Originally posted by Navmaxlp So, I think this is what I was thinking originally but I want to make sure I understand. It seems like the conversion from FF to APSC has to do with multiple things. The main difference is image size on the sensor for the same picture. Meaning, the smaller APSC will capture a smaller portion of what comes through the lens. In order to stand in the same spot and achieve the same sized image on the sensor, you would need a lens with a wider field of view. 1.5 times wider to be exact. If you wanted to use the same lens on APSC as on FF for it's distortion properties, you would have to move further away from the subject. The only problem with that is, moving away from the subject creates additional distortion in the image that may or may not be desired. Is that all correct?
Basically, what I'm getting from this is; stop trying to figure out which lens is "best" and figure out what I want to accomplish within the photo. Then use the lens and distance that best captures what I'm looking for. Is that right? Like I said earlier, right now, I've got an A 50mm 2.0 and a 100mm 2.8 Macro. It seems like these may be fine for certain portraits I'm looking for but not for others. Would you all say adding a DA 70 (because there's no way I can afford the FA 77) would kind of round things out for me? It will certainly be some time before I can pick it up (months to years) but I like knowing where I'm going so I can plan for it. In the mean time, I can also use my 18-135mm for anything needed between the two primes. The DA 50 1.8 might be in the future as well but it will depend on how the manual focus of the 50 2.0 goes. I know it's not a highly regarded lens but, for me, I think it will do fine.
Thanks for all the feedback. I love nerding out on a lot of this stuff. It definitely helps me to understand how it all works.
That sounds about right.
The only issue is that when you say "distortion," you might be talking about two different issues. Some lenses do have an intrinsic distortion. They distort straight lines into curved lines. That makes buildings, windows, and other rectangular objects look like inflated barrels or pinched-in pin-cushions. That type of distortion is a property of the lens that independent of the sensor format (although barrel or pincushion distortion is less obvious on a crop sensor). Note that some zooms have barrel distortion at the wide end and pin cushion at the long end.
But if you are talking about "distortion" of facial features from using different lenses, then that is only a matter of lens-to-subject distance, not focal length. Stand 10 feet from the person and photograph them with an ultrawide angle lens and you'll find that their face in the center of the picture does not show any "wide angle" distortion. Stand 1 foot from a person's nose and photograph them with a 200 mm telephoto and their nose will become a bulbous protuberance in front of an overly-rounded head.
Thus, you want to first set the distance to the subject to set the look of the shape of the face and then pick a focal length that creates the width/height of frame width you want around the face/torso/body.