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Tag Archive: f-stop

  1. When and Why to use Variable ND Filters

    We created a test of the 77mm Promaster Variable ND filter on a Zeiss 50mm Planar T* f/1.4, to help demonstrate the impact the filter can have on the image, as well as test for any color shifting, fringing, or “x-darkening” common with poorer quality filters.  We’ve tried some of the other common variable ND filters, including budget filters and Polaroid, but now we rent exclusively Promaster, due to its far superior performance.  This is also a good example of how adjusting f-stop can impact the depth of focus.

    f13-no-nd

    Exposure with no filter.

    f-7-min-nd

    Exposure with Promaster Variable ND set to MIN, approximately 1 2/3 stop reduction in light.

    f-1-8-med-nd

    Exposure with Promaster Variable ND set nearer MAX, approximately 5 2/3 stops of light reduction.

    The shots were all done with the same settings, 125 ISO, 1/50 shutter.

    The only drawback of the Promaster variable ND is endemic to all variable NDs and polarizers, as well.  It tends to cut reflections, even the good ones.  While this makes for a more vibrant sky, it can also make flesh tones more flat, and organic surfaces like the skin of the apple (or a person) lose luster under the higher ND levels.  For staged shots where we have time to change filters, we’ll still use classic ND, but with more shoots getting smaller, we think the variable ND has earned a place in our camera bag.  After using Promaster variable ND filters in the real world for almost a year, we’re happy to include them in our rental catalog.

    Posted by Jon Kline

  2. Stops, F-stops, and Lens Speed – Exposure for Video

    This is part one in our “Exposure for Video” series

    When shooters talk about speed, they’re really talking about light.  Light is measured in stops.  If lens A lets in twice as much light as lens B, lens A is one stop faster than lens B.  If it lets in four times as much light, it’s two stops faster.  So, if we “stop up” or “open up” we are adding more light and if we “stop down” or “close down”  we are taking light away (relative to where we started).  All lenses have a maximum aperture (also called iris size or f-stop), but can be stopped down to reduce the amount of light coming in.

    Photo: flickr/sodaniechea

    Photo: flickr/sodaniechea
    In some images, you can actually see the iris in the photograph. Here, you can tell the iris has six sides, making hexagon-shaped lens flares.

    Lens speeds are measured in f-stops.  Don’t confuse a stop with an f-stop.  A stop is half or double a certain amount of light.  An f-stop is the measurement of the diameter of the iris in the lens.  They are related, but not the same.  I’m going to explain this with some math, please bear with me.  If you can commit it to memory, you’ll be halfway to being a professional shooter.  If it bends your brain too much, just skip to the chart and try to memorize the pattern.

    The F-stop is a ratio that measures the iris diameter relative to the focal length f.  That’s why it’s represented as a fraction, f/x.

    The cool thing about exposure is that as long as f is a constant, it doesn’t matter what it is. We can ignore it for this example, just remember that the number we’re focusing on is the denominator in a fraction, so a bigger denominator is actually a smaller number.

    If I double the diameter of a circle, the area of a circle will become four times as large (remember that πr² equation from geometry class?).  So if I double the diameter of the iris by going from f/8 to f/4, the iris gets four times bigger, and will let in four times as much light.  Four times the light… that’s two stops (because 4=2×2).  Eight times the light?  That would be three stops brighter.  One sixteenth the light?  Four stops darker.

    Now, as promised, the chart of f-stops:

     

    <--Opening up            Stopping down -->
     .      .      .      .      .      .      .
    f/1           f/2           f/4           f/8
          f/1.4         f/2.8         f/5.6
    
    

    What does it mean?  It means that there is a one-stop difference between f/1 and f/1.4.  There is a three-stop difference between f/1.4 and f/4.  And you can extend the imaginary chart out as far as your mind wants to go by just doubling or halving the f-stop ratios.  If you want numbers in-between…. you’d better bust out your geometry textbook (or use an f-stop calculator).  To be practical, we’ve never seen an f-stop beyond f/0.7 on the open side or f/32 on the closed side, and optics tend to get a little crazy as you approach extremes, anyway.

    Remember, the iris (aperture/f-stop/whatever you might call it), is just one of the four parts of exposure.  You will need to combine f-stop, shutter speed, sensitivity, and the amount of light in the scene to get the full equation.  The good news?  F-stops are the tricky part!  The rest gets much simpler.  And now you have one of the tools to compare two lenses side-by-side.  We’ll have more information on exposure in our next installment in the Exposure for Video series.

    Posted by Jon Kline


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