Photo Tip: Understanding Depth of Field – A Must on Safari

One of the keys to success in the bush which I find important is knowing your camera.  Since I’ve been into photography for quite a number of years, I have a sound understanding of the basics and these have been invaluable in the field.  Also invaluable is practice.  And I thought the next most important item was more practice.  Well, after my first safari, I learned that these last two as well as the first need a little modification.  I went into the bush with a camera I could set in the dark.  I had a long time history of knowing the relationships of ISO, aperture and shutter speed as well as ( I thought ) a good understanding of Depth of Field, often noted as DOF .  What I learned the hard way while trying to take critical photographs is that big glass, i.e. lenses of 400mm or greater follow the same rules as all other lenses, except all of the little details become much more important.  So my new and improved mantra is:

  • know your camera
  • know your camera with the lens you will be using in the field
  • practice
  • practice a lot with the lens you plan on using in the field.

When it's just before sunrise and you can't see any of your camera settings - you better know your equipment, including that big old lens - in this case a Canon 500mm - Copyright P. B. Eleazer

With big lenses, technique becomes very important.  On safari, you will often use a bean bag and few of us practice using this support method and in low light, shutter speed often suffers, so good technique is critical to sharp images.  All of these are worthy of discussions, however, for today’s discussion I want to remind everyone of the factors that effect DOF.  You see, on my first safari, lighting was often sunrise or sunset.  Since I wanted the shutter speed with my telephoto lens to be as least as large as the inverse of the focal length of the lens size ( i.e. 400mm lens  would be 1/400 second), I let the DOF go to ‘wide open’ for my lens’.  The problem is these are big animals and often you may want to get a pair or more of them at staggered depths.

Okay, it’s time to learn about DOF.  Three things effect DOF:

  • aperture (often referred to as the f-number or f stop) – simply stated, the bigger the f stop, the greater the DOF
  • distance from the subject – the greater the distance from your subject, the greater the DOF
  • focal length – as focal length gets larger, the depth of field gets narrower

Aperture

Aperture of f-stop is probably the most widely known and used method of controlling DOF. Most intermediate/advanced cameras have “aperture priority” which allows one to set the f-number. Generally speaking, this is where my camera lives.  I recommend this setting to most novice to intermediate photographers going on safari. For the more advanced photographers, I recommend that you ‘step down’ the lens to check DOF.  This is also known as “DOF Preview” and is well discussed at one of the links at the article end.  A tip to remember: If the f-stop is doubled, then the depth of field is also doubled. Note that “doubling” here literally means doubling (e.g. from f/4 to f/8), it does NOT mean one whole stop (e.g. from f/4 to f/5.6).

Distance to the Subject

For many on safari, this is the factor that saves them.  That may fit to those going to Kenya or Tanzania, but in Botswana and Chobe in particular, you will be getting quite close to your subject.  Furthermore, some of your more memorable memories may be the close-up details of an animal while looking through the lens.  You don’t want the disappointment of that great lion close-up only to find that the tip of the nose is not as sharp as the ears or eyes. A tip to remember: If the subject distance is doubled, then the depth of field is increased by four times (depth of field is proportional to the subject distance squared).

Focal Length

Many ( I dare say most) of us do not shoot many shots with a lens of 300mm or greater.  Many will buy, specifically for that once in a lifetime safari, a 70-300 zoom lens or the famed Canon 100-400L IS.  Those that can afford to will buy or rent a 500mm lens.  Further, most that use the 500 or 600 will use them in conjunction with a 1.4x teleconverter (known as a TC), making the 500mm lens now a 700mm equivalent.

Here is where I screwed up my first time. I comfortably shot the Canon 70-300mm zoom at soccer games, field hockey games and for wildlife.  For my first safari, I jumped to the 100-400 lens and assumed there would be no difference.  There was!  400mm is a lot different than 300mm.

Here is where I screwed up a second time.  By the time I took my second safari, I had mastered the 100-400 shooting at 400 and wanted to move up in image quality, speed and ability to get closer to the target.  I bought the Canon 500mm f4 lens.  This is a fantastic safari piece of glass.  Since I had mastered the 400, I assumed the 500 would behave similar.  Well the 400 had a minimum aperture of f/5.6 and the 500 had a minimum aperture of f/4, so when shooting wide open, I had 2 factors making things different – smaller f-stop and longer focal length.  I should have practiced the combo more prior to entering the bush.  At article end is a link to a ‘calculator’ at Bob Atkins site, however from his calculator I want to show a simple example if the impact.  For a 100 ft. away target:

  • my 300mm shooting wide open at f5.6 lens had a DOF of 6′ 11″
  • my 400mm wide open at f5.6 had a DOF of 4 feet
  • my 500mm at f4 had a DOF of only 1 foot!  Wow!

I’m rambling, so let me get back on topic. The last factor in your control for DOF is the focal length of the lens you decide to use. Telephoto lenses have a shallow depth of field as compared to their wide angle counterparts. Anybody out there have a sub-20mm lens? It’s pretty hard to get background blur, right? Any super-telephoto shooters out there? Just the opposite.  I’m going to be a little lazy and use some photos from a similar topic blog called ‘epicedits‘.

TRY THIS: Use a zoom lens that reaches from wide angle to telephoto (something like an 18-200, 28-135, etc.) or use two lenses (wide angle and telephoto). Again, find a subject that has some background element in view. Position yourself approximately 5-10 feet from the subject and set your aperture in the low-mid range (f/4-8, but make sure to find something that can be used for both lenses). Take the first shot with the wide angle lens or at the shorter focal length of the zoom lens. Now, hold your position and your f-number, and switch to the telephoto or use the longer focal length of the zoom lens and take the same shot with focus on the same subject. You should see a wider depth of field with the shorter focal length. Copyright - Brian Auer

This example is only looking at 20mm compared to 105mm at f/5/6 and at 4 feet yet one can see impact.  Trust me when I say that the DOF gets very small when shooting short distances with a 500mm at f/4.  A photo tip to remember: If the focal length is doubled, then the depth of field is decreased by four times (depth of field is inversely proportional to the focal length squared).

You will find that some photos appear sharp when viewed as a full image on the monitor, but when zooming and viewing at 100%, that DOF was a little less than you thought.  An example is this Cheetah photo.  Looks pretty sharp … until you look really closely.  Note the soft nose compared to the tack sharp eyes.

Is this Cheetah sharp? Looks Okay as a small, 800 pixel image.

100% crop - note that the nose is not quite as sharp as the eyes. One more f-stop and this would probably have been perfect.

A few links related to the subject (in no particular order):

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