ND filters for outdoor people TTL shots

[herke]

Hi,
I'm wanting to get an ND filter so I can use TTL outdoors without resorting to High speed sync.
I will want to darken the background by 2 stops and let the TTL light the subject. I cant work out how many stops of an ND filter I will need.
My max shutter speed is 1/200 (canon 550d)

Does anyone else do this? how do you get on?
I was thinking a 4 stop, but that doesnt give me too much wiggle room as 2 stops will be for darkening the background. Can you stack the screw on filters ok? So maybe I could get the 4 stop, and if its not enough get a 2 stop and stack them.

I was looking at the hoya pro ones (77mm) which are around 60 quid on ebay. Has anyone used them?

Thanks
Neil

[mstphoto]

Have you tried underexposing the whole image by 2 stops and setting the flash to overexpose by 2 stops.
This in effect darkens the B/G but leaves the main subject properly exposed.

Mike

[herke]

Hi,
yeah thats what i'm trying to achieve, but within my max sync without relying on HSS. Its ok if its overcast/in shade but in direct sun the shutter speed it WAY over the max sync.

has anyone heard of light craft filters? they do a variable filter for about 100 quid. The UK Home of Light Craft Fader Neutral Density Faders
dont know what the quality is like though.