Hi,
Question ive wondered about for a while... when using a speedlight is it best to use the auto shutter speed or a pre-set speed?
In custom function Ive got it set to 1/250sec fixed speed when using a flash in AV mode.. however there are two other options, Auto and 1/250-1/60. When using Auto inside I get insane shutter speeds like 10 seconds.
What circumstances are best for each setting and which should I be using?!
Also, no doubt pretty simple question but, when using a speedlight for example on the fixed 1/250sec setting does it adjust the power of the flash so it exposes correctly?
Thanks.
The shutter speed normally doesn't make any difference to flash exposure, provided it is long enough for the shutter to be fully open when the flash fires. The highest shutter speed when this is possible is known as the max x-sync which is 1/250sec on your camera, although any longer shutter speed is also okay.
You will understand why this is if you google up on how a focal plane shutter works. Basically the flash is extremely brief and it is varied to adjust the power output and exposure. The flash duration then becomes your effective shutter speed and it can be anything between about 1/1000sec and 1/40,000sec or so. (The only exception to this is High Speed Sync which is a special feature of some guns. Your Speedlite probably has it - separate topic really.)
If the only light you want on your subject is the flash, then it's best to use the highest shutter speed you can, ie 1/250sec. However, flash illumination falls away very quickly over distance and this will often give a very dark background, but usually there is also some ambient light about and it is generally desirable to try and get some of this into the exposure to lighten the background. This is what the camera is trying to do with Av and P modes.
What it's trying to do here is balance the ambient exposure and the flash, by using a longer shutter speed to let more light in. Obviously if it's dark, the shutter speed can get very long, causing bluring etc. You can shorten that by using a lower f/number or raising the ISO but there comes a point when you run out of options and it may be better to just let the background go dark. This is what's happening in P mode when the camera will let the shutter speed drop to 1/60sec to try and balance the ambient, but then it will stop.
Av on the other hand will track the ambient light for as long as it takes, possibly many seconds if it's really dark. On the other hand, there is also a custom function on Av which will lock the shutter speed at 1/250sec, as you have discovered.
Between these various auto options, and of course full manual, you have a lot of choices. Use whichever one suits your method of working, or the subject. Most people go for Av (with the ambient tracking enabled, which is standard default) and use +/- compensation on the camera to alter the brightness of the background, and +/- compensation on the gun to alter the brightness of the foreground subject with the flash. Between the two you have a lot of control, but then there are also full manual and auto/manual options. Hardly anybody uses
all these options as they are pretty much just different ways of doing the same thing, but give them a try to see what suits.
