Please help me understand flashes

[pearce_jj]

Hi All

All help greatly appreciated!

I'm struggling with my flash. Canon 5D (classic) and a £30 Jessops canon-mount TTL flash, Av mode.

Problem - camera seems to select really long shutters in dark rooms even when the flash is on.

The images come out correctly exposed, but to my mind surely the shutter only needs to be open just as the flash fires?

Many thanks
James

 

[CT]

Your camera is is actually doing the best it can for you and it will try to balance ambient room light with the flash output if you use it in AV Mode. In AV Mode the camera 'assumes' you really want that longer shutter speed and adjusts it's output accordingly.

If you use the camera in 'P' mode, then when ambient light levels drop below safe hand holding speeds, the camera will default to at least the minimum flash sync speed., but will make no exposure allowance for ambient light - it will use the flash as the whole source of illumination in much the same way as a point and shoot compact will. This will result in well exposed subjects, but in a large room, with the room tailing off into darkness behind the subjects, although with your shoe mounted flash you may be able to bounce the flash for a better light spread.

AV is the better mode to use indoors if you want the flash exposure balanced with the ambient room light, but you have to be aware of the shutter speed selected which might be quite slow, so subject movement can become an issue and possiblly even camera movement without a tripod, in which case you'll get a sharp image from the flash exposure, but ghostly secondary images from the ambient light after the flash has died away.

I dunno if that's any clearer?

 

[pearce_jj]

Many thanks for this CT.

So if I were to select manual (say f/2 1/80th) then the camera would adjust the flash output depending on ambient light as required?

 

[CT]

Well - the technique is to measure the ambient light first. Let's say it's a room - not very bright, and you're getting 1/25th. of a second - aperture wide open. This will give you a properly exposed shot for the room using room light alone, but it wont be very punchy, and any people in the shot are likely to have their faces in shadow or have those black eye socket shadows from the overhead lights.

If you now turn on the flash it will still sync with your 1/25th shutter speed, and used in TTL Mode, it will fire the flash just sufficiently to light your foreground subjects and probably do a decent job without any interference from you, but if anything it may be a bit too strong, and dialling down the power by a stop or two usually gives a more pleasant result.

If you wanted to use Manual Mode then as long as you metered for the ambient light you could use those settings just as easily.

You can learn a lot about how flash behaves, just by practising and taking some shots of family etc in room settings and that's the best way to go really -lots of practice rather than scratching your head on the big day.

 

[pearce_jj]

Thanks CT, much appreciated.

 

[CT]

I have to say - if in doubt use P Mode - it will do a good job of fill flash used out of doors - lots of pros use it for outdoor wedding shots where they just want fill flash, it doesn't demote your standing. Even when you move indoors it will do a decent job except that it defaults to flash sync speed if light levels are low and act as the main or only source of illumination, which isn't 'by the book' the best way to do it, but if they're imprtant shots, at least you get a result.