This was a tough shot to get although it certainly doesn’t look very hard since most of it is blurry and out of focus. Here is the story followed by a wonky discussion that strays off into the uses of “blur” and ends with a two nice photos, one of which is both in focus and excitingly blurry (the wonky stuff is really wonky so I understand if you skip it but at least check out the photos below).
Coming home from our recent trip to North Dakota, I was able to experiment with motion and focus but in an unusual way — I wanted to get a photo that captured the truck’s motion while both I (in a car my wife was driving – thanks Sweetie) and the truck were moving down the highway towards each other. This is as close as I was able to get to what I was “shooting” for – a truck that was at least partially in focus, but also seemed in motion relative to both itself and to me.
I got to thinking about posting my experiment because of this week’s weekly photo challenge, which is about “focus.” Focus was of course one of the primary problems facing me in getting any shot like this. I took this particular shot along a divided highway with a Sigma telephoto lens set at a 210mm focal length. The truck was about 150 feet away. Because I wanted as much depth of field as humanly possible while not freezing the truck’s motion, I had been experimenting with a wide range of aperture and shutter speed settings to find out what would work. The details of this shot: 1/30th sec.; f/18; ISO 400; aperture priority. This gave me a depth of field (or “DOF“) of 121 ft. (beginning at 111 ft. and ending at 232 ft.) You might wonder about going to f/11. The problem is that it almost halves the DOF (to 70 ft.) while moving the shutter speed to somewhere between 1/60th and 125th of a second. All things being equal, an aperture of f/16 (DOF of 104 ft.) would have been a better choice, but water under the bridge (or perhaps over the highway). Given the relative closing speed between our two vehicles (I am assuming 120 miles per hour), the truck moved through the Circle of Confusion (the space that defines the depth of field) in 5/8th of a second. Since I couldn’t mount a tripod where I was sitting in the car, I had to hand hold the camera. While the Sigma lens has good to very good optical stabilization, my hands are not very steady and that plus the car’s movement introduced more complexity into getting this photo in focus and without unintentional blur.
The other thing I wanted to see as the truck moved into and out of the Circle of Confusion was some blurring representing its motion. Blur is a relative of focus (e.g., when something is intentionally or unintentionally out of focus because an object is outside of the lens’ Circle of Confusion). Blur is often introduced into a photo by camera motion, which typically occurs when a photographer uses a shutter speed that is too slow for the photographer to hold the camera steadily. When blur is intentionally introduced into a photo, the most common reason is to impart a sense of motion to an object in a photo.
When using blur to indicate motion, the photographer is usually in a more or less fixed position and follows the speedy object (from a marathon runner to a Formula One race car intending to either freeze or nearly freeze the runner or car or whatever against a background that blurs. Getting the object in focus while the background is visibly blurred is the key to imparting motion/action to the object. As an example of where you don’t see that background and thus don’t appreciate the motion of an object in a photo, take a photo of a passenger jet in flight. You know that the plane is traveling very rapidly since it is flying, but if the photo of it is against a blue undifferentiated sky (no clouds), for all you really know, the plane could be parked up in the sky motionless. See another example of what I mean here:
One way to get around this is to show the plane(es) with contrails or artificial smoke, something that is often done at an airshow as shown in the photo below (both photos are of the Canadian Snowbirds, the Royal Air Force’s aerobatic flight performance team).
Finally, blur can be added artificially (here via Filter Forge) to what is otherwise an in focus photograph as shown in the two photos below.