When teaching new Speeliters how to dim the sun with High-Speed Sync (which I wrote about here in Part One), the question is often asked “why not just shoot at the normal sync speed with a really small aperture so that you can use regular flash?” My response is “you should go out and try.” [This article, by the way, also applies to all types of flash that won't work with your camera in High-Speed Sync: third-party speedlights, monolights and location/studio strobes.]
As a reminder, here’s the short version of sync speed: it is the fastest shutter speed at which you can nornally shoot flash. In a DSLR, the sync speed is limited to 1/200″ or so because the shutter is comprised of two curtains in front of the sensor—the first-curtain being fully closed and the second-curtain being fully open when the camera is ready to fire. In order for the flash to illuminate the entire sensor, it has to fire after the first-curtain has completely opened and before the second-curtain begins to close. That defines the sync speed. If you shoot faster than the sync speed, then the second-curtain will be covering some of the sensor and you’ll have a dark bar along one edge of the frame.
So, if you want to stay in normal flash mode, your shutter speed is limited to the sync speed. On my 5DM2, that’s 1/200″. Unless you are shooting a 1D-series camera, your sync speed is probably 1/200″ too.
If your shutter speed is capped by the sync speed, then you can only dim the ambient light via the aperture. O.K., so how dark will it be if I shoot at 1/200″ and f/22. Let’s head outside at high noon and see what it looks like.
On a warm spring day in California, with the sun straight overhead, my faithful Minolta Flashmeter IVF tells me that the ambient sunlight measures EV15. At ISO 100, EV15 equates to f/22 at 1/50″. That sounds good. If the metered shutter speed is 1/50″, then I have two additional stops (1/100″ and 1/200″) of shutter speed that I can use to dim the ambient. Along the way to shooting a series of demo shot, because I’m always curious, I started the series at one-stop brighter than the meter suggested. So the first frame below is ISO 100, f/22 at 1/25″. Then I increased the shutter speed in one-stop increments (-1EV) until I got to the sync speed for my camera.
There you have it—the photo above is the darkest I can render the ambient light when limited by my sync speed of 1/200“. Is it dark enough? That depends upon your vision. As I see it, the answer is “no, it’s not dark enough.” If I’m going to dim the sun, I want to make it darker than this. So I continued my one-stop changes in shutter speed: 1/400″, 1/800″, 1/1600″, 1/3200″, and 1/6400″. On my screen, I can see a hint of the white van in the shot at 1/3200″. At 1/6400″, the frame is completely black.
Why I avoid f/22 when I can
For all practical purposes, I use the widest aperture that I can to get the depth of field that I need. Stopping down beyond that aperture for the sake of controlling ambient light should be avoided. Why? Depth of field is really an illusion. In an optical sense all that is happening is that ‘circles of confusion’ are getting smaller. When CoCs are small an image appears to be in focus because we cannot distinguish the blur. Thanks to physics, what this also means is that, much beyond f/11, the sharpness of the image degrades significantly. So, while f/22 might make an image appear to have more depth of field, when you enlarge that image you’ll see that it was just an optical illusion.
For aesthetic reasons, as well as optical reasons, I typically try to stay under f/11. Look at the images above and go three frames back up the line from the bottom. The frame ‘Meter -4EV’ is what I want my ambient exposure to look like—f/22 at 1/800″. But, I want to shoot at an aperture wider than f/11. So, let’s count backwards from f/22 to f/8: f/22 > f/16 > f/11 > f/8. That’s 3-stops. By opening up my lens from f/22 to f/8, I have to stop the shutter down 3-stops in order to maintain the same amount of ambient: 1/800″ > 1/1600″ > 1/3200″ > 1/6400″.
So essentially at midday, without using an excessively small aperture, I can get to 1/6400″ at f/8. This is enough to dim the sun with my shutter so that it looks like a moonlit night. Details about the location and the background will disappear. It’s no longer the outside of my house with a couple of cars and a pre-teen son. It’s whatever I want to make of it. As a Speedliter, I can come in with my lights, modifiers, and such to create an image that expresses my vision.
Now, to get back to the fast shutter/tiny aperture vs. High-Speed Sync question, as I mentioned in Part One, switching the Speedlite into high-speed sync costs me about 2.5 stops of power. That’s o.k. with me. I’d rather have the creative freedom to control the ambient with my shutter than to accept the limitations imposed by a sync speed of 1/200″.
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