The Importance of Lens Focal Length

The importance of lens focal length and why it’s not something left up to chance. And why “Zooming with your Feet” is misconstrued and only partially correct.

I thought these two images I shot yesterday really showed how different an image could look just by changing focal lengths or zooming if we have a zoom lens.

Both images were shot at the same location with My Canon 17-40L 4.0 (as featured in my profile at the Canon Lens Experience/B & H Photo & Video) The first was shot at 17mm very close to the Billboard, The other was shot at 40mm across the Highway. As we can see, while the same subjects, the images are very different in how they appear.

17 mm Image

17 mm Image

The 17mm allowed me to get close, But a couple things occurred: The Mountains in the distance seemed very far away, farther than the eye would perceive the perspective to be. The next thing that happened, since I was so close to the sign, was I needed to tilt the lens up causing a lot of Geometric distortion. I minimized the total distortion by keeping my Left/right swing parallel to the background leaving just the up/down non level part. As you can see the top of the sign “Falls over” and the fence bends in. Also while it is natural for something farther away to appear smaller, this exaggerates the phenomenon. You can also see the non parallel  of each end of the sign. Now this could have been helped if I had a ladder, climbed up and kept the camera level to the subject. But of course I didn’t

Now there’s nothing wrong with these distortions and sometimes we use them for “Whimsy” and playful fun in an image .

40mm Image

40mm Image

Next, I stepped across the street and I shot with a 40MM, Which accomplished quite a bit. First, since I was farther away I was able to keep the Lens closer to level and keep the sign from getting any up or down distortions. Next, It compressed the Background Perspective and brought the mountains more into view.. What didn’t happen quite is that I didn’t quite maintain the “Magnification” of the sign in the field of the image.

That could have been done in one of two ways. One, I could have “Zoomed with my feet” This is the only situation where Zooming with Your Feet is actually possible – Changing magnification. All the other qualities of changing lens focal length or ‘Zooming”; Angle of View, Perspective Compression, Perspective geometric distortion or lack there of…are not possible with “Zooming with your feet”

Or the other possibility was to change lenses and shoot with something like a 70mm from where I was. The reason I didn’t zoom with my feet is that would have put me in the middle of a busy highway between me and the sign.

Now one thing you may notice and may confuse you. It looks as if the 40mm has a wider field of view than the 17mm. The reason for this is…we moved. If we stayed in the same place, the 17mm has a much wider angle of view than the 40mm (93º vs. 50º). You’ll find in photography, physical movement has a lot more to do with a lot of the physics than anything else

Also don’t take away from this that 40mm is better than 17 mm because of the distortions in this image. All lenses distort, some just do it in a quicker manner (this is not talking about distortion with in the lens design)

So the take away from this is not that there is necessarily  a perfect lens for every image but more just to realize…  why you would change lenses. The actual reason you do may vary depending on your artistic viewpoint. You may need precision and exacting or you may want the playfulness that a different lens focal length can bring.  But the point is just knowing that changing focal lengths is not just about magnification or bringing that far object near.

Which also clarifying a bit the whole Zooming with your Feet. It’s not really possible except in one way.