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Photography Question 

When purposely blurring, do i use a lower f.stop?

If I were to take a photo of a waterfall with the water thats falling blurred to make it smooth, wouldn't that let too much light in as well?
If so should I give it less light in addition to a longer exposure?...

Do I make anysense?
Well I understand this all come with experimenting, and trial and error, so I will. But I was just wondering if someone could please explain this to me.
I am a new student of Photography in college and I have a real interest in it. I don't know how far i'll go but I want to find out.

Anyway, thanx for any input.

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2/26/2003 11:07:24 AM

Jon Close
BetterPhoto Member Since: 5/18/2000
  Yes, if you choose a longer shutter speed then you compensate with a smaller aperture so that the exposure remains the same. For example, each of the following combinations gives the same exposure (same amount of light on film) but with increasing motion blur due to slower shutter speed, and increasing depth of field due to smaller aperture:

f/2 & 1/1000
f/2.8 & 1/500
f/4 & 1/250
f/5.6 & 1/125
f/8 & 1/60
f/11 & 1/30
f/16 & 1/15
f/22 & 1/8

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2/26/2003 11:38:37 AM

John A. Lind
BetterPhoto Member Since: 9/27/2001
  Jon has explained well how you can trade shutter speed settins and aperture f-stops to maintain the same exposure.

Regarding waterfalls in particular . . .
To get the "bridal veil" effect you mention requires a very long exposure; on the order of several seconds. I recommend experimenting with times in the range of 2 to 8 seconds. Outdoors the primary problem is too much light, even in deep shade (not always possible and may not be desirable) using ISO 50 film such as Fuji's Velvia, which many shoot at EI 40 (sadly, Kodachrome 25 and Ektar 25 are discontinued), and stopped down to f/16.

The secret weapon for lengthening shutter speed to several seconds or more during broad daylight is called a neutral density (ND) filter. This is a neutral gray filter that will not affect color balance. All it does is reduce the amount of light entering the camera. They come in various values, or strengths, that allow varying how much the light is reduced.

To get exposures of 2 to 8 seconds during the day requires losing 4 to 10 stops of light, depending on what film speed you're using and whether the scene is in bright, unobscured daylight, overcast, open shade, etc. If you're serious about doing these typse of photographs routinely you will need several of them in different strengths. I don't recommend stacking them for a couple of optical reasons . . . although it can be done with some risk of flare and vignetting.

At these shutter speeds, it should be obvious you will also need a very sturdy (rigid) tripod and a cable release. I also recommend doing them on a very still, calm day when there isn't any wind to flutter leaves or other foliage. Even a slight breeze can move portions of plants significantly.

You will also have to pay attention to the reciprocity failure characteristics of whatever film you choose. The primary effect of reciprocity failure with very long shutter speeds is the need to make the exposure even longer than calculated or metered for the rated film speed. Although it varies for each specific film, it's something to consult the film data sheet about whenever making an exposure longer than a second. The data sheet will have a table in it with recommended exposure compensation factors.

-- John

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2/26/2003 5:38:53 PM

Vanessa     Wow, john this is just a little bit above my head, ha ha! But I think I got what your saying about everything being just right in order for it to work well. I think I should have mentioned im still working with only Black and White film so I think the gray card you metnioned would help me out.

Jopn C., very clear. Thanks.

Both of you have given me a load of information I needed.

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2/27/2003 9:06:51 AM

John A. Lind
BetterPhoto Member Since: 9/27/2001
I did dump out a lot of information. Let me summarize it a little and then I'll walk through an example of how to actually shoot it.

1. Technique:
In order to get the blurred water you asked about, shutter speed needs to be over one second. Usually the desired speed is in the 2 to 4 second range. This results in two technical problems that must be dealt with.

2. Too much light during the day!
As Jon showed in his example, you can trade shutter speed for lens aperture and get the same exposure. This is called the "reciprocity" principle. The problem during the day, even with the slowest of films available, is inability to get a proper exposure with a shutter speed long enough for the moving water to blur sufficiently. This requires cutting down the amount of light reaching the lens (and film) using an ND filter to about 1/10th to 1/100th of that normally encountered during the day. How much depends on sky conditions (direct sun vs. overcast vs. shade or deep shade) and the film speed you are using.

3. Reciprocity Failure:
The recprocity principle works with shutter speeds between about 1 second and 1/10,000th second with most films. With exposures longer than this, the reciprocity principle may no longer apply. It's as if the rated film speed starts to get slower. This requires compensation by increasing the exposure time beyond that which would normally be used for its rated film speed. The shutter speed at which this occurs and the compensation required is different for each film. Some are 1 second, some at 1/10th second, and some at 10 seconds or longer. The data sheet for the film has information about the shutter speed at which reciprocity failure starts to occur and how much to compensate for it. Agfa, Fuji, Ilford and Kodak all have data sheets for their films available on their web sites. See their product support areas for them.

If you haven't looked at a data sheet before, it provides technical details about the film. This is one on Kodak's site for Plus-X Pan, a B&W film I've been using for a very long time:
The second part of the "exposure" section shows when reciprocity failure occurs and the required compensation. A very old film, it runs into reciprocity failure at shutter speeds longer than 1/10th second. Don't fret about the discontinuation notice, it's been replaced by a "new" Plus-X and the only thing that changed was developing time . . . slightly.

Example (using Plus-X Pan):
In direct sunlight with a clear sky, and an aperture of f/16 with ISO 125 film, you would normally use a shutter speed of 1/125th second. Knowing about a 2 to 4 second exposure is required, look at the compensation required and work it backwards. For 2 seconds after compensation, the calculated exposure (at ISO 125) should be about 1 second. This means the light must be reduced by at least 100X, or 7 f-stops. An ND 3.0 reduces light by 10 stops. If the lens is opened up by 3 f-stops to f/5.6, this gets to the 7 stop reduction needed (you could also open up to f/8 and use an exposure of about 5 seconds). Put camera on tripod, set shutter speed on "B", attach cable release, compose the photograph and focus the lens. Then put the ND filter on (it's very hard to see through one). Trip shutter using cable release and count the exposure; 2 seconds if aperture is set at f/5.6 or 5 seconds if set at f/8. You can use the second hand on a watch or, as I do at night with long exposures, count it out. With exposures this long, if you're 1/4 second off, it doesn't matter. If the sky is overcast or the subject is in shade, you wouldn't need to reduce the light as much and a weaker ND filter would be used.

-- John

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2/28/2003 4:26:32 AM

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