[liukaitc]

Hi, I just want to buy one ND filter that meet my need
I want be able to use flash in a sunny day with f2.8 lens. so shutter speed should be slower than 1/180.
What stop of ND filter do I need?

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Last edited by liukaitc; 05-08-2012 at 02:07 PM.

[drougge]

I assume you mean wide open. Well, assuming f/11 without filter, that's four stops from f/2.8. So four stops, aka ND16.

[liukaitc]

I assume you mean wide open. Well, assuming f/11 without filter, that's four stops from f/2.8. So four stops, aka ND16.

Thanks, is it also know as 1.2 ND??

[steve1307]

Yes 1.2 ND = 4 stop ND = ND16 same thing.

I think the "Sunny 16" rule says bright sunlight is f16, 1/125, iso100 so you'd need 5 stops from f16 to f2.8 at 1/125 , iso100.
(OK that would be 4-1/2 stops to f2.8 at 1/180, iso100)

[drougge]

Thanks, is it also know as 1.2 ND??

Wikipedia seems to think so. I don't think I've seen them sold with such names, but that doesn't mean much. (I'm not exactly famous for my memory.)

[abacus07]

3 stop filters (0.9 or ND8) are more common and should work within one stop. The best answer is to look at your photographs that you have taken without a filter and see what the exposure is. One of my recent sunny day pics was iso80, f2.8(DA40) and 1/1500. a 3 stop filter gives you 1/180s.

What exposures does a flash need to be used at? I thought it was only 1/180th of second or faster. What would happen if you used a flash at 1/1500s? Shouldn't it work pretty well?

The other issue you have is that your lenses need two different size filters. One solution is to buy a 67 to 77mm step up ring and use one filter. Or you can buy a cokin or lee square filter system that fits all sizes.

[drougge]

What exposures does a flash need to be used at? I thought it was only 1/180th of second or faster. What would happen if you used a flash at 1/1500s? Shouldn't it work pretty well?

Or slower actually. The flashes themselves are usually good for at least 1/500, often even shorter, but the shutter in the K-5 is not. Anything faster than 1/180 needs to strobe the flash, which gives much less light per picture area. (And also requires a flash with support for this, which liukaitc doesn't seem to have.)

[MSM]

Don't forget the ND filter will also eat 3 or 4 stops of light from your flash. It is hard to over power the sun. You need a faster sync which Pentax (or others for that matter) doesn't have. Or you need a more powerful strobe. If you use a single stobe, you will be at full manual power with long recycle times and quick battery depletion.

[twitch]

Actually flash+ND at sync speed is more efficient than a flash in HSS.