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03/04/2005 03:50:12 PM · #1 |
I want to take a shot of a busy place downtonw during the day using a long exposition to eliminate moving poeple and car. Even when setting iso 200 and f/22 the exposure time is too fast. Will using ND filter to cut down the light will help me here? I understand ND filters are graduated 2X, 4X, 8X. What type of filter would I need? Do I need to stack a couple of filters? I think I need a 30 seconds exposure; how much ND filter "power" do I need? And finally, do you think it will work?
Thanks, I know it's a lot of questions, but since I am an amateur, I don't want to run out and buy tons of filters not knowing the chance of success. |
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03/04/2005 04:00:28 PM · #2 |
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03/04/2005 04:00:46 PM · #3 |
Yup ND will help for sure. just have to figure out what number of stops you need to reduce. you didn't say what shutter speed you're using now. If you have a polarizer that is good for 1.5 to 2 stops. so if you're correct exposure at f22 and no filter is a shutter speed of 1/2 second then 6 stops (6x ND filter or polarizer + 4 stop ND) will bring you to 32 seconds at f22. If it's 1/8 second bare, then you'd need to go to 8 ND. Don't go beyond 8 ND, it causes weird things to happen with our CCD's or at least that's what I'm told.
Good luck!
Bill
Cloudspot.com |
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03/04/2005 04:01:42 PM · #4 |
An ND filter will multpily the amount of time it takes to expose.
Example, if without a filter, you get ISO 200, 1", F22; an 8x filter will give you 8" instead. If you want 30 seconds, I think you'd need to experiment with them.
Also, a long exposure will not render moving object invisible unless they are darker than the background. If a car goes by, any reflection from the sun will be caught, and you'll see streaks wherever those reflections were.
Message edited by author 2005-03-04 16:03:06. |
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03/04/2005 04:01:59 PM · #5 |
just roughly (someone may correct my math)
using sunny day rule f16 1/200 iso 200
1 stop so f22 1/100
1/50 1/30 1/15
1/8 1/4 1/2
1 2 4
8 16 30
and for each of these lines you would need a 8x (.9) or a 3stop ND
that is 4 stacked & that would be nasty
try evening or night time instead ...
Message edited by author 2005-03-04 16:02:25. |
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03/04/2005 04:03:03 PM · #6 |
Yes, they will work. Set your ISO as low as it will go, and set your exposure compensation (using aperture priority mode) as plus as it will go. Use the darkest ND filter you can find. Stacking them is ok, but there may be some degradation of image quality. The camera will be metering through the filters, so no calculation is needed.
If the camera meters it at, say, 2 seconds, and you use a +2 exposure compensation, that would give you 8 seconds (+1 = twice as long, +2 = 4x as long). Of course, these may be overexposed, but you can bracket. You ought to be able to handle them in PS, unless there are truly bright highlights in the image.
The way the filters work is, 2X reduces light by one stop (requires twice as long an exposure), 4X reduces light by two stops (requires four times the exposure) etc etc.
They can also be used in shutter priority (or full manual) to allow a wider aperture and less DOF.
Robt.
N.B.
One of the earlier posts is unclear on the difference between "4 stops" and "4X": a ND filter is calibrated by the increase in exposure it requires. "Stops" are a geometric progression: if nominal exposure is one second, one stop more exposure is 2 seconds, two stops more is 4 seconds, 3 stops more is 8 seconds, 4 stops more is 16 seconds, 5 stops more is 32 seconds, 6 stops more is 64 seconds. Each "stop" requires double the exposure of the previous stop.
Therefore, to have "6 stops" of ND filters would require 64X ND. a 6X ND filter is actually 2 1/2 stops.
Message edited by author 2005-03-04 16:12:47.
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03/04/2005 04:07:37 PM · #7 |
With ND 4x and above you can't count on your AF or meter to work properly. make test shots and check the histogram to see if you're getting good exposure.
Bill
Cloudspot.com |
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03/04/2005 04:18:50 PM · #8 |
Wow, so much information so quick, thanks everybody. Plexxoid, I was kind of worried about sun reflexion on moving object (or worse, stationnary ones). Do you think adding a polarizing filter would help with those reflexions?
Thanks |
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03/04/2005 07:01:34 PM · #9 |
Originally posted by fplouffe:
Do you think adding a polarizing filter would help with those reflexions?
Thanks |
CPL will help on, at least, some of the reflections. You can adjust it for the stationary ones, but the moving ones will be more difficult to predict. A CPL will also work as a ND filter. I would avoid stacking too many in daylight because of risk of flare. Use only HQ filters, multil-coated.
If part of the photo is brighter than the rest you might want to use a graduated ND filter. Google for Cokin (or Leaf etc)
Good luck
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03/04/2005 07:46:10 PM · #10 |
?
Message edited by author 2005-03-04 19:46:52. |
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03/04/2005 07:56:04 PM · #11 |
i've tried that actualy and the ND will work great for your propuses
the ppl. in my shot where very faint and gost like if not gone and car blured i think i didn't go long enough
GL
_brando_
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