Just one ND filter?

[Yno]

I am going to be in Yosemite and want to play with different styles of waterfall photos. In order to get a long enough exposure to make the water smooth out I need an ND filter. I don't think I will use one enough to get the Singh-Ray variable, but was thinking of a B+W multicoated 0.6 (2 stops). Would I be better off with a 0.9, or could I stack a CPL on it to get 4 stops?

[argyle]

You'll need to get down to a shutter speed of at least a 1/2-second to 1-second (if not longer) to get the effect you're after. Guessing which ND filter is best is kind of a crapshoot since a lot depends on the lighting at the time. If you already have a good polarizer, you'll have about 1.5 to 2-stops of ND already, so buying a 2-stop ND filter won't accomplish much considering the shutter speed that you'll need to get the silky effect.

If you set the camera's ISO to 100, and figure in the Sunny 16 rule (1/100 or 1/125 shutter speed at f/16), 6-stops of total reduction will get you down to a 1/2-second shutter speed. On a bright day, I'd estimate that you'd need about 8-stops (+/-) of total reduction to get to the 1-second or slightly longer shutter speed...this means that you'd need to combine a 6-stop ND filter with your CPL. You could always stop down the aperture an additional stop to f/22 in order to reduce the amount of ND filtration, but diffraction will start to rear its ugly head and effect the overall IQ.

TBH, its generally very difficult to get by with a single ND filter because of the various lighting conditions that can be encountered. I'd recommend carrying two...a 4-stop and a 6-stop in addition to your CPL (which is pretty much a 'must' for waterfall photography).

[genzbenz]

I have 4 NDs. I have the 1 stop, 2 stop, 3 stop, and a 10 stop. Before I had my DSLR, I had a Canon SX10 IS that would only go to F8. In order to get decent waterfall pix, I used all three of my NDs at the time (1, 2, and 3 stops) in addition to the Circular Polarizer (got an extra stop out of it), so I got 7 stops extra from f/8. The pix were decent, but my T1i works much better. Of course, having 4 filters on the camera can reduce sharpness, and at it's widest, there was some vignetting on the SX10 IS.

I recently got the 10 stop ND and have only gotten to play with it once. It's REALLY dark. It's a lot easier to setup the shot, get your focus on (then turn the lens to manual so focus doesn't change when you hit the shutter) and then pop the filter on. This is the Hoya ND400. You have a lot of flexibility with long exposures with this filter! A few seconds if you'd like!

[Yno]

Looking over B+W's site and some of the resellers' sites, it looks like that means a 103 and a 106. The 103 is only available as a single coated filter, and the 106 as a non coated filter. Any ideas why, and if this will cause me problems? I use a hood, and generally try to position my camera out of direct sunlight.

[Jon]

Well, I suspect it's the very nature of ND filters that they don't need multicoating.

Every time light passes through the filter, it's attenuated by that filter's factor. In the case of the 103, 3 stops worth of light are being absorbed; in the 106, 6 stops are absorbed. Now if there's internal reflection (which is one of the things multicoating is supposed to prevent), that will be attenuated by 6 stops (in the case of the 103) or 12 stops (in the case of the 106), because the reflection from the inner filter surface will have to travel through the ND filter again, be reflected off the inside of the outer filter surface, and travel through the filter yet again before it can reach the sensor. So any possible filter-induced flare is being attenuated by 6 or more stops, and probably moves it out of the dynamic range of your sensor, or at least well down into the low end of its response.

Now the other big reason for multicoating filters is to reduce lght loss due to reflection and improve the transmissivity of the filter. Sort of counter-productive to try to improve light transmissivity in a filter that's designed to reduce light transmissivity across the board, isn't it?

[argyle]

Looking over B+W's site and some of the resellers' sites, it looks like that means a 103 and a 106. The 103 is only available as a single coated filter, and the 106 as a non coated filter. Any ideas why, and if this will cause me problems? I use a hood, and generally try to position my camera out of direct sunlight.

One last thing to consider, and it won't cost you any money :D . When using ND filters, especially on rather long exposures, its best to keep your eyepiece covered...this will prevent any stray light from entering and 'ghosting' the image. Your 40D also has live view to help aid in focusing if the filters cut out too much light. I just generally place my baseball hat over the camera to cover the eyepiece during long exposures.

[lazer-jock]

One last thing to consider, and it won't cost you any money :D . When using ND filters, especially on rather long exposures, its best to keep your eyepiece covered...this will prevent any stray light from entering and 'ghosting' the image. Your 40D also has live view to help aid in focusing if the filters cut out too much light. I just generally place my baseball hat over the camera to cover the eyepiece during long exposures.

That's what the little ignorable piece of rubber on my neck strap is supposed to be used for I assume. I knew that it was "the eyepiece cover", but I couldn't imagine why anyone would want to cover it. I think that I just had an epiphany!

[Jon]

It's essential if you're using a tripod and will be squeezing the shutter when your eye is not to the finder, as the metering is done in the pentaprism housing so stray light through the eyepiece can seriously mess up your reading. The mirror should form a light-tight seal over the focusing screen while the exposure is actually under way. If you look inside the mirror box you should see the gasketing material that forms the seal.

[Yno]

Somewhere I have two of those things, but they don't fit on my aftermarket straps. I don't know where they have hidden themselves. I have used a hat in the past, and I will put my Tilley to good use in Yosemite!