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Tuesday, September 16, 2008

Back to Basics - Intro / Lens Compression

Back to Basics

As I mentioned in yesterday's blog, we're going to be adding a new feature to our regular line-up. From time to time, friends and family (actually clients too) will pull me aside and ask for a little deeper explanation of this or that having to do with photography. Usually it's a tidbit that would benefit a larger audience, or at the very least, be interesting to write about. That got me thinking, and it is from this inspiration, "Back to Basics" was born.

Back to Basics will focus on "Essential Elements of Photography" which means I'm going to try to stay away from camera or hardware specific questions and keep the focus on broader topics that apply to photography in general. If at any time, you have an idea, suggestion, or request for topics to be featured in this new category, just post a message to the blog, and I'll see what I can do.

Well that's it for the introduction, let's move on to our first article.

Lens Compression

While leading the Charlottesville, VA walk as part of the Scott Kelby Worldwide Photowalk, one of my fellow photowalkers overheard another photographer comment, "I love the compression I'm getting out of this lens." A little confused by what he meant, she pulled me aside a few minutes later and asked for an explanation. Here's an expanded version of what I told her (with a little more geometry than most of us like to think about when we're out shooting).

Lens compression describes a perceived phenomenon that a lens with a longer focal length appears to decrease depth perspective and shorter focal length lenses increase depth perspective. That is a wide angle lens makes the distance between a subject and the background appear to expand, and telephoto lenses appear to make the distance between the subject and background contract.

Sounds pretty cool, right? Well it is, and it's useful when composing your shots. I headed across the street from my neighborhood again to try to get the shots I couldn't get yesterday. They show the extremes of lens compression in action.

The first shot was done with a Nikon 24-70mm lens, at 24mm, f2.8. Notice that the statue in the foreground takes up about one-third the vertical height of the frame.

The second shot is of the same statue, this time taken with a Nikon 70-200mm, at 150mm, f2.8. The statue doesn't appear to have grown any larger (still takes up about a third of the vertical height of the frame) but suddenly the church is much, much closer. Voila!! Lens compression!

So what the heck is actually going on here? Well, it's only partially the lens that's making the magic happen, the other key factor involves where you're standing. But first, we'll take a couple steps back... (pun intended) Sparing the gory details of the mathematics the govern the optics, what you need to know is a basic concept called "angle of view". Angle of view is essentially the vertical and horizontal angle that is visible to the sensor (or film) through the lens. In wide angle lenses, you have a very wide angle of view, conversely in telephoto lenses the angle of view is very narrow. If you click on the diagram below, you'll see a little better what I'm saying.

Looking at the two figures, the top figure shows that the statue is completely within the angle of view (as shown in the first example photo) and the angle of view lines expand far above the church. Its for this reason that we're able to see so much of the sky in the first photograph. Now, when the telephoto lens is put on the camera, with its narrower angle of view, the camera has to move much farther back for the statue to take up the same portion of the angle of view. In essence, I had to move back to make the statue take up the same amount of the frame. As shown in the lower diagram, these narrower angle of view lines extend to include only a portion of the church, giving the illusion that the church is actually closer in the second photograph. It was, however, the fact that I moved back to take second shot which caused the illusion.

So what should you take from this? For a photographer, the mathematics are interesting but pretty useless. (I know someone is going to make me eat those words, but for now I'm going to stick by them.) What is important is that lens compression is a very real phenomenon and is a useful tool for composing your photographs. Remember this, the smaller the focal length of the lens the greater the perceived distance between the subject and the background, and conversely the larger the focal length the smaller the perceived distance.

That's all for now everybody. Love to hear your comments.

Update: After talking to with one of our regular readers, Emilio, it was noted that the article might imply that the lens swap was required to acheive the lens compression effect. This isn't the case. The only reason I swapped out lenses between the two shots was to have a wider range of focal length. Had I used my Nikon 18-200mm VR, I could have acheived the same effect just by "zooming" between 24mm and 150mm on the same lens. That may have been a little less confusing, eliminating one of the variables above... hope the overall message still came through. Thanks Emilio!

Oh, and by the way... The font for "Back to Basics" is actually Emilio's handwriting, generated using the ScanAHand tool discussed in last Friday's post.

All the best, Rob 

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Comments

Thanks for that explanation. Now a question relating the the photographer's comment "I love the compression I'm getting from this lens". Do some lens compress more at the same focal length? And also, is it just a stylistic thing or is there a rule of thumb as to when compression is desirable and when it isn't (i.e. portraits, landscapes, etc...)?

Julie,

Lens compression is a result of focal length, and hence should not vary for a particular focal length from lens to lens.

Generally speaking, the use of lens compression is a "stylistic thing" but many photogs ascribe to a rule of them that long focal lengths can provide a more flattering portrait.

Thanks for commenting and for the great questions... let me know if you have any more.

All the best, Rob

I enjoyed the scan a hand, but 30 bucks for
my handwriting...i don't think so.

The explanation between telephoto and wide angle
was informative and fun to comprehend...

You definitely have a natural talent!!!

dad

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