Lens Focus Questions & Answers


Active Member
I just finished reading the chapter of my photography book that covers lens focus. At the end of it was a page that held many questions on the topic. While the multiple choice answers were given, I always have a lot more to say about things. I thought I'd share the questions below with my answers underneath each one. If you've got any concerns or if you'd like to know more about something, please ask in a post here. Enjoy.

Q. Which autofocus mode constantly updates the focus distance to track a moving subject?

A. To start off, I'd like you to notice something in the question. Do you see how the phrase "focus distance" was used? When focusing a lens, what you're actually doing is moving what's called the focal plane nearer or farther away from you. The focal plane is like a pane of glass that's perpendicular to you. I just wanted you to understand that. Regarding the question, there are three options. They are Single, Continuous, and Program. Single focus is when you press your shutter button down half way and lock the focus in. You can move the camera and your subject can completely walk out of the frame. It's no matter; the focus will stay where it originally was. Continuous, on the other hand, updates as things move. It'll automatically update the focal plane as the situation changes. So if your subject moves farther away from you, the lens will take that into account. Finally, Program is a camera shooting mode that allows certain types of focusing. It doesn't describe how the lens itself focuses. so the answer is Continuous.

Q. The farthest distance a lens can focus on is known as what?

A. When you focus on infinity, you are focusing on forever. Some lenses come with an infinity symbol that makes life a lot easier. For those less expensive lenses, it's up to the photographer to locate infinity. A good trick is to find something very far away and focus on that. It's essentially the same thing as infinity. Also, if you're engaging in nighttime astrophotography, a handy trick is to set your lens to focus on infinity during the daytime and the tape the lens focus ring so it doesn't move. That way, you'll be ready to go when the stars come out.

Q. How would you lock focus in Single AF mode?

A. Single AF mode has different names for different camera makes. For Nikon, it's called Single Area AF and for Canon, it's called One-Shot AF. They're basically the same thing. Essentially, when shooting in this mode, you'll choose one point in your scene on which to focus and when your camera determines where the greatest contrast between pixels is, it'll lock the focus there. That is, after you press your shutter button (or otherwise dedicated focusing button) down half way. After pressing this button, the camera will lock focus there and not change it until you let go of the button to repress it later on.

Q. Which symbol represents infinity on a lens?

A. It's a sideways eight. It looks like this: .

Q. Which part of someone's face should always be in focus and sharpest?

A. The eyes are windows to the soul and therefore should be in focus.

Q. What is the closest distance a lens can focus?

A. There is no universally set distance. It depends on which lens you're using. Some have closer minimum focus distances and some have farther ones. The answer to this question, though, is minimum focusing distance.

Q. In automatic AF point selection, where to cameras usually focus?

A. If you aren't familiar with this mode, you'll know you're in it when you see multiple small boxes on your LCD screen or through your viewfinder when focusing, as opposed to only one. But to answer the question, cameras will generally focus on the closes object in the scene while in this mode.

Q. A skylight filter reduces the effects of what type of light?

A. This type of filter is no longer popular, due to the onset of digital cameras. Neither are UV filters. Some folks still use them for protection on the ends of their lenses though. Back when skylight filters were popular, photographers used these them to reduce the effects of ultraviolet light, which sometimes caused colder bluish casting in some sky shots as well as shadowed areas of snow. The filter is slightly magenta colored, which corrects for an altered color balance. Today's cameras can correct for this automatically.

Q. What is Continuous AF sometimes called?

A. It's sometimes called predictive because the camera predicts where the focus should be.

Q. Which camera control can be used to change the AF point?

A. This is a trick question. There are multiple methods for changing the autofocus point. It's commonly done with the arrow keys on the back of the camera, which are also called the multi-controller (multi-selector). You can also change the focus point by using the dial on the top of the camera.

Q. What secondary use to UV filters have?

A. As I mentioned above, many photographers swear by using these inexpensive filters to protect the ends of their lenses. Breaking or scratching a UV filter is a lot better than breaking or scratching the glass in a lens.

Q. What name is Single AF mode sometimes referred to?

A. Again, as I mentioned above, Canon uses the phrase One-Shot and Nikon uses the phrase Single-Area.

Q. At a sporting event, what kind of knowledge helps you anticipate fast moving action?

A. Since you'll always want to know what's to come next while photographing sports, it's helpful to know the rules of the game.

Q. If you over-sharpen a photo, what visual effects do you see?

A. In general, when sharpening an image in Adobe Camera Raw or Photoshop, you increase the amount of contrast that is between pixels or groups of pixels. If you over-sharpen though, halos can form around these same areas.

Q. Skylight filters are which color?

A. This type of filter is a very light pink or magenta.