1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

Lens mm conversion on APS-C camera

Discussion in 'Help Team' started by Chris Bell, May 16, 2017.

  1. Chris Bell

    Chris Bell New Member

    Sorry to be a pain on this subject that I’m sure many ask about, but after watching lots of videos and searching over the web I’m more confused than when I started

    If anyone is talking about focal lengths recommended for certain scenarios, are they normally always talking in full frame sensor lens MM unless otherwise stated?

    On my a6000 with its APS-C lens does that mean I always have to take the x1.5 crop factor into account? For example if someone recommends a 50mm focal length, I need to look at 75mm aps-c lens

    What about if I put a full frame lens on aps-c camera? What calculations do I have to do then?

  2. EightBitTony

    EightBitTony Well-Known Member

    When lenses are advertised, they're advertised with 35mm equivalent focal lengths.

    So a 50mm lens, is always a 50mm lens.

    When you take the shot on an APS-C camera, you will get a field of view equivalent to 75mm on a 35mm camera.

    So if you want a 35mm equivalent 50mm field of view, shooting with an APS-C lens, you'll need to use a focal length of ~33mm.

    So, lenses are sold / advertised with 35mm equivalent focal lengths. No matter what you stick them on, they remain the same, so a 70mm lens is always a 70mm lens.

    However, the actual image circle as presented to the sensor, varies depending on the sensor size, which affects the resulting apparent field of view of the image, and you need to do that maths if you want to get equivalent looking shots with all other things being equal.

    Your example above is the wrong way around. If someone recommends a 50mm focal length, on an APS-C camera you'll need to be shooting at ~33mm.
  3. Roger_Provins

    Roger_Provins Well-Known Member

    To achieve the same field of view on an aps-c sensor as quoted for a 35mm full frame you need to divide by 1.5.

    e.g. 50mm divided by 1.5 = 33.33333 nearest available lens would be a 35mm

    Edit I was a few seconds to slow. :)
  4. Chris Bell

    Chris Bell New Member

    Ok, thanks for the speedy reply!

    So the kit lens the a6000 comes with shows 16-50mm on the front, the lens will always be 16-50mm but on my APS-C sensor I will get a field of view equivalent of 24mm to 75mm of a 35mm camera

    So if someone recommends 50mm, I use 33mm on any lens type that is on my a6000, be it a APS-C or full frame lens?
  5. PeteRob

    PeteRob Well-Known Member

    It is a bit of a throw-back in time to when photographers were used to 35 mm film cameras and they were familiar with the field of view you would get from, say a 50 mm lens. Because an APS-C sensor is smaller than a 35 mm frame only part of the image was captured - the angle of view was smaller. So you have to be careful of context when talking about focal lengths as to whether you are referring to the true focal length or the equivalent [field of view] focal length. Lenses for interchangeable lens digital cameras are marked with true focal lengths so a standard for APS-C camera (DSLR or CSC) would be an actual 18-55 mm zoom. Compact cameras are often marked with equivalent [field of view] focal lengths - my wife's bridge camera has marks on the lens showing out to 600 mm when the lens is certainly a lot shorter than that.

    As Tony said you have your conversion upside down. For a so-called crop factor of 1.5 the field of view of a 50 mm lens would be that of a 75 mm lens on a 35 mm camera.

    I don't know why they persist with this. The vast majority of people these days have never used a 35 mm camera and it is only really useful if you are trying to make an equivalence with something you are familiar with.
  6. EightBitTony

    EightBitTony Well-Known Member

    Yes. Exactly.
  7. PeteRob

    PeteRob Well-Known Member

    Yup. But remember that these recommendations are echoing past practices. 35 mm SLR cameras in the main used to come wth a 50 mm lens as standard. It is a good all-rounder lens, it can be used close up without too much perspective distortion, you can get a fair bit in the frame, they were easy to make, had a large aperture so they were useful in low light and relatively cheap to buy. There is nothing particularly magic about 50 mm.
    EightBitTony likes this.
  8. Roger_Provins

    Roger_Provins Well-Known Member

    It's just as relevant for comparing with full frame digital cameras.
  9. Andrew Flannigan

    Andrew Flannigan Well-Known Member

  10. Benchista

    Benchista Which Tyler

    Not really - I use the same lenses on two different formats, full frame and APS-C. How would I refer to them?

    Focal length is a permanent property of a lens, angle of view ain't.
    Roger Hicks likes this.
  11. El_Sid

    El_Sid Well-Known Member

    So not really better then... At least with focal length you are always dealing with a fixed definition...[​IMG]
  12. Andrew Flannigan

    Andrew Flannigan Well-Known Member

    Yes. For those of us who know it's not a problem but when people talk about equivalent focal length and real focal length at the same time the uninitiated do have a problem which is where this thread started.
  13. Benchista

    Benchista Which Tyler

    Be even worse if you used angle of view, though.
  14. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    when people used Medium format and 35mm cameras, no one talked about the medium format equivalent lens. this was even more so for those who used various large format cameras and lenses.

    You just learned the appropriate focal lengths for your cameras to suite your purpose.
    Andrew Flannigan likes this.
  15. Benchista

    Benchista Which Tyler

    Quite a few people even then talked about the 35mm equivalent, though. at least for MF.
  16. Roger Hicks

    Roger Hicks Well-Known Member

    Dear Nick,

    Quite. I have 300mm lenses for Nikons (full frame and DX) and 4x5 inch, 5x7 inch and 8x10 inch (where it's a "standard" or "normal" lens, with about the same angle of view as 45mm on 35mm). Whereas 15mm is a "standard" lens on the Minox 8x11mm format.

    Not so sure about equivalents, though. Most people had a rough idea of the equivalents for the common formats, though of course different image proportions make these of limited use. Surely most people knew that on 6x6cm you normally had 75-85mm, usually 80; on 6x9, 100mm or 105mm; and after that, guesswork and interpolation. There's a piece on equivalents on our .eu site.


  17. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    I never heard that.... And I was using medium format from 1945, Perhaps it was an amateur thing.

    When I started medium and large format were the Norm, not 35mm.
  18. Benchista

    Benchista Which Tyler

    I've still got various books with conversion tables, too, but it was mostly in magazines when they talked about MF focal lengths often in terms of their (nearest) 35mm equivalent, taking into account that could be inaccurate because of format differences, as Roger says.
    Roger Hicks likes this.
  19. Terrywoodenpic

    Terrywoodenpic Well-Known Member

    I suppose my bibles from that era was the Ilford manual of Photography, the BJ Almanac, and the Focal encyclopedia of Photography, while format and paper sizes were certainly compared, I do not recall any of them ever comparing focal lengths.

    little more was said on the subject except that "Normal" was around the diagonal of the format.
    Which would make 35mm about 42mm; 2 1/4 sq about 85mm; and 5x4 around 160mm; none of which are common focal lengths. However as 2 1/4 negs were normally cropped and printed on to 10x8 paper (as if taken 6x4.5 mm.) The 75 to 80 mm was very close.
  20. Jacques Rebaque

    Jacques Rebaque Active Member

    I would have thought the angle of view was a fixed property of a lens, and a thereby a function of it's focal length, just because we only use a percentage of that image circle doesn't mean it isn't there.

Share This Page