Variable ND filter & long exposures problem

[gurtch]

I just tried using a variable ND filter to get blurred waves on the beach. The vignetting was because the filter is thick, and I was using a 24-70 lens at 24mm. But that is not my problem. The shorter exposures, such as 1/13 second, posted here are OK. But the long exposures such as 15 full seconds, also posted here, have broad vertical stripes. can anyone explain what is going on? Are they selectively "polarized"? I used a tripod, mirror lock up, and eyepiece blind. Camera was a D800E with Tamron 24-70 f2.8 VC lens with VC set to "OFF"
Thanks in advance. This technique is new to me; more difficult than I thought it was going to be. BTW to filter is a "cheapie".
Thanks in advance
Dave in NJ

 

[BlinkingEye]

Did you have the filter cranked close to its maximum? It is my experience that variable ND filters should not be use beyond about 70 percent, especially inexpensive models. It is my experience that a variable ND filter is not an item to skim on cost. I rarely use a variable ND anymore for still photos much preferring to stack filter to get the "darkness" I desire. But I will use an ND filter for video.

IMHO the behavior you show in the photos is a variant of the "dreaded X" exhibited by cranking an inexpensive ND filter far toward the maximum.

 

[gurtch]

Did you have the filter cranked close to its maximum? It is my experience that variable ND filters should not be use beyond about 70 percent, especially inexpensive models. It is my experience that a variable ND filter is not an item to skim on cost. I rarely use a variable ND anymore for still photos much preferring to stack filter to get the "darkness" I desire. But I will use an ND filter for video.

IMHO the behavior you show in the photos is a variant of the "dreaded X" exhibited by cranking an inexpensive ND filter far toward the maximum.

Thanks....yes, the variable ND filter was set at maximum for the stripes phenomena to occur, and yes it is an inexpensive filter. But what is happening? Curious to know.
Dave

 

[segedi]

The way I understand those filters to work, they combine two polarizing filters together. The variable effect works by combining the polarizing effects in different degrees. The cheaper ones tend to not be neutral and give off color casts. And might not have even polarization, especially in wide angle lenses. But there's most likely a better expert out there with better info.

 

[Jack]

Segedi hit it on the nose -- it is an undesirable effect of twin polarizers giving off the uneven mirrored sine-wave pattern.

Your best bet for digital heavy ND is an ND-IR cut. After about 3 stops, the non IR versions pollute the frame with excess IR which most modern digital cams read as a funky yellow-brown color cast. On our workshops, we advise a pair of ND-IR filters, a 3-stop and a 6-stop -- you can still see well enough through a 6 to compose, and can add the 3 after composition if you need longer exposures for more blur. The 3 is a good density for blurring moving water in earlier dawn or later dusk light.

 

[gurtch]

Thanks all for your explanation
Dave

 

[thedruid]

Another tip is not to use Auto WB, I've found that a good starting point was to set my temp at around 4000K and go from there. Also cover your viewfinder to stop stray light which you'll see as a form of ghosting on your image.

 

[Shashin]

The dreaded black X. Dave, it is caused by an interference pattern generated by the polarizer gratings being slightly displaced and the angle of the light going through them. The wider the lens, the worse it is. The expensive ones do this as well.

Hoya has come out with ND filters with a metallic coating--I think called ProND, but they have a number of other products with close namings, so double check the specs. I have a 6-stop one and it is very good in regards to color cast--this is with AutoWB. They are supposed to have the same color across the range of ND.

And with water:

Both with the Pentax 645D.

BTW, I have other ND filters. The B+W filters give me the worst color cast. I have a Hoya NX400 9-stop filter that is better than the B+W, but still can have a strong color cast. Foliage usually gives me the biggest problem as it is very reflective in IR. Tiffen has announced a series of ND filters with increasing amounts of IR cutoff.

Also, I like the 6-stop filter as I can still focus and compose under most lighting. 9-stop allows me to do that in strong daylight. 10-stop is just bulletproof and I need to take it off to do that.

 

[gurtch]

The dreaded black X. Dave, it is caused by an interference pattern generated by the polarizer gratings being slightly displaced and the angle of the light going through them. The wider the lens, the worse it is. The expensive ones do this as well.

Hoya has come out with ND filters with a metallic coating--I think called ProND, but they have a number of other products with close namings, so double check the specs. I have a 6-stop one and it is very good in regards to color cast--this is with AutoWB. They are supposed to have the same color across the range of ND.

And with water:

Both with the Pentax 645D.

BTW, I have other ND filters. The B+W filters give me the worst color cast. I have a Hoya NX400 9-stop filter that is better than the B+W, but still can have a strong color cast. Foliage usually gives me the biggest problem as it is very reflective in IR. Tiffen has announced a series of ND filters with increasing amounts of IR cutoff.

Also, I like the 6-stop filter as I can still focus and compose under most lighting. 9-stop allows me to do that in strong daylight. 10-stop is just bulletproof and I need to take it off to do that.

Yours are exactly the effect with water I was trying to achieve. About how long was the exposure times to get that look? I have a 6 stop ND filter I can try, but the exposures in bright sun may be too short.
Thanks again. Great shots
Dave

 

[Shashin]

Dave, thanks.

To get the smooth or luminescent water starts around 5 seconds--it is not a hard number as it depends on the motion of the water, it can be a bit shorter or longer. I am shooting those images at f/16 at ISO100 with exposures or 15–20 sec., but I am not under direct sun. Under bright sunlight, you will need stronger ND. I have found that 9–10 stops are needed for that.

 

[gurtch]

Thanks Will