JPB the elder wrote in post #17897747
I am still learning my camera . I have tried doing longer exposures on waterfalls and cascades with disappointing results on the longer ones . Anything longer than 1/50 or so results in blown out whites . Is there a ratio of ISO to shutter speed that I need to experiment with ?
Exposure is exposure, be it long or short, doesn't matter. You can get the same exposure both ways. You first need to figure out exposure and how to control that before worrying about extending it for a blur effect. Asking for a ratio of ISO to shutter is a big red flag. The relationship is the exposure triangle, and there is no ratio, there is each property and what it effects in the exposure triangle and how you use it to get exposure as well as other effects you desire.
Waterfalls and water in general are best when not being hit by high-noon sunlight. You want over-cast, or under canopy hidden water that is not ultra-bright and reflecting. You won't even need an ND filter likely for those, and can simply use aperture to stop down ambient light, base ISO, and open up the shutter to get your exposure with some blur. I find water blurs very nicely while retaining action and motion at 0.5s to 0.3s shutter speeds, an of course, up to a full second. That's if you want to retain some form and motion. If you want it to be a hazy blur, just go longer.
CPL's can be very helpful with water to calm down reflections.
ND filters can be helpful to stop down ambient light when aperture on it's own is not enough.
Here's a quick and dirty method to exposure and long exposure using your camera in manual and using the meter. I use an old camera without live view or other fancy things. Just the basic in-camera reflective light meter.
1. Camera in manual mode. You control all.
2. ISO is 100, leave it there.
3. Aperture is set to both ambient light and depth of field.
4. Set metering mode to evaluative.
5. Compose your image. Meter with the meter and alter shutter speed until the meter is where you want it (ie, 0 or -1/+1 around zero based on what you want).
6. That's your base exposure. Take a shot. Look at the histogram, make sure you don't have blown highlights and that you have enough detail in the shadow areas. Alter exposure based on that.
7. You now have your final exposure values. You can get a long exposure with the same exposure, by applying a ND filter of any strength. The ND filter will drop overall exposure by however many stops you use. You then regain the exposure by getting light back from opening the shutter longer, for every one stop you lose to the ND filter, you should gain back a stop in long exposure from the shutter. So if you use a 10 stop ND filter, you will want to make up for +10 stops of light with the shutter, and that's where long exposure comes from.
8. An easy way to know what shutter speed you need to compensate for the strong ND filter dropping ambient light is the following example:
If your metered exposure values are: 1/50s, F11, ISO 100 and you want to go long exposure for the blur effect and you have a 10 stop filter, you would simply divide 1/50, which is 0.02 seconds, multiplied by 1000 (which is why a 10 stop filter says 1000x), so 0.02 x 1000 = 20 seconds. So your shutter goes from 1/50s, to 20 seconds, when you use that 10 stop ND filter, to get the same exposure value, but with a blur effect. That example applies to anything. If your shutter was 1/100s, that is 1/100 = 0.01s, 0.01s x 1000 = 10 seconds. Shutter of 1/13s, 1/13 = 0.076s, 0.076 x 1000 = 76.9 seconds. You get the idea. Simple way to do it. Each ND filter will have a value like that, that you use to figure out exposure in the field.
A 3 stop filter is 8x (this is the multiplier of light reduction), and 0.9 density.
A 4 stop filter is 16x (this is the multiplier of light reduction), and 1.2 density.
A 6 stop filter is 64x (this is the multiplier of light reduction), and 1.8 density
A 10 stop filter is 1000x (this is the multiplier of light reduction), and 3.0 density.
That will help you to know what the numbers mean when you're looking at them. The light reduction value is more useful though, as it directly helps you calculate exposure time from the shutter speed you meter originally prior to filter use.
Let's say you're in canopy cover, or over cast skies, and you are shooting a waterfall. You set ISO to 100, you set aperture to F8, F11 or F16 based on depth of field and ambient light needs. You meter with evaluative and change shutter speed until you center the meter, or are at +1/-1 stops around the center of the meter. You meter, say, 1/60s. You want to blur the water a little bit, but not turn it into mist/glass, so you pick out a 3 stop ND filter. To know the new shutter speed with the ND filter applied (3 stop is 8x multiplier), you take your 1/60s shutter and divide it, so 1/60 = 0.016 seconds. 0.016s x 8 (the multiplier) = 0.133 seconds. 1/0.133 = 7.5 (let's round that to 8), so your new shutter is 1/8th of a second. Or you could just move your meter +3 stops from 0 (1/60 -> 1/30 -> 1/15 -> 1/8) since that's really easy to see. Multiplying is an easier method with higher power ND filters, like 10 stop filters, where counting out stops can be more of a chore.
I would suggest a 3 stop filter for water in motion, under foliage cover, or over cast skies, like water falls or waves.
I would suggest a 6 stop filter for water that you want a little more mirror-like blur, or haze, or in brighter light.
I would suggest 10 stop filters for turning water into glass or mist, even in broad day light.