[Just1MoreDave]

If only actual focal length matters, the DA* 55/1.4 was a mistake. I give the lens designers credit for knowing more about this than me.

Besides the technical issues of certain focal lengths, the relationship with your subject is different at 5 feet than it is at 15 feet.

[Giklab]

The OP brings up an interesting point. If we focus (lol) on the 'flattering/unflattering' properties of focal lengths, we discover that the most used lenses are short to medium telephotos on FF.

Food for thought: There was a thread somewhere about "normal" lenses, and I got a lot of flak for saying that 28 (or whatever) mm's are not really that normal on APS-C because of barrel distortion. Now answer this: On a Q, a "replacement" for the classic portrait lens would have a FL of 15mm. What if you used a 15mm fisheye?

[Cannikin]

Perspective is a function of camera distance only, nothing else. Focal length, whether true or "equivalent", has no direct effect on perspective.

Now if you're concerned about having the same framing with the same perspective, obviously "equivalent" focal length is the one that matters, as you would be standing in the same spot at "equivalent" FLs. True focal length is entirely irrelevant as far as perspective goes.

[HSV]

Thanks to y'all for the input. I agree and understand that there's a relationship between distance, FL, framing, and distortion.

Let me see if I'm understanding this...let's say that you put a 50mm lens on a m4/3 body and a100mm lens on a FF body. If both are placed at the same distance from the subject, then the results will be equal? (in terms of unflattering distortion) How about putting a DA15 on a Q, and a FA85 on a FF?

I'm putting the example with the DA15 because it doesn't have much "native" distortion. Would (potentially) this mean that every lens can be a so-called portrait lens if I'm willing to crop? (without regards to other factors like DOF, resolving power, sharpness, noise, etc.)

Would this apply also to Giklab's fisheye example?

[fuent104]

Let me see if I'm understanding this...let's say that you put a 50mm lens on a m4/3 body and a100mm lens on a FF body. If both are placed at the same distance from the subject, then the results will be equal?

Yes, this is correct. At least, it is theoretically correct (often what manufacturers list as focal length is not exactly precise).

this mean that every lens can be a so-called portrait lens if I'm willing to crop? (without regards to other factors like DOF, resolving power, sharpness, noise, etc.)

Yes, you are correct, again. But that is very theoretical, and you would need to overlook most of the properties of lenses and sensors in order to operate in that manner.

One thing I have found helpful is to actually think in terms of field of view. I.e., when you look at something (from wherever you are), crop an image of it in your mind. Pay attention to what is in the crop (in your mind). Then, move closer or farther from the object, and do the same thing. You will notice the difference.

[HSV]

Thanks for the clarification.

Just curious, how about the "compression effect"? Does this also depend on distance or there's a true correlation to focal length?

I would assume that this one should be correlated to true focal length.

[Ikarus]

Thanks for the clarification.

Just curious, how about the "compression effect"?

Same thing. That's just what everybody was referring to by 'perspective (distortion)'. It's what flattens or elongates the face in these portrait shots that Eastman already pointed to above.