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I was reading a forum post by someone, who said, "I prefer prime lenses, and would rather zoom with my feet." I also read a blog post by someone that was comparing lenses of two different focal lengths, and they said, "If I want a wider angle, I'll just back up." Now, of course we've heard these references to "zooming with our feet" for years, but does it really work? We'll explore that in today's blog post.
First off, don't get me wrong; I love prime lenses. I lust after the Zeiss 18mm 3.5, daily. But in the end, I do use a lot of high-quality zoom lenses—mostly because of what I shoot, and the conditions I do it in. I spend a lot of time in hostile deserts with high winds and blowing dust, or with those same winds—exchanging ocean spray for dust. So changing lenses in the field is NOT my favorite thing to do. I have already had to replace one digital sensor low-pass filter in my camera, and they are not cheap.
Also, for the sake of this post we are going to assume that zooming a zoom lens and changing lenses for a different focal length amount to the same thing.
So let's see if we really can zoom with our feet. Today my super-models are on vacation, so I enlisted the aid of my flower baskets as models for the day. I start off 8 feet from my model, and I shoot with my lens at 24mm.
This is okay, but that's not the framing I want on my subject—the flower basket. So I replace the 24mm with a 96mm. (I zoomed, but it would be the same if I changed fixed focal lengths.)
Here is the shot now framing the basket top to bottom in the frame height:
That looks good. So now, let's not zoom from the 8-foot location; let's go back to our 24mm lens and instead "zoom with our feet" by moving right up to it. If zooming with our feet is indeed possible, the next image shot from 2 feet away with our 24mm lens should look the same as our image at 8 feet away from our 96mm lens.
Huh? Where did the house behind the flower basket go? I shot from the same angle—I just moved forward towards the basket. Oh, it's there behind the basket, but everything except the framing of our subject has changed.
So obviously, you can't zoom with your feet. You keep the size and position of your subject the same, but everything else changes. Let's look at what changes as we change focal lengths:
• Angle of View – First and foremost, our 24mm on a full-frame camera would have a horizontal angle of view of 73.7°. The 96mm would have 21.2°. (A 17mm would be 93.3° and a 200mm would be 10.3°)
• Height Perspective – Objects in front of and behind our subject will appear different in height. Where is that tall house behind the basket now?
• Depth Perspective – A longer focal length lens makes objects behind our subject appear closer, even though in reality the distance has not changed. Again, look how close the house or fence appears in the 96mm shot.
• Perspective Compression – Even though the distance has not changed in the above point, we can use that depth-perspective compression to our advantage, and make the background appear more out of focus even if the actual DOF has not changed.
Now some of the "zoom with your feet" proponents may argue, "Yeah, but the only thing I care about is the subject, and that has not changed". Well actually—it did. Look at the size and shape of the flowers. At 96mm, the flowers all appear about the same size. With the 24mm, the flowers closer to the camera appear larger than the ones farther away. (Remember that when photographing faces, so you don't make people's noses big.) Again, just one more reason why we can't zoom with our feet.
So take this information and use it. Use it the next time you are selecting a lens for a shoot, or buying a new lens. What is the look YOU want for your images and what it is that YOU shoot. It's not just a matter of getting closer or farther away from your subject, or just magnification.
Let me also take a moment to dispel...well—not a myth—but a misconception. As photographers, we know that depth of field is affected by three things; aperture, focal length, and distance to subject. But in practice, when framing our subject the same way in the frame, focal length and distance to subject cancel each other out because WE MOVE proportionally. With our movement, our DOF actually remains the same. In this instance, moving to keep our subject framed the same, the only thing that changes DOF is...aperture.
Want more of Peter Tellone? Check out his website where he showcases much more of his amazing work.