F is for Focus, F-stop


In the old days we used to focus our cameras manually. In the really old days that meant sticking your head under a black cloth and looking at the upside-down image on the ground glass and seeing if the image was in focus. Even before the advent of autofocus (AF) cameras, most rangefinder cameras has some kind of mechanism to assist when focusing. Single lens reflex (SLR) cameras allowed the photographer to see the image coming through the lens, and most were equipped with a split circle that allowed adjustment of the lens focus until the top half and the bottom half were aligned, confirming proper focus.

Nearly every camera sold these days is autofocus, which is an advantage, and a disadvantage. For me, I can’t see through the viewfinder on my DSLR very well, because I wear glasses and my eyes are tired. I don’t think I could focus manually if I had to… So AF is how I focus when using my camera, and for that I’m really grateful. But AF makes for lazy photography if you let it. Lazy because you point the focus spot–usually the center of the image screen–at what you want in focus and expect (hope) it is indeed in focus. And most of the time it is. There are times when it’s not, and that could be due to the subject moving, or because you accidentally lifted your finger off the shutter, and then depressed it again. Most cameras a set up so that when you depress the shutter button either half-way, or fully, the camera sets the exposure and releases the shutter once focus has been confirmed (I call this one-button AE/AF). Pressing the shutter halfway sets the AF on most cameras. You can use this to focus off-center, then holding the shutter down halfway, recomposed the image and make the photo by fully depressing the shutter. It’s simple and both proper exposure and proper focus are achieved in one move by one finger.

Recently I have starting using two fingers. Weird, I know! It’s called Back Button Focus (BBF) and it appeared first on a Canon camera in 1989. What Canon did, and many other cameras allow this now, is offer the option of separating the exposure function from the focusing function. The shutter button will still control the setting of the exposure when pressed, but the camera will not start autofocusing until another button (set by the user) is pressed. Most users assign this function to a button on the back of the camera, close to where it can be pressed and held by the thumb. This usually means one of two possible buttons. Using BBF, the photographer must set the exposure and focus in two explicit moves–which might seem to defeat the purpose of an AE/AF (modern) camera. But it seems to give me more control over where I focus, and when I choose to set the focus. The camera I use has two types of focus modes: one shot and Ai Servo. This sounds pretty fancy, but really it just means the camera either sets focus and sticks to it, or sets focus and then constantly adjusts it if the subject moves, which is great for sports. With “one-button AE/AF” the only way to refocus is to lift off the shutter button. With BBF, focus can be set, and reset, by pressing, holding or releasing the assigned button.

You can read more about the technique here at Canon’s digital learning center.


D is for Dynamic Range

Dynamic Range is a measure of how much “light” is captured by the digital sensor and resulting image. It’s the range from the darkest black to the lightest white, or relative to an 8-bit scale, 0 is darkest and 256 is brightest. If the actual scene has a greater range of light intensity, then some parts of the image will be “blocked” at either black (0), or white (256) or both. Enough of the techno-babble, even though there’s a whole lot of interesting techno stuff related to this, often mixed into the discussion of high dynamic range (HDR) processing.

The sensor’s dynamic range is a physical limitation. Each part of the sensor can only “hold” so much light (or photons) before it becomes full, so you can “expose for the shadows” and risk blocking the highlights, or expose for the highlights and lose detail in the shadows. (More on this at E is for Exposure.) The luminance range of the scene can be measured in units called eV (exposure values) where each eV is double the amount of light of the lesser value. It’s believed the human eye has about a 10 to 14 eV range, whereas many camera sensors have a 8 to 10 eV range. The newest Sony sensors in Nikon and Sony cameras might have a 14eV range, and seem to show a much greater dynamic range than any Canon sensor. (Check out this Sony AR7 review with many comparisons of shadow detail between Sony and Canon.) Whether this is essential, or critical, or even important, is another matter, and the subject of much debate, especially considering most amateur photography is expressive rather than documentary.

Recently interest in high dynamic range (HDR) processing has increased, and some cameras even come with HDR built in. Essentially it’s a simple process of making multiple images at various exposures, some under-exposed and some over-exposed (relative to what the camera might determine is normal exposure.) These images are merged using software to capture the wider range of luminance in one image. I like to think this is similar to the process Ansel Adams describes in chapter four of his book “The Negative.” He called it the Zone System and it relates subject luminance range to the scale of grays (white to black) on the final print. For Adams the method by which the visualization of the final print came from the exposed negative was both a technical and creative/artistic process. Although Adams worked with film and paper and chemicals, the idea that the scene has some dynamic range, and one of the photographer’s choices is how to represent that dynamic range in the final image, still remains relevant today.

If the camera sensor has a greater capacity to record detail in shadows and detail in highlights, then that’s a boon! But even without having that super sensor, there are many ways to achieve a particular artistic outcome, including deciding to give up the shadow detail for more highlight detail, to make multiple exposures and process using HDR software, or to use other software manipulations (Photoshop has a “shadow and highlight” tool that can expand or compress these areas of the image after the fact, even though it sometimes makes the image look flat. A photographer can learn how the camera sensor “reads” light, and experiment to develop processes to achieve the desired image.

Professional cameras

Also referred to by members of my family as “paparazzi cameras,” these large black beasts seem to YELL “I’m very serious about taking photos.” But there is more to it than simply appearances. They are built like tanks. I’m sure you’ve seen the photographer with the huge white lens on the sideline get taken out by the wide receiver at an NFL game? That’s a pro-body being badly treated. This is what’s inside Canon’s latest pro camera:


That’s magnesium–light, strong, (but not good around open flames.) Most consumer cameras have plastic bodies, although pro cameras and lenses use a lot of plastic as well.