Closet Photographer: What the heck is Depth of Field?

Last week we covered the basics of what Aperture really is. Now that we have a sense of what the size of that opening in the lens does (lets in the light and determines shutter speed based on the amount of light being let in) let’s move on to one of the visible affects of aperture called Depth of Field (DOF). Let’s start with a definition of Depth of Field (again from Webster Dictionary Online) “The range of distances of the object in front of an image-forming device (as a camera lens) measured along the axis of the device throughout which the image has acceptable sharpness” This one is a little easier to understand straight from the definition but I’m going to explain it out anyways. Depth of field refers to the amount of the image that is in focus directly in front or behind the object which you are focusing on. The varying degrees of depth of field are determined by your aperture or f-stop setting. A shallow depth of field refers to an image where there is very little of the image in focus whereas a deep depth of field shows sharp focus in most or the entire image front to back.  

How does this work...well let’s try something. If you remember from before a low f-stop number (such as F1.4) means a large opening letting in lots of light all at once therefore requiring a faster shutter speed to have the image properly exposed. A large f-stop number (such as F22) produces a much smaller opening which lets in smaller amounts of light meaning the shutter speed will be slower (shutter open longer) in order to get enough light to expose the image properly. So, for this exercise we are going to use our eyelids as our shutters. Stand in front of a window or look towards an area with a lot going on, close your eyes, then open them then close them again very quickly. How much of the scene in front of you can you remember in detail? Now do the same thing, looking the same direction but keep your eyes open and look at the scene for a longer time (say 15-30seconds) before you close your eyes. How much of the scene can you remember now? The first time you opened your eyes you likely noticed the first thing your eyes focused on and a few details close to it but only general shapes further in front or behind. This is like a shallow depth of field, because the shutter is only open for a short period of time it can capture only the details close to whatever object you have told it to focus on. The second time you went through this exercise you were likely able to pick up a lot more detail throughout the entire image as your eyes were open longer. This is similar to a deep depth of field; because your eyes were open longer you can recall more of the details of the scene from front to back.

 

Here is another representation of the different depth of fields but this time a more visual exercise. Below are photos of the objects in the same locations and lighting. I have my camera set to Aperture Priority mode so that I can choose the aperture setting and allow the camera to choose the shutter speed to ensure my image is properly exposed. The only thing that changes in these images is the aperture or f-stop. Now that you can see what the change in aperture does for the image let’s talk about where you would use these settings.

This is by no means the only f-stops to use or the only situations to use them in but I wanted to give a little perspective on when I use these settings to start you thinking about the settings you use.

 

F2.8 or lower – Great for portraits in locations with busy or cluttered background where you just want the person to stand out

 

F4.5 – wedding photography David Ziser (www.digitalprotalk.com) calls this his aperture of convenience as is gives more depth in most situations but is still large enough to give a flattering softness to the background and allow for faster shooting in a faster moving situation

 

F8-F11 – for any situation where you want both the main subject and their environment to be clearly recognizable but no huge distance between the subject and the background

 

F16-22 – this is fabulous for landscapes where you have greater distances between different parts of the scene that you want in focus such as a winding river up into distant mountains (F22 gives a fabulous starburst effect to a sunset or sunrise) – This aperture range is also often used in macro photography for greater clarity and detail

 

The other thing to keep in mind when dealing with aperture is that it’s not always constant. Here’s what I mean by that let’s say for example that at F2.8 on my 50mm lens I can have one inch in front and one inch behind the point I’m focussing on in sharp focus (I don’t know if that’s the actual distance it’s just an example). Now let’s say I switch lens and start using my 105mm macro at F2.8 will the inch in front of and behind my focusing point still be in sharp focus? NO! See the examples below.

This brings me to your homework this week. Yes, that’s right, I’m giving you homework. Take out your camera and set up a similar group of objects to my first set of photos (they could be soup cans, your kids stuffed animals or even beer bottles – whatever you have a few of sitting around). I recommend an uneven number so you can focus on the middle object and that you put them on an angle so they are approximately 2inches apart. Set your camera to aperture priority mode (A on Nikon, Av on Canon) go through and try at least the apertures I tried above or if you are feeling adventurous dial it down to the lowest f-stop number and try every f-stop your camera/lens combo will allow. Pull the photos up in your computer and compare so you know exactly what your camera/lens combo is capable of. If you have multiple lenses repeat the exercises with each lens or with different focal lengths in your zoom lens.

 

I can’t wait to hear how you made out! See you next Monday for a quick lesson on bokeh (and no it has nothing to do with flowers)