Scene Compression Uncompressed

I read a lot of blogs, follow a lot of YouTube channels and subscribe to many “handy tips” postings that come into my mailbox daily.  One such recent posting was from Tim Grey, a respected Photoshop expert and professional photographer.

Grey Learning

Viewers had posted questions about the long standing belief that as you use lenses of longer and longer focal length, and compare the same scene shot through these different lenses, the apparent separation between foreground and background diminishes with focal length.  In fact, this has been a long accepted “generality”, passed on from photographer to photographer, that scene “compression” occurs with telephoto lenses.  But as with many things, the details get somewhat “blurry” (pardon the pun) and the specifics of the effect are often not explained.

First, what is scene “compression”?  In photography, we are converting a three dimensional space into a two dimensional image.  In creating that image, we define a relationship for the one dimension we lose – the front to back relationship between foreground, midground and background elements of the image.  By positioning ourselves relative to these elements, and selecting a lens and settings, we create a unique composition.

Over time, photographers started to notice that the relationship between foreground and background elements could be altered apparently by changing the focal length of the lens.  Shorter focal lengths appeared to create more separation; longer focal lengths appeared to bring these elements closer together.  To prove it, various pundits did “tests” where the same foreground and background elements were shot with different lenses, ensuring that the foreground element always remained the same size in both frames.

SLR WarsLo and behold, the effect was real.  The background appeared to be much closer to the foreground subject when shot with a telephoto lens.

Any scientist will tell you that an experiment, to be meaningful, has to identify and control for factors other than those being tested, in order to validate the result.  Read any scientific journal and there is clear information on these factors and how they were handled.

Unfortunately, the first proponents of lens compression didn’t do that.  Yes, the resulting images had foreground subjects consistently of the same size.  But how were those consistent sizes obtained?  They were obtained by shifting the PHYSICAL position of the photographer to RECOMPOSE the image and counter the larger foreground image size of the subject when using a longer focal length lens.  Photographers physically stepped back to get the shot.

Say what?  That means all compositional elements were not the same with only the lens focal length changed.  As it turns out, stepping back to recompose and keep the foreground subject consistently the same size CHANGES THE PHYSICAL RELATIONSHIP and perspective of the photographer relative to the scene.  Because the foreground subject is closer to the lens than the background, any changes to the relative positon of the foreground subject will be magnified in relation to changes in the relative position of the background.  This gives the appearance that the background has shifted forward toward the subject when a longer focal length lens is used.

MythDebunkedAs proof, Tim (and others, I might add now that I’ve checked) noted that when photographers shot the same images again, this time without physically moving themselves or any element in the scene and only switched lenses, the effect disappeared.  To correctly provide a meaningful comparison, the image with the smaller focal length lens was cropped to the same framing as the telephoto image and enlarged.  This resulted, you guessed it, two images that were exactly the same (except for the loss of resolution in the cropped image).

Duh!  This result proves two things:

  1.  That a flawed experimental design can produce flawed results.
  2.  That all rules, principles or stated behaviours should be thoroughly understood before accepting them as fact.

I was one of those who believed what I had heard.  Now I know better.


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