The fallacy of 35mm "equivalent focal lengths"

Started 3 months ago | Discussions thread
OP Tony Beach Forum Pro • Posts: 11,934
Re: The fallacy of 35mm "equivalent focal lengths"

jrtrent wrote:

Tony Beach wrote:

jrtrent wrote:

Tony Beach wrote:

Preface

Ironically, even here at DPR equivalent focal lengths are often divorced from a complete understanding of what is or isn't equivalent. There is this idea that if a lens covers a certain FOV on a given format then it is somehow equivalent to whatever focal length that would cover that FOV on a 35mm format camera.

And I thank them for divorcing other issues from this basic and useful concept.

Then apparently you stopped reading after the first paragraph.

I read the whole thing,

Then you read the part where I wrote that, " 'Reach' is one of the factors I was considering, " in deciding which camera to buy for my girlfriend's birthday.

I just disagree with you that optical resolution, pixel density, and aperture diameter have anything to do with equivalent focal length.

It's more fundamental than that. There is no such thing as an equivalent focal length, there is just a focal length.

I currently use two different digital cameras, a crop sensor DSLR and a smaller-sensored compact, but whichever one I shoot, I choose a focal length for its field of view, and 99% of the time my chosen focal length is about a 50mm equivalent.

Again, the only thing that is equivalent is the FOV. Changing the focal length to achieve that has ramifications for image quality as well as DOF. As I wrote in the OP, for many it's not a big deal, so if that includes you then you're done here.

I compose in the viewfinder and do not crop my images later.

So I'm sitting with my girlfriend at an Agility event and she sees a coyote on the hill and wants me to get a photo of it (this actually happened). Do I use the S95 I have in my pocket that I mostly use to shoot videos of her running her dogs, or do I use the 85mm lens sitting on my D800 that's sitting nearby? If you believe the marketing the compact camera is the better tool, but the marketing is often fallacious.

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