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Speedlites and Umbrellas (Related to my Minolta Meter Post)

Finding out my light meter was incorrect was an add-on to to me trying to figure out the best zoom setting on a Speedlite when using a reflective umbrella.

My thought was that normally you would try to illuminate as much of the umbrella as possible without a lot of splillage.

But, along the lines of zooming my flash to 105 mm when I needed the most pop out of bouncing, I thought it would work the same way - the tighter the zoom into the umbrella, the more light output you would get. It just wouldn't be spread as much.

I'm not getting anywhere with this. It could have something to do with the meter registering too low. But I wrote readings down, and it really didn't change much as I went from 24 mm to 105 mm.

Am I putting too much thought into this? Any one have any comments?

Thanks - Dave

Comments

  • rs_eosrs_eos Member
    edited July 2016
    When using light modifiers, the zoom setting is typically set to evenly distribute the light.  But, you can also zoom in if hot-spots are desired.  When the light from the flash isn't direct (bounces or passes through one or more surfaces), the exposure between zoom settings will be very close if not identical.   The light is really bouncing around in that case and you effectively end up with the same amount of light hitting your target.  With a bare flash, it's easier to picture the cone that is formed.  So when focal length is at say 50mm and your flash is zoomed to 20mm, much of the light is being wasted.  If zoomed at 200mm, unless you want a spot-light effect, that's not good enough to illuminate the entire image.

    Umbrellas have a very wide angle of output and so you'll definitely lose light, but the amount lost at different zoom levels will effectively be the same (since the the resulting angle of output remains the same after the light bounces around a bit). 

    I just concluded an experiment with a Canon 600EX-RT speedlite.  I used the light bare, then with three different umbrella modifiers.  A Westcott 43-inch umbrella with cover was used in a bouce configuration.  The same without the cover was used in a shoot-through configuration.  Finally, a Westcott 43-inch Halo was used (which is a shoot-through configuration, but the light first bounces off a reflective backing.

    For each setup, I used two zoom settings: 20mm and 200mm.   My lightstand was in a fixed location at 45-degrees and 6 feet away from my target.  I used a Canon EOS 6D with 50mm f/1.2L lens.  All shots were manual-everything.  Common settings were ISO 100, 1/100s, f/2.8 and Kelvin set to 5500.  Manual focus used to set focus once.

    For the bare flash, there was a 1.33 EV difference in exposure between the two zoom settings.  Shadows were identical.   I had power set lower here as compared to using the umbrellas (down about 3.67 stops)

    For all other cases, the EV difference between the zoom settings were either zero or just one-tenth of an EV.   Shadows were softer when using a zoom of 20mm which makes sense since I had less of a hot-spot.   For all umbrella shots, I had the flash at 1/4 power.  The Halo images are not as bright since the Halo makes the light deal with two surfaces.  The shoot-through and bouce just involve a single surface.

    In summary, the zoom settings when using a light modifier will not really change exposure much at all.  What they will change though are subtle other aspects such as more/less specular highlights, harder/softer shadows, etc.


  • dbrunodbruno Member
    Hey, rs - nice work, and it is what I was finding. So what I will probably do going forward is set my zoom to 24 mm, and adjust the separation between flash and umbrella to minimize spillage to "use it all". I am now beginning to question if zooming to 105 mm when bouncing off a surface behind me to get more reach is really doing anything. I'll have to check that out when I can set up something reasonable.

    Dave
  • rs_eosrs_eos Member
    edited July 2016
    The reach of a speedlight really only comes into play when the light will be direct.  See the guide number table which should be published in your user manual.  Once the light bounces or passes through a surface, the light source becomes much larger and ultimately changes the math.   Changes it such that the difference in zoom settings no longer has as a dramatic difference on exposure.
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