

[Steveo31]

Ok, that makes sense. I have the EOS 3, so the synch speed is 1/200, or the high speed sync goes to 1/8000. So given that on Av mode, the camera will choose a shutter speed, and the flash will just balance it out, right?

If so, got it. Thanks for your help :)

[Ken Fong]

Yep, in Av mode, the camera is doing what it is told when you select the aperture. It's best to practice this at home before going out...I usually get caught without a tripod when wanting to use flash in Av mode.

[stevelew]

I have shot in this mode. In P mode the shutter speed and aperture is automatically decided on by the camera. If you don't like the aperture you can change it by turning the wheel to a higher or lower aperture. Shutter speed usually stays about 1/30th or faster. In A mode, you pick the aperture and the camera figures out what shutter speed for the correct ambient light. In a dark place like a reception hall the shutter speed could be 1/4th of a second because the camera decides what speed will pick up the background.

I would rather shoot in M. Pick the shutter and the aperture and the flash usually compensates for the background. Works most of the time.

Steve

[Steveo31]

Hmm... just seems that the whole TTL thing is obsolete if the Flash is just going to over power the natural light. Would putting the flash at -1 be any better?

I shot a roll, but haven't been able to get it back. I'll post the results if anyone cares to see... :D

[Tom W]

In "P" mode using flash, the camera will pick the aperture and shutter speed, but will not go slower than 1/60 second. I'm not sure, but I think the maximum aperture using flash in "P" is f/4.0. The e-ttl will automatically expose the image from there. If the setting gets too bright, the camera will begin to stop down the aperture and/or speed up the shutter so that the ambient light doesn't overexpose. At some point of exposure level, the flash will begin to shift to auto fill-flash reduction mode (unless you have that feature turned off with a custom function).

When you shoot in Av mode, the camera and flash expose separately. The camera will expose for ambient light, based on the aperture you choose. If this requires 1/4 second, then that is what the camera will do. It will "pretend" that the flash doesn't exist. The flash, however, will operate in the normal way and will provide proper exposure for the main subject. This allows for obtaining a brighter background, but at the risk of blurred movement if the setting lacks light.

Tv mode works in a similar way, with the camera again setting exposure based on the shutter speed you select (if you go over 1/200, the camera will stay at 1/200). It will choose an aperture that will expose properly for the shutter you've chosen. The flash still exposes separately. This gives you the ability to use a higher shutter to freeze action (well, not real fast action, but moderately fast movements) while allowing the camera to expose for ambient lighting. Again, the flash provides the proper light for the subject.

In manual, you set the aperture and shutter as desired, independent of the camera's metering. This way, you can control ambient exposure, DOF, and can drag the shutter or stop action as needed. The flash meters by itself and fires the proper light to expose the subject.

Its actually pretty sophisticated, but its shortcoming is the lack of old-style photosensor auto flash. User overriding is a bit less than some would like. And, the results can sometimes be a little unpredictable, particularly with very bright or dark main subjects.

[maderito]

Tom - Good, condensed explanation.

Back to the original posted question.

. . ."the camera will choose the correct sync speed" should read...
The camera will choose the correct shutter speed" (to properly expose the background). The duration of the flash controls the exposure of the subject (assuming you're not in fill-flash mode).

[scottbergerphoto]

Good post Tom!
Scott

[Tom W]

Good post Tom!
Scott

well, thanks, both of you. Every now and then, I actually think before I hit the "submit" button. :)

[Steveo31]

Yeah.... good info there. Bookmark!

[Steveo31]

Ait, I got some back, and to tell you the truth, I can barely see a difference. I think I like the AV/M option just cause it's allows for my control. The P tends to stop at 1/200, regardless of if High speed sync is on, and goes higher only when the f/ maxes out. Grr.

Thanks for the info though. Very helpful!

[Tom W]

Here's some good reading for your spare time. Although its geared towards the digital side of things, it does a great job of explaining how the E-TTL system works:

http://photonotes.org/articles/eos-flash/

[Steveo31]

Here's what came back... just some test shots.

Black subject, great exposure IMO.
http://img78.photobucket.com/albums/v340/Steveo31/Public/psycho.jpg

It was pitch black outside. AF assist was right on.
http://img78.photobucket.com/albums/v340/Steveo31/Public/box3.jpg

Flash pointed straight up, so in this case it was pointed at a wall/window. Wide panel was up, so slight catchlight would happen.
http://img78.photobucket.com/albums/v340/Steveo31/Public/huuuuhh.jpg