Question about Av mode?

Tonight I went to shoot a Bikini Contest at a local Hooters, and I wanted the fastest shutter speed possible, so I set the camera to 4.0 since I was using my 17-40L. But I was getting some motion blur, even with the use of my 550EX. So I tried P mode, and all of a sudden my shutter speed jumped from 1/16 to 1/60. Why would there be a difference especially since it is still at 4.0?

It sounds like Av mode exposes for the ambient light and uses the flash for fill to lighten the shadows. That is generally what you want in daylight photo situations.

P mode apparently sets the shutter speed for the flash synch. speed and uses the flash as the main light source. You will probably notice that subjects in the background are darker, but at a bikini contest who cares.

You should read your manual for details of how flash exposure is determined in both modes.

When you are in AV mode, your shutter speed will still act like there's no flash present hence the low shutter speeds, but when you switch to Program mode, the camera automatically bumps the shutter speed up to 1/60 when the flash is on.

http://photonotes.org/​articles/eos-flash/#faq7


BTW, the whole article is a must read for EOS Flash.

Remember that with the Canon flashes, if you set your camera to Tv mode you can adjust the shutter speed anywhere up to the maximum shutter-sync speed for your camera (250 for 20D) and the flash will sync. So you can get better stop-action shots.

You can even set the shutter speed faster and switch the flash into High-speed Sync mode but you lose flash range.

If you got any good pictures give us a link!

Does P mode change the ISO on a Rebel?

Auto mode changes ISO, in P you select it.

EX flash units work in program and manual mode. If you set your manual mode to 1/60 - or 1/90 /or 1/125 of a sec and set ypou F stop at F4 or F5.6 the Camera and flash will adjust the exposure for the proper exposure needed.

For some reason in TV mode you select the shutter speed and it will default to F4 and blink improper exposure.

In Av mode you set the F stop F4 and it will set a slow shutter speed it does not recognise the flash.

But if you use manula it will give you more latatude but you will use up batteries faster.

Almost exactly correct . In P mode, the camera tries to meter for the ambient light, but the program doesn't let the shutter speed drop below 1/60. If you go into a well lit area, the shutter speed will rise to match the ambient light. In Av, Tv, and M, there is no such artificial limit and as a result if you are in a dark room and don't use a tripod, you will get some blur from things that are moving and reflecting ambient light as well as camera movement. P mode is set up this way so that most people can get some ambient light recorded in their image with a shutter speed fast enough to prevent camera shake with most short to normal lenses.

In addition When evaluating the resulting image from a flash picture, you need to evaluate two things:
1. The flash exposure of the subject in the foreground. This is controlled by the flash and the cameras flash metering. In ETTL, use FEC to get what you want. If you are bouncing flash off the ceiling, make sure that the total distance to the ceiling and from the ceiling to the subject doesn't exceed the max distance for the flash. Max distance at ISO 100 = Guide Number (55M for 550EX)/f stop. Multiply by 1.4 for ISO 200 and by 2 for ISO 400. Diffusers like a StoFen Omnibounce will reduce flash disatnce.
2. The ambient/background exposure is controlled by the shutter speed and aperture. If you want to capture the ambient light in a dim room you need to get out of P or Auto Mode and use a tripod. P and Auto only allow the shutter to get to 1/60 and no slower. That will result in a dark background in low ambient light.
The flash picture is a combination of 1 and 2.

Scott

Thank you all for your help, I think I understand it now. And I have read that long article on canon flashes, but i think I need to reread it a few more times. So when this particular Hooters has their next show in dec. I will be better prepared. I did get some decent shots, but i have some work to do on them still and will post a few for you all. I just have to work on the noise removal (used ISO 400), I am trying the free version of noise ninja, so I will see how it goes.

Noise on the 300D/10D should not be much of an issue at ISO 400. That suggests some underexposure. If you get you exposure closer to the right of the histogram, most of it should disappear.
Scott