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Tag Archive: exposure for video

  1. ISOs, sensitivity, noise, and grain

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

    When shooting video, the third piece of the exposure puzzle is ISO, also called sensitivity, ASA, or gain.   All the other ways to adjust exposure are physical changes, they change the number of photons involved in your exposure.  In a DSLR or video camera, the ISO adjustment is electronic.  Adjusting your ISO will change the quality of your image in a few critical ways.

    Much like shutter speed, doubling or halving your ISO is a one-stop change in your exposure.  So going from ISO 50 to ISO 100 is one stop brighter, and going from ISO 400 to ISO 100 is two stops darker.

    Digital cameras use one or more CMOS or CCD sensors to collect light.  Each one is made up of thousands of tiny light-sensitive cells in an array, working together to make the image.   These cells are called pixels. They convert the photons of light into electrons, much like a teeny-tiny solar cell.

    When recording a video, each of these cells “dumps” its collection of electrons to the computer once each frame.  When you’re in a dark situation, there are only a few electrons to dump, and the margin of error goes up.  These inaccuracies show up as noise, and (since we perceive light logarithmically – we’ll save this for another blog post) they are most visible in the darker portions of our image.  Noise can be distracting and make an image look cheap, especially in moving pictures. Most of the time, we want the least amount of noise possible.

    The more gain you use (the higher your ISO), the more noise you will introduce into your image.  Using less gain will reduce noise.  Noise is blocky and unnatural-looking, and most noticeable in the dark areas.  Compare the green areas of the photo below to see varying levels of noise in the 100 and 1600 ISO exposures.

    iso-example

    Click the image to enlarge

    The amount of gain you use will also affect the latitude of the exposure.  Latitude is the camera’s ability to record detail in the extreme highlights and shadows of an image.  Most digital cameras will gain a half a stop or more of latitude as you increase your ISO.  If you reduce your ISO, you’ll reduce your latitude and the image will appear more contrasty.  Most of the time, we want to maximize our latitude, since we can always increase contrast in post, but we can’t reduce it.  Look at the details in the center of the flower in the image above.  You can see the texture in darker areas much more clearly in the 1600 ISO exposure.

    So, we want the lowest ISO possible to reduce noise and the highest ISO possible to increase latitude?  Not exactly.  For digital cameras, the manufacturer determines a particular ISO that gives superior noise performance and acceptable latitude performance.  This is called the native ISO, and is dependent on a lot of factors, including pixel size, heat handling, in-camera processing, and aesthetic preferences.  Most DSLRs are rated around 100 or 160 native ISO.  Other cameras vary, but nearly every camera available in 2014 has a native ISO between 100 and 800.

    Most photographers and cinematographers find a few settings on a camera that they like, and stick with them when they are in certain situations.  On a camera like the Canon 5D mark III, I usually find myself somewhere between ISO 160 and 640, depending on circumstances.  I suggest experimenting and finding what looks you like with your camera.  Eventually, you’ll learn what to expect as you adjust your ISO and the rest of your exposure settings.

    Posted by Jon Kline

  2. Shutter Speeds and Frame Rates – Exposure for Video

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

    Shutter speed is one of the variables that helps control exposure and the overall look of your video.  If you’re an amateur photographer, you might be familiar with shutter speed, but there are some key differences between photo and video.  Frame rate also factors in for moving images.  Let’s start with frame rates first.

    The video frame rate is the number of images (frames) shown per second.  We abbreviate this FPS.  This can be anything from around 12 (the minimum to “fool” the eye into seeing motion) to more than 1000.  But film and video have converged around a few standards.  Here are the ones you should try to remember:

    Frame Rate    Most Common Use
    15 Old web videos
    23.976 Digital cinema
    24 Classic cinema
    25 PAL (foreign) TV
    29.97 Almost all SD TV in the US, lots of HD TV
    30 SD TV in the US before color
    48 Peter Jackson’s movies
    50 Some HD TV outside the US
    59.94 Some HD TV in the US

     

    I’m sure whole books could be written about why some of those frame rates seem so ridiculous, but the difference between 30 fps and 29.97 fps is actually really important, I promise.

    If you’re making movies or web videos in 2013, I strongly suggest shooting in 23.976 fps.  If what you do ends up on television, you’re probably better off with 29.97 or 59.94.  But let’s get back to shutter speed, which is the whole reason we started talking frame rates in the first place!

    motion-blur-shutter-speed

    Photo: Flickr/wwworks
    Longer shutter speeds make moving things streakier.

    The shutter speed is the amount of time the frame of video is exposed to light.  The longer the exposure, the more light is recorded.  Of course, under normal circumstances, the shutter speed can’t be any longer than the time between one frame and the next.  So if you’re shooting 24 fps, the most light you could let into each frame would be 1/24th of a second.  Makes sense, right? Your shutter speed can be a lot faster, however.  most cameras will let you adjust your shutter to 1/8000 or so.  This would let in a lot less light.

    Photographers have used fast shutters in millions of photos to freeze motion.  Since even fast-moving things appear relatively stationary over 1/8000th of a second, they can be “locked in place” without any visible motion blur.  Other times, photographers can use very long exposures to intentionally blur backgrounds or leave trails to indicate motion in a still shot.  But cinematographers have a different challenge.  Since video and moving pictures are all about capturing motion, each of the still images has to work together with the ones around it to create the illusion of motion.  The motion blur that photographers are often trying to avoid is the same blur that we can use to make motion seem fluid and natural.  The illusion of motion in moving pictures depends in part on the smooth transition from one frame to the next.  In modern video, we usually set a shutter speed at about half the frame rate.  In an homage to the film cameras of the 20th century, this is called a 180-degree shutter.

    There are times when the 180-degree shutter might not be the right choice.  Sometimes you may be shooting something that flickers, such as a televison or LED clock, and don’t want to emphasize the flicker. Other times you may be shooting in such darkness that you’d rather have the extra motion blur and get all the light in every frame possible.  There is no shutter speed that is right 100% of the time, but I usually start at 1/50th (when I’m shooting at 23.976fps) and make adjustments as necessary from there.

    There are artful ways to use faster shutter speeds, too.  In Saving Private Ryan, for example, the action scenes are shot with a 1/200th shutter, even though they are 24fps.  This tends to make the motion look “strobey” or stuttery, and reminiscent of old 16mm newsreels.

    Remember that shutter speed is just one of the four variables that affect exposure.  We’ll talk about the rest as we get further into our “Exposure for Video” series.

    Posted by Jon Kline

  3. 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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