What is the Depth of Field Preview Button?

If you use a DSLR camera, you likely have a couple of buttons on the front of the camera alongside the lens mount that seem a little mysterious, and as such, they can be easily ignored.  

These buttons are typically programmable and are often called the custom functions or Fn buttons.  They are pretty handy because you can use them as a shortcut to certain camera settings or functions.  This way you can avoid having to paw through the menu system to get to a function that you use often.

One function that I like to use when doing outdoor photography is the depth of field (DOF) preview, and so I have one of these programmable buttons on my DSLR set as the DOF preview button.  On many camera models, this function is symbolized by an aperture icon and simply called “preview”.

Depth of Field, Aperture, and Optical Viewfinders

Depth of field is the area or region between two points before and after your focus point that is considered acceptably sharp.  

If you want to try to get as much of the scene as possible acceptably sharp, as with many grand landscape images, then you would want to use a deep depth of field.  

However, if you would like to separate or isolate your subject from the background, as with intimate landscape scenes or wildlife portraits, then one way to achieve that is to use a shallow depth of field.  

Aperture is one setting that affects the depth of field, and if you want to learn more about how aperture controls DOF, then be sure to read this OPS article, What is Aperture in Photography: Key Concept Explained.

If you use a DSLR with an optical viewfinder, then what you see through the viewfinder is often not what you get when you shoot in either of the priority modes or in manual mode.  This can be very frustrating to some budding photographers trying to move beyond auto or programmed mode.  

What some photographers don’t realize is that the camera doesn’t actually apply your chosen aperture until the shutter is released.  Until that moment of shutter release, the aperture is set to the widest or maximum aperture (lowest f-stop number) of the lens you are using.  

So, let’s say for example, you are using a 24-70mm f/2.8 lens in manual or aperture priority mode.  You set the aperture to f/16 or f/18 because you would like to achieve a deep depth of field to include a nearby foreground element in focus with some mountains off in the distance.  

What you see through the optical viewfinder is actually the scene at f/2.8, and so it’s impossible to judge what the DOF would be at f/18 and whether it gives you the DOF you desire unless you take the shot first and assess it on the LCD.

Why Do Cameras Do This?

The reason the cameras do this is because the more the aperture is stopped down, the smaller the opening, and thus, less light is let into the camera.  This can make the scene appear very dark in the optical viewfinder, which can make it difficult to properly focus on your focus point.  By keeping the aperture wide open, more light is let in, and so focusing is a lot easier.

So there are obvious advantages to this setup.  However, one disadvantage is that when you look through the viewfinder, the scene you see has a very shallow depth of field because the lens stays at the maximum aperture regardless of your settings.  

If you want to test this on your camera, set it to manual mode and just play around with your exposure settings of aperture, shutter speed, and ISO (not auto ISO), you’ll notice that the brightness of the scene in the viewfinder does not change as you change exposure settings.

Now, let’s go back to our mountain example for a moment.  What if for that mountain scene, f/11 would have been sufficient to get a similar depth of field given your focal length and distance to your foreground element?  If f/11 gives us the DOF we desire, then we have the option to use faster shutter speeds and a lower ISO, which can sometimes result in sharper and cleaner images.

Wouldn’t it be nice to be able to know that ahead of time instead of taking a ton of test shots at different apertures?  

Similarly, let’s say you were trying to make an image of a small, intimate nature scene, like colorful fall leaves on the forest floor.  What aperture would give you the depth of field you want to create the image you have in mind – wide or narrow?  How close should your camera be to the leaves?  The DOF preview button can help you make those decisions.

What Does the Depth of Field Preview Button Do?

You can program one of those mystery buttons on the front of the camera to work as the depth of field preview button.  When you press it, it will stop down the aperture to the f/stop you have chosen for that scene.  

Note, if you are already shooting wide open (using the lowest f-stop number on your lens), then the DOF preview button won’t have any effect. 

If there is enough light when you press the button, you will be able to preview the area in focus through the viewfinder and make any settings adjustments before taking the shot.

If there isn’t sufficient light to preview the DOF through the viewfinder, then you will have to either use live view or just take test shots and review them on the LCD.  

On many DSLR cameras, live view behaves differently than the optical viewfinder because what you see *is* what you get.  

The reason for this is because the reflex mirror flips up and you see whatever the CMOS sensor is measuring from the lens.  So in live view, your selected aperture (and shutter speed and ISO) is displayed in real time, and you are able to see the effects of aperture on depth of field.  

One advantage of mirrorless camera systems is that the electronic viewfinder (EVF) and live view on the LCD are one and the same, and you can view the depth of field and changes to exposure in both cases.

Cameras With DOF Preview Button

Wondering whether your camera has the depth of field preview function?  Here’s a list of widely used DSLRs that have the capability.  

If your camera is not on the list, check with the manufacturer or the user manual before you give up on the idea of using it.

How to Program the Fn Button to DOF Preview

Each camera manufacturer has their own method for programming the customizable buttons on the front of the camera bodies, so the best approach is to look it up in your camera’s manual.  

That said, if you want to play around with the menus and see if you can find it, the custom functions settings are usually found in the menus under the Custom Settings Menu → Controls (or Custom Functions).  

If you scroll through those, you should see an option similar to “Assign preview button” or  “Assign Fn button”, or something along those lines.  

If you select this, you should be given a handful of options of what function you can assign to these programmable buttons, including DOF preview – sometimes just called “Preview” and symbolized by an aperture icon.

My Trick for Focusing in Low Light Using the DOF Preview Button

What I’m about to describe is a little trick that I use to nail focus in low light situations that takes advantage of live view and the DOF preview button. 

It’s *not* for viewing depth of field, as you’ll see in a moment.  I’m not suggesting that you use the DOF preview button in live view to assess the DOF (because it won’t work that way), but rather to use this as a tool for focusing.  

I’ve used this approach with mid-level and pro-level Nikon cameras, so please note that this trick may or may not apply to your DSLR camera.  It’s still worth a shot (pun intended…) to see if it might work for you.

In live view, one might expect that the DOF preview button wouldn’t be needed because the camera is already applying your chosen aperture setting in real time.  

However, on some cameras, the DOF preview button still “works”, but instead of stopping down to your chosen aperture (since you’re already there), it opens the aperture up to the maximum aperture and lets in more light. 

This is handy for focusing in low light situations where it may be difficult to focus using live view, and autofocus may struggle and hunt around.  

Sometimes, when you use longer shutter speeds or higher ISOs to account for the low light, the view on the LCD screen can get noisy.  By opening up the aperture all the way, the DOF preview button lets in more light, and the view on the LCD screen is cleaner, and so it is easier to set focus.

Here is how I use the DOF preview button for focusing in live view:

1. Switch to live view
2. Put your camera’s focus point on your chosen focal point (which could be at the hyperfocal distance)
3. Zoom in 1:1
4. Press the DOF preview button – the aperture opens up to the widest setting (on certain cameras)
5. Set focus using manual focus (autofocus turns off the DOF preview)
6. Press the DOF preview button again to return to your chosen aperture
7. Zoom back out
8. Take the shot

Now, not all DSLRs with live view and the DOF preview function will work this way.  

For example, my entry level Canon Rebel SL2 has both live view and DOF preview functions.  When in live view, the exposure settings are applied as expected and viewable in real time.  

However, when you press the DOF preview button, the aperture opens up to the maximum aperture (as described above), but it doesn’t change how the exposure appears in real time on the LCD (it doesn’t get any brighter or cleaner).  So, there must be a way that the camera is compensating for the change in exposure settings, but I’m not sure how (possibly through the ISO).

Also, some entry level DSLRs do not have a programmable Fn button in the front of the camera, but some do still have the DOF preview function as a menu setting, so be sure to check your manual to find out.

I hope knowing about this handy function opens up some new creative opportunities for you on your next photography outing!