Long Exposures

[dmg]

If you are new to ND filters, be careful with the density specification.  The list above is for ND filters labeled by f/stops.  It is common for ND filters to be labeled by "optical density", which is the base-10 logarithm of the transmission.  So for example a filter with ND = 3.0 would attenuate by 10^3 = 1000 = 2^10 = 10 f/stops.  This filter would enable you to increase the exposure time by a factor of 1000, while a 3 f/stop filter would only give you a factor of 8.
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I think it depends on the manufacturer. I have in front of me a B+W, which is labeled as you explain (1000x, and ND 3.0).

But I also have a Hoya ND 400, which is 400x, or ND 2.6 using the B+W notation. I am also looking at a 3 stops hoya one that is labeled ND 8 (hoya notation).

dmg

[DigitalTasmania]

I'm starting to test a Vari-ND right now, the wide angle version without front threads.  The widest I've gone so far is 24mm on a 1.3 crop Canon 1D2 or about 31mm equivalent.  I'll test wider lenses next.

The only time I got artifacting (cross bar effect) was when I went beyond the maximum marked setting, and Singh-Ray warns about this.

Paul
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Paul et al.

Thinking about the Vari-ND for my 5D on 17-40, 24-105, and 70-200.

Any further comments you might have on the "thin" Vari-ND filter, it's "vignette-safe" focal lengths, and it's performance on Canon camera's would be very much appreciated.

I assume for the price that the build quality is top notch?

Many thanks from Tasmania,

Thomas

[EgillBjarki]

How would i go about calculating how much ND i need and what aperture to select when i want to make an exposure past 30 seconds, all the way to 30 minutes or more? is it up to mathematics at that point or is there a formula? which i guess goes back to mathematics but anyhoo..

Anyone know?

Thanks,
Mike
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This is how I do it...

Set my camera to its highest ISO and biggest apeture, then the cameras light meter show how long shutter time I should use.

Lets take an exemple :

1Ds Mark II on ISO 3200 f/2.8 with 5second shutter time.

f/2.8 ISO 1600 = 10sec
f/2.8 ISO 800 = 20sec
f/2.8 ISO 400 = 40sec
f/2.8 ISO 200 = 80sec
f/2.8 ISO 100 = 160sec
f/4 ISO 100 = 320sec
f/5.6 ISO 100 = 640sec
f/8 ISO 100 = 1280sec
f/11 ISO 100 = 2560sec
f/16 ISO 100 = 5120sec
f/22 ISO 100 = 10240sec
f/32 ISO 100 = 20480sec

Then I just devide resault into 60 to get minuts.

Might be many other better ways to do this, but this works well for me...

Here is a shot I made here in Denmark on a Phase One P30 digital back, 45sec exposure.