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This feels like it should be obvious, but I'm failing to figure it out.

I own a Sony a5100 -- an APS-C camera with a 1.6 crop factor.

I have long owned the SEL50F18, a 50mm f/1.8 lens designed for APS-C.

I wanted a slightly-more-telephoto lens to add to my kit, and after having tons of fun reading reviews, I decided to buy an old manual Minolta MC ROKKOR-PG 50mm f/1.4 and an adapter to E-mount. It was cheap (about $75 for the set), and the idea of a fully-manual lens with some "character" sounded like fun.

Since the Minolta MC was for a 35mm film SLR, I anticipated the 1.6 crop factor making this lens have the same angle-of-view as an 80mm lens designed for APS-C (since the 50mm lens should produce an image circle much larger than my APS-C sensor)

The adapter is a cheap "Fotasy MD-NEX" adapter which has no optical elements.

So, I was very surprised when my gear arrived today and the new ROKKOR-PG produces images that have the same angle of view (or, at least, close enough that I can't tell the difference)

Where is the hole in my logic? Why do these two lenses produce images with similar angle-of-view?

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    \$\begingroup\$ Because they are both 50mm lenses. \$\endgroup\$
    – Michael C
    Commented Jun 3, 2017 at 4:36
  • \$\begingroup\$ I read both of those possible duplicates. Both are reasonable candidates, but the answers didn't click for me. But, I think I understand now: The 50mm APS-C produces an APS-C-sized image circle that's about 25deg AoV horizontal. The 50mm FF produces an image circle that's 1.6x the size, but is about 40deg AoV horizontal. My camera only captures the inner 25deg of this image circle -- which is the same angle as my 50mm APC-S produced. Have I got it right now? \$\endgroup\$
    – Andrew Shelansky
    Commented Jun 3, 2017 at 4:48
  • \$\begingroup\$ The a5100 has a 1.5x crop factor. Only canons have 1.6x crop factor \$\endgroup\$
    – Janardan S
    Commented Jun 3, 2017 at 11:46
  • \$\begingroup\$ Since I can't add an answer and the current answer is very confusing, the short answer is this: all lenses are marked with the actual focal length, not the 35mm equivalent, regardless of the sensor size that they're intended for. I was initially confused by this when researching which DSLR to buy. \$\endgroup\$
    – micheal65536
    Commented Feb 21, 2018 at 8:21

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Here's what you are missing: The crop factor multiplier only applies if you want to compare the same focal length lenses on two different cameras with different sensor sizes.

Used on the same camera, any lens with a 50mm focal length will give the same field of view (assuming both lenses cast an image circle large enough to completely cover the sensor). That is because focal length is a property of the lens, not of the size of the sensor. Angle of view is a property of both the focal length and the size of the sensor.

For more, please see these existing questions here at photography.stackexchange:
Why do Full Frame lenses and crop body lenses exhibit the same crop factor when used on a crop body camera?
Does my crop sensor camera actually turn my lenses into a longer focal length?
Does an APS-C lens label itself with effective or true focal length?
Shooting 50mm EF vs EF-S
What is crop factor and how does it relate to focal length?
What is "angle of view" in photography?
Is an EF 50mm f/1.4 the same as 50mm with an EF-S lens on a Canon 550D?
Will a 50mm EF lens give the same result as a 50mm EF-S lens?
What does 'Equivalent to 1.6x the focal length of the lens' mean?
FX glass on DX body
Why don't lens manufacters just label their lenses in 35mm equivalent terms?
Why are crop lenses indicated with focal lengths they don't have?
How does "designed for APS-C" affect focal length?

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    \$\begingroup\$ Yes, the edit "assuming both lenses cast an image circle large enough to completely cover the sensor" is sort-of the bit that was eluding me. The 50mm APS-C on a FF body would not have the same AoV as the 50mm FF would on a FF body -- because where the FF lenses would fill the edges of the frame with stuff that my APS-C camera can't see, the APS-C lens would fill those edges of the frame with darkness. But the middle 2/3 of the image would be +/- identical. \$\endgroup\$
    – Andrew Shelansky
    Commented Jun 3, 2017 at 5:25
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    \$\begingroup\$ @AndrewShelansky more like half, but yeah. A 1.5X crop factor is a linear measure. The area of the large sensor would be 2.25X as large as the area of a sensor 1/1.5X as wide/tall. \$\endgroup\$
    – Michael C
    Commented Jun 3, 2017 at 5:29
  • \$\begingroup\$ I included some illustrations to my answer to this question linked above. \$\endgroup\$
    – Michael C
    Commented Jun 3, 2017 at 5:31

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