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So this is a question about aps-c and full frame 'lenses' and not sensor format. My sensor here is constant - a Canon APS-C.

This might be an already answered question but I couldn't find any that completely clarified me. There are so many confusing answers to this I got from multiple sources. Many people on YouTube keep mentioning the zoomed in effect of the aps-c sensor which got me confused. So apologies for asking the same thing again.

As I said I have a Canon crop format (1.6x) and till now I've been using the kit zoom lens (EF-S) for astrophotography. I've decided to switch to a fast prime lens and I know that I like the FOV I get at around 50-55mm with my current EF-S lens.

Now the cheapest fast prime i could get for this job seems to be the Canon Nifty Fifty and I can get hold of one easily. But this is the 'EF' lens and not 'EF-S'.

My question is, to get the same fov at a certain focal length of an APS-C lens on an APS-C sensor, should I get the same labelled focal length on a full frame lens, or should I get an EF lens 1.6x wider? (50mm EF or 30mm EF to mimic 50mm EF-S?).

In other words, are APS-C (EF-S) lenses labelled with a focal length that gives equivalent FOV of a full frame lens, or labelled with its actual focal length?

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    Focal length is focal length. You will get the same FOV with your sensor with any 50mm lens that you can mount. EF-S lenses are normally labeled with the actual focal length. They will then say xxx effective focal length, which represents the focal length on a full frame camera that gives the same FOV as this lens on APS-C. Your 50mm will have an effective focal length of 80mm.
    – Ross Millikan
    Jun 22, 2022 at 4:00
  • @RossMillikan so I should get the 30mm EF lens to replace my 50mm EF-S?
    – NRB
    Jun 22, 2022 at 5:02
  • No, you should get the 50mm EF lens to replace your 50mm EF-S
    – Ross Millikan
    Jun 22, 2022 at 5:03
  • @RossMillikan alright thanks. I seem to be confused still but I'll try to understand this.
    – NRB
    Jun 22, 2022 at 5:04
  • Have a look at this - it's dealing primarily with perspective, but in that journey compares the same lens on two bodies, crop & FF. When I first started out, this puzzled me too, so this is how I came to understand it. photo.stackexchange.com/a/97747/57929
    – Tetsujin
    Jun 22, 2022 at 7:09

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Field of view is the real-world angle captured on the image. It is determined by the quotient of the sensor size and the focal length.

That's basic geometry. If you want to keep the angles when reducing one part of the geometry (the sensor size), you have to reduce the other parts of the geometry (here: the focal length) by the same factor.

As you don't change the sensor size (always use a "crop" body), all 50mm lenses will give you the same field of view, no matter whether the focal length comes from a prime lens, a zoom lens zoomed to 50mm, regardless of the type of mount being EF or EF-S or whatever.

If you wanted to switch over to a full-frame-body, this would mean a 1.6x enlargement of the sensor size. To get the same field of view, you'd have to enlarge the focal length by the same factor 1.6, meaning you'd need an 80mm lens then.

Regarding EF and EF-S lenses:

  • EF lenses are optimized to produce a good-quality image over the full 24*36mm area. Using an EF lens on a crop body works perfectly well, as the smaller crop sensor is well within the "quality" area of the lens.
  • EF-S lenses only cover a smaller area with good quality: the cropped 15*22.5mm. Trying to use an EF-S lens on an EF body would give you images where the corners experience all kinds of quality degradation. That concern is the reason why Canon chose to make the EF-S lenses not fit onto the EF body mount, they are only mechanically compatible the other way round.
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    So in essence I have to worry about angular FOV only when I'm dealing with different sensor sizes, not with lenses of the same 'labelled' focal length. Which boils down to the fact that focal lengths labelled on all lenses are true focal length and only the image circle and backfocus changes.
    – NRB
    Jun 22, 2022 at 13:13
  • @NRB Correct. I've never seen an interchangeable lens labelled with something else than the real focal length.
    – Ralf Kleberhoff
    Jun 22, 2022 at 15:36
  • Alright thanks I think I finally understand it
    – NRB
    Jun 24, 2022 at 14:52

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