Depth of Field: Your Camera’s Viewfinder Cannot Be Trusted
Depth of field is one of the most coveted tools in a photographer's toolkit. Creating a perfect depth of field – one with the right depth and focused just perfectly – is an essential skill. Unfortunately, for many, learning how to predict and use depth of field most effectively is a difficult task. The task is only made more difficult by the misconceptions about your viewfinder. In short: Your viewfinder cannot be trusted for Depth of Field accuracy.
Why It Can’t Be Trusted…
The viewfinder is a dark and cavernous space. Well, not really, but it eats light all the same – even those with good quality pentaprims – the glass prism that bounces the view through the lens up to your eye. Some entry level cameras use a pentamirror (mirror box prisms) – a reflection system made up of mirrors instead of glass – which tends to be less efficient with the light. I could easily turn this into a discussion as to why you should get a camera with a good viewfinder, but that is besides the point. While a good viewfinder certainly helps, the truth remains the same: Light in the viewfinder is still a limited resource. We can't afford to cut any light, and so your camera is designed to provide as much light as possible through the viewfinder. The best way to do this? Open the aperture as wide as the lens will allow.
Contrary to a popular misconception, depth of field is not a function of time: It is purely affected by the size of the aperture. So when you're looking through the viewfinder – which again is wide open – you are looking into the smallest depth of field the lens can provide. Unless you're using an aperture set to the maximum size, the finished photo will have a larger depth of field.
This all matters because we really need to know the limits of our depth of field as it falls inside our photo. Where is the focusing plane? Where should the subject be placed? We of course aren't concerned with a tiny aperture like f/22 – rest assured, the focus is going to fall where you want it with a large margin of error. But it's those middle apertures, like f/4, where the actual focusing point is important. Imagine focusing on a tiny flower in a field of other similar flowers. The easy approach is to focus on our subject, but that would place the subject at the exact center of the entire depth of field. That may not be the most impactful composition. Perhaps an element rests between the subject and the camera that is a piece of the story – you wouldn't want to leave that out.
Solving the Puzzle of Depth of Field
If you are one who is hung up on depth of field, worry not – there are solutions to be had. If you are fortunate enough to own a camera that has a Depth of Field Preview button, it will become your friend with those tricky shots. The catch is that it work by stopping down the aperture to the same aperture you've set to use for the photo. Available light will be reduced, and the image will appear much darker through the viewfinder. It's a learning curve and you may find yourself looking into the stopped down view much longer than you might expect. But it is inevitably the best and easiest way to verify that the DOF is the size you want it and that elements within your composition are focused (or not focused) exactly the way you would like. Fortunately for the consumer, DOF preview is a feature that is become much more common on most modern SLR cameras. It's a feature that is at the verge of becoming standard (like power windows in cars).
For the rest of you – or if you have Obsessive Compulsive Disorder – there are tools that can be purchased to help you calculate the depth of field. One of our favorites is the ExpoAperture 2 from ExpoImaging. Applications are also available for your phone – be it a Blackberry, iPhone or Android device – that can help you to calculate the depth of field. For those of you with older gear, many older lenses with aperture rings, these lenses have a depth of field calculator built in as well (check your manual for how to use it).
I will admit that I'm at a loss for DOF calculator applications for the phones. If you've found an application for your phone that you would recommend, please respond via the comments below. Let us know what application you'd recommend, and include a link if possible.