Originally posted by careaga I have a K-50 and have been toying with the idea of getting an 85 mm lens for portraiture. At B&H I was looking at some K mount lenses (manual) and it said they were for full frame and were equivalent to APS-C Focal Length: 127.5 mm. I understand that my K-50 isn't full frame and I have read about the math used to convert full frame (lens length) to APS, which might as well be Chinese to me, math not being my strong suite.
My question is this: I have a 135 mm manual lens that works well with my K-50. Would I be wasting money on getting a lens so close to what I already have? Bear in mind I am NOT a pro. I am barely an enthusiast.
Any help/advise would be greatly appreciated.
Ignore what they say about equivalence, it is just an 85mm lens and that's it. You can pretty much ignore full frame equivalence math if you aren't switching between different systems, just get used to what focal length means in terms of your camera and you'll be fine.
Full frame equivalence math only came about because people were used to how their lenses performed on film (full frame, a.k.a. 35mm 'sensor' size) and wanted to wrap their heads around how lenses would perform on their new digital cameras with smaller sensors. A good analogy would be to think of it like inches and centimeters. If you think well in terms of inches, you take measurements in centimeters and convert them to inches so the information is easier for you to wrap your head around, or vice versa. (Just like you might convert someone's height measurement from 175cm to 69 inches or 5'9" since that's the format you are used to seeing.)
If you never switch between systems, you never need to worry about the math. (Though if you do switch to full frame in the future, you'll probably have a good understanding of what focal lengths mean on APS-C and you may end up doing the math in reverse.)
These ranges aren't set in stone and everybody has different tastes, but here's a rough guide:
Ultra wide angle.....10mm (APS-C) / 16mm (Full Frame) .................... Creative architecture, landscapes, starry skies, etc.
Wide angle lens.....18mm (APS-C) / 28mm (Full Frame) ..................... Commonly used for landscapes, group pictures
Normal lens...........35mm (APS-C) / 50mm (Full Frame) ..................... 'Normal' being closest to the way you see the world with your eyes
Portrait lens......50-90mm (APS-C) / 70-135mm (Full Frame) .............. 'Portraits' can range from full body to head shots, so the range is quite subjective.
Telephoto lens.... >70mm (APS-C) / >100mm (Full Frame)