The fallacy of 35mm "equivalent focal lengths"

Started 2 months ago | Discussions thread
Chris R-UK Forum Pro • Posts: 19,212
My opinions
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I am going to state some of my opinions but this is a complex area and others may disagree or I may make some errors.  Please feel free to disagree or correct errors.

As others have said, the usage of "equivalent focal length" on this site and elsewhere is well defined and useful.  The concept of "reach" put forward by the OP is not well defined and its usefulness, to me at least, is limited.

"Equivalent focal length" is useful for questions like:

  • What is the best focal length for head and shoulders portraits?
  • What focal length do I need to photograph the interior of Notre Dame in Paris?
  • What focal length do I need to photograph soccer games from the sideline?
  • What focal length do I need for a safari in East Africa/South Africa/India?

"Reach" is not useful for the first three questions and only of limited use for the fourth because other variables such as light, terrain and atmospheric haze affect the answer.

I think that "reach", as used on this thread, seems to be based on extreme situations where you need to go to 1:1 resolution to see the target.  Birders have a rule of thumb that in these situations you need to use the camera that gets the most "pixels on the target". So birders would be familiar the idea of "reach".  The OP's example (in one of his later posts) of trying to photograph a coyote on a hill would also be subject to this rule.

However, very few of us use 100% crops in, for example, competitions.  Why?  Because, if you need a 100% crop, then image quality is going to be poor anyway.  So we switch to a longer lens or get closer if we can.  In my case, if I can't do either of those, I don't take the shot.  The OP's coyote on a hill some distance away would be an example of this, especially if it was moving.  I would take a shot and blow it up if I needed to confirm that it was a coyote, but I wouldn't expect that shot to be usable for anything else other than that.

So, for me "equivalent focal length" as used on this forum is a very useful concept.  The OP's concept of "reach", if it could be defined and measured, might also be useful for some situations, but it wouldn't replace "equivalent focal length".

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Chris R

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