A great question came up in my Twitter feed a while back, and felt it was worthy of putting together a short post to help explain the whole concept of zoom on cameras. The original tweet asked:
“What’s the best focal length on a lens to reach 400 yards so I can see a 1 inch square at that distance?”
We started talking about different long length lenses, such as the 400mm, 800mm, and 1200mm lens options from Canon. All were (are) very pricey and beyond the budget for my friend. he then started asking why a 35x zoom point and shoot wouldn’t be an option. So, the discussion turned to a teachable moment! How exciting for a teacher! The recap is two simple points:
Point #1 When manufacturers refer to the zoom of a lens, whether it’s a P&S camera, binoculars, or digiscopes, they are referring to how much of a magnification one can get over “normal viewing conditions”. Notice how I put the last part in quotes. What are normal conditions anyway? What kind of vision is normal? 20/20 vision? And what are we looking at? Something 10 yards away or 400 yards away. The point here is that there are so many subjective factors, the “zoom” isn’t really has hard and fast a number as one would think. We can approximate sure, but it’s not set in stone, and certainly a measurable distance is nothing more than a rough guess.
Point #2 The zoom of an SLR lens isn’t at all on par with the zoom of other equipment – it actually is a hard and fast number. Simply put, the zoom of a lens is the ratio of it’s longest reach to it’s shortest reach. A 70-200mm lens starts with a focal length of 70mm and ends with a focal length of 200mm. This is the distance from the focusing point to the sensor. So, a 70-200 lens has a “zoom” of 200/70 or almost 3x. A 100-400 lens would have a zoom factor of 4. it’s simple math for SLR lenses.
This is a classic example of where the same term can mean different things to different people.
So, my answer to him? Well, to see something at 400 yards large enough to take a decent picture you would likely need an 800mm lens or 1200mm lens. You also probably need a 1x or 2x TC to really get far enough. The problem with taking a picture at this distance is that heat, atmospheric conditions, and just the physical limitations of optics would not make for appropriate conditions to capture decent images. Most wildlife photographers I know of like to get closer than 400 yards from a subject to take their picture, and for good reason. The distance to subject is of prime importance in capturing wildlife photos. Zoom or no zoom (prime lenses), there’s no substitute for proximity! Photography all too often comes down to something along the lines of real estate mentalities…location, location, location!
My best example? A hummingbird I shot from a mere 15 feet away!
