Freezing motion

i get that if you are using a flash, its the flash that freezes the motion as it is so fat (not the shutter speed) IF flash is the main source of light

what if your flash is not the main source of light and its a 50/50 mix of ambient and flash (or just a bit of fill flash)

do you still have to work on fast shutter speeds? or can you still use the flash exposure to freeze the subject?

You need atleast 2:nd curtain for that you get a sort of blur up to the point where the flash fires and sharpens the image.

It's most obvious on Cars where their lights look like glowing stripes in the air while the car titself might be perfectly sharp at the end of that due to flash.

Flash and Shutter speed both have an impact. With a really slow shutter speed you get blur with the flash light stopping the motion at the beginning or end of the exposure depending on how you have sync timed.

If you want to stop the motion with balanced light you have to rely on a fast enough shutter speed. That can be problematic since there is s maximum sync speed that is generally relatively slow for motion stopping.

High Speed Sync is available but it severely reduces the speedlite output which means the speedlite has to be quite close to the subject.

im talking about taking pics of people

a) in a dark club, where flash is the only light source,

b) taking people in a room with ambient light where flash is not the only source

im just trying to work out if you have to use a slightly faster shutter speed with b as the ambient light might show some movement and not look sharp, compared to a

In a word yes.
When balancing ambient with flash, you need to have a shutter speed that will keep camera movement at a minimum. As well as good shooting technique. I try to keep the shutter minimally at 1 times focal length. If you are using a crop camera then it should probably been even faster.

thank you, makes sense now

dynamitetony wrote in post #14359417
b) taking people in a room with ambient light where flash is not the only source

im just trying to work out if you have to use a slightly faster shutter speed with b as the ambient light might show some movement and not look sharp, compared to a

If you are talking about a case where the ambient is too strong to overpower but not strong enough to make a nice fast exposure, yes. But your flash power gives you some control over how much of the ambient is allowed. If you have a ton of flash power, you can overpower the ambient. Usually you don't want to go THAT far but the flash lets you get a lot faster. Say you'd be shooting at 1/60 to get an all-ambient exposure, and that's just not enough to be sharp in your situation. Indoors, flash would easily let you get to 1/125 or 1/200 without looking too 'flash-y' which is probably enough shutter speed to keep the ambient from showing motion blur. So you still have ambient, but you're controlling it and therefore controlling its unwanted side effects.

In cases where you have a lot of ambient, things are different because you're not using the flash to get enough light, you're using it to get light where you want it (like fill flash on someone under a tree with a bright sunlit background). This is where the outdoor portrait people are using ND filters, fast shutter speeds and HSS.

I took this shot and swept camera during the shot. I made sure the camera was moving when I released the shutter. The t shirt as about 5 feet away and the xmas tree about 10. The flash was more dominant than the ambient at the point within the t shirt area but then fall of by the point of the tree made the ambient more dominant.