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Author Topic: Questions from a beginner  (Read 2088 times)

b.e.wilson

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Questions from a beginner
« on: October 16, 2002, 06:27:27 pm »

[font color=\'#000000\']1. The term 'circular' in circular polarizer referes to the polarizartion of the light that leaves the filter. It's made of two elements: a linear polarizer, followed by what is called a 'quarter-wave plate', a piece of film that recirculizes the light. The term 'circular' doesn't mean in circles, but 'unpolarized'. The filter needs to rotate because the polarizer must be aligned (or anti-aligned) with the incident polarized light.

2. The ND grad filter works because it is placed some distance in front of the lens. The incident light from above will still pass through the dark upper half of the ND grad, and the light from below passes through the clear half of the filter. In fact, if you are shooting a lens with a small lens opening you may need to move the filter away from the lens to gain the needed separation. Using a soft-edge filter on a small digital camera lens can result in very little grad effect until the filter is moved far enough from the lens so that the incident light will take different paths through the filter. If you aren't sure, get in front of the lens and look through the grad toward the aperture. Move your head up and down to see just what incident angles are passing thru which part of the grad. What you see of the aperture is what the aperture sees of you (or the scene behind your eyes).

3. I meter the scene through the properly-oriented polarizer first, then put the filter on the lens in the same orientation. Polairzers block between 1.5 and 2.5 stops of light, depending on conditions, and sometimes it's enough to know how many stops it's blocking by meteirng bare, then with the filter, and calculating the difference and applying that difference to the exposure.

4. Here's how I use an NG grad: I meter the sky and the ground to determine the exposure difference and select a grad that will compensate. Then I mount the grad, and position it, and observe if I am getting the effect I want. If not, use a softer- or harder-edged filter. Or if I am at the extreme I carry, I move my grad to a slot in the holder farther away from the lens. Most of the time I find that isn't necessary.  I expose the shot at the exposure I got from the ground. By the way, I use a Cokin holder, and I carry two of them: one  as is and the other with the outer two slots cut off (for use with wide-angle lenses).[/font]
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pedz

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Questions from a beginner
« Reply #1 on: October 16, 2002, 05:03:25 pm »

[font color=\'#000000\']Some of these questions may shock you...

1) Why does a circular polarizer rotate?  I'm not 100% sure what a circular polarizer is but I assume the grating is concentric circles around the center of the filter (lens).  So, why rotate it?  I have let to see any difference.

2) How (or why) does an ND filter work?  Here is my confusion.  Assume the ND filter half way covers the lens with the top being the dark half and the bottom the clear half.  Light (lets say three seperate beams of light) from a point that is in focus on the subject travels to the camera and hits the lens at the top, center, and bottom of the lens.  The three beams go through the lens and reconverge at the same point on the film plane.  One of these beams has been filtered (the top one), one has not (the bottom one), and one half way has (the center one).  So why is it that the beams of light that come from the top half of the frame get filtered more than the beams of light that come from the bottom half of the frame.  At the point they hit the lens (or filter) they are not still seperated.

3a) On a less theoretical side: if I'm using a spot meter (or incident meter) how do I compensate for a polarizer filter?

3b) Is the same question: If I'm using a spot meter but in this case I'm using an ND filter that half way covers the lens?  If it is a 3 stop filter, will the sky be down 3 stops?  It seems to change based upon the apature of the lens.  This is really why question 2 bothers me so much.[/font]
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Kevin

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Questions from a beginner
« Reply #2 on: October 26, 2002, 02:25:31 am »

[font color=\'#000000\']To add to Bruce's comments ...

If you are using TTL metering, just meter the same with a polarizer as you do without one.  The result will just be an increase in exposure time (or a larger aperture) to compensate for the loss of light (1-2 stops).

Using an ND grad can be complicated.  If, for example, you want the sky to be rendered as a medium tone, you can point your spot meter to the sky and release the shutter.  If you put an ND grad over the lens and do the same, the sky is still the same tone.  What will result is a brighter foreground.

Bruce's methods work just fine.  Point your meter at the highlighted area (e.g., sky or sunlit mountain), and then at the shaded foreground.  If the difference is, say 3 stops, then use a 3 stop ND grad IF you want both areas rendered at the same tone.  Chances are you want the highlighted area to be a little brighter, so perhaps a 2 stop ND grad might be more appropriate.  In this case, however, base your exposure on your reading of the foreground; the ND grad will then hold back the highlighted area(s).

As for placement of the ND grad with particular apertures, a more effective way than to control its distance from the lens is to use your depth-of-field preview button.  Hold it down while you move the filter up and down to position it where you want it.  This can be difficult with a small aperture, especially with a 2 stop ND grad.  Just give your eyes a few seconds to adjust.

In short, it isn't very difficult if you have 1) TTL and 2) depth-of-field preview.[/font]
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