Telephotos: Length Matters

Why do photographers lug around big telephotos? Wouldn’t it be much better to have a really sharp shorter lens and be able to crop the shot? In a word, “no”.

Nikon D610 550mm f/7.1 from 17 feet away

The shot above, taken with the Sigma 150-600 Contemporary at 550mm, was at a distance of about 17 feet (5 meters). The depth of focus is 0.11 feet, or just 33 millimeters. The aperture was f/7.1 to try to get the whole bird’s head in focus, but no more. The distracting bushes behind the bird would have ruined the shot if they were in focus. The bird was nervous enough at 17 feet; it would have simply flown away if I had approached any closer. Yes, I did try getting closer, and yes, it did fly away.

No matter how sharp your lens is, and no matter how much resolution your camera sensor has, you can’t get the “look” that a long focal length provides merely by cropping your shorter-focal-length-lens shots. Even if you could buy something like a fast 200mm f/1.0 lens (which doesn’t exist), you still couldn’t get the look that a “slow” 600mm telephoto gives you at long distances.

The narrow depth of focus that a long lens gives you cannot be replicated by shorter lenses, no matter how large the aperture that shorter lens has. This last statement implies that you take your shots from the same distance for each lens being compared.

Here’s an example: I have a 600mm f/6.3 lens, and I shoot a subject from 50 feet away. The depth of focus is just 0.77 feet, or about 9 inches. You might not think that such a slow lens could manage such a narrow focus depth, but that’s the reality.

A 200mm f/2.0 lens is the fastest existing telephoto of that length that I’m aware of (and almost $6,000.00). What’s the depth of field for that lens at 50 feet? It’s 3.82 feet, or 5 times as large as that slow 600mm f/6.3 lens! By the way, this 200mm lens weighs 6.5 pounds, or almost 50% more than the 4.5 pound Sigma 150-600 Contemporary.

Now, let’s imagine we have that mythical 200mm f/1.0 lens and shoot at the same 50-foot distance, wide-open. Let’s even imagine that our lens is infinitely sharp, and our camera sensor has infinite resolution, so that we can crop any amount we want. How narrow is that depth of focus? It’s going to be 1.13 feet, or nearly twice as much! And I can only imagine what that f/1.0 lens might weigh.

So why do I even care how much the depth of field is? I care because of the “look”. Subject isolation with telephotos and their creamy, de-focused backgrounds are what it’s all about. Nothing screams ‘amateur’ like photos that have busy, distracting in-focus backgrounds with fences or telephone poles.

This desirable telephoto narrow-focus effect is quite the opposite of what you want with nearly all super-wide-angle lens photos, where you’re after near-to-far being sharp. But that’s another story.

It’s no fun hauling around big glass all day, but there’s a real reason that people do it. It’s to get “the look”.