Usage Question on a Neutral Density Filter

[Cory]

<p class="MsoNormal"]Ok, let me give an example of what I would be wanting to do,
explain how my camera settings and a filter would accomplish that&hellip; and see if I
know what I&rsquo;m talking about. This isn&rsquo;t the exact scenario I ran into, but it
draws a more clear picture (heh, no pun intended).<o></o>
<p class="MsoNormal"]If want to take a picture of, say, a baseball player up to
bat and I want to have the batter in focus, the background blurred, and catch
the &lsquo;motion&rsquo; of the bat swing (i.e. the bat itself would be blurred, hopefully
without blurring the batter much), then I&rsquo;d want to set a fast aperture (say,
2.8) but have the shutter remain open long enough to catch the blur. Now, if
the shutter stays open too long I overexpose, so I want to darken things&hellip; this
is where a darker filter comes in (Neutral Density 1.8? 6 stop equivalent). It
darkens the target shot, allowing me to use a 2.8 f/stop, thereby blurring the
background, yet keeps the lens open long enough to catch some blur without
overexposing. (I know ISO will probably play a part too, but I think it isn&rsquo;t
the main factor and I would want to keep it low for quality purposes
anyhow?).<o></o>
<p class="MsoNormal"]<o></o>So, do I have all that correct?<o></o>
<p class="MsoNormal"]<o></o>Main reason for asking is I now have (what I believe will be)
my &lsquo;walk-around&rsquo; set of lenses being (all Canon&rsquo;s) an EF-S 10-22, EF 24-105 4.0
L IS USM, and EF 70-200 2.8 L IS USM, all use 77mm filters. I&rsquo;ve got (all
B+W&rsquo;s) a Slim Polarizer for the 10-22, and also have a good
(Kaeseman/MRC/X-Pro/etc) Polarizer, UV/Haze filter, and 8-Star effect filter.
I&rsquo;m looking at picking up one of the neutral density filters to complete the
basic &lsquo;set&rsquo; of filters I might find useful with all these.

[wickerprints]

You may not require an ND filter. But you will probably want to use manual flash + second curtain sync, or else the batter will be blurred.


My thinking is that a batter swinging the bat is an event that happens so quickly that it is unlikely that you will need a really slow shutter speed. I don't know how much blur you need, but the key here is to experiment. My suggestion is to try doing some sample shots; get a model (someone with a bit of experience swinging a bat) and go to a local park to test out the appropriate settings.


Another thing is that you may not need f/2.8 unless for some reason you are shooting with short focal lengths or there is not much separation between subject and background. For instance, if I'm at 200mm near MFD, I could shoot f/9 and objects 20 feet away will be very blurred. DOF is not solely a function of f-number. But in case you do need f/2.8, I would again test it out. You may end up needing the ND, but it's impossible to say how much additional density you need until you get a sense of the amount of light and amount of blur desired. Too much and you will sacrifice image quality (due to having to compensate by increasing ISO); too little and you cannot get the desired effect.

[George Slusher]

Cory:


You shouldn't need an ND filter to do this. Just choose your ISO low enough that you can have a slower shutter speed with a wider aperture.


Here's a way that you can figure out if a ND filter will help, using just what you have. Your circular polarizers will lose at least 1 stop, probably more. That would be equivalent to an ND2 or ND 0.9x. Stack them (the slim filter will have to go on top) and you'll get roughly what an ND4 would do. (If you're observant, you may notice that the color changes when you rotate one polarizer relative to the other. The density may also change a bit.) If you want an even more dramatic test, put the Kaeseman CPL on the 24-105mm or 70-200mm on your camera. Mount the camera on a tripod and set up your shot as wickerprints suggested. Take the slim CPL and flip it over, so that the threads point AWAY from the lens. Now, hold the CPL in front of the lens + CPL. You vary the density by rotating the CPL in your hand. You can't do this by stacking them the usual way, as you can with linear polarizers. CPLs have 2 layers--a linear polarizer and a "quarter-wave plate." The quarter-wave plate essentially "unpolarizes" the light so that auto-focus SLRs will work properly.


Stacked in the usual way would put the layers:

1. Linear polarizer
2. quarter-wave plate
3. Linear polarizer
4. quarter-wave plate

Thus, the light coming to the second CPL is essentially "unpolarized." If you reverse the top filter, you'll get:

1. quarter-wave plate
2. Linear polarizer
3. Linear polarizer
4. quarter-wave plate

When you rotate one of the polarizers relative to the other, you can go almost all the way to opaque.


If you decide to get one or more ND filters, the ND4 or ND 0.6x, which reduces the light by 2 stops, is usually the one to get. As I said, a CPL can act like an ND filter (with some effect from polarization, of course). Hoya makes very nice ND filters, particularly their Hoya Multicoated (HMC) versions, which are considerably cheaper ($57) than the B+W MRC filters ($103), which, IMO, are greatly overrated. If you want a slim filter, you can get the B+W slim for $113 or the Hoya Pro1 Digital ($122), which is probably a better filter, overall. (If you check on eBay or with , you can get the Kenko Pro 1 filter, which is made by the same company as Hoya, but not sold by US dealers.) However, except for CPLs, the 10-22mm lens doesn't need slim filters. A "regular" filter's ring is about the width of the slim CPL filters. You could also get a cheap set of ND filters (ND2, ND4, ND8) on eBay just for experimental purposes, then get the one or two you decide will work best.

[Cory]

Now there's some food for thought. Heh.


Thanks guys, I'll have to play around with things and see. I hadn't even thought of stacking my polarizers to get, basically, the same effect.

[George Slusher]

Your circular polarizers will lose at least 1 stop, probably more. That would be equivalent to an ND2 or ND 0.9x.

[:$] (Insert sound of me dope-slapping myself!)


That should be "ND 0.3x." ND 0.9x would be an ND 8 (3 stops). At least, later, I got ND 8 = ND 0.6x correct.