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I'm having issues wrapping my head around how to get crop factor based on 35mm. I understand what crop factor is but my math is confusing me.

For example, everyone knows that aps-c is a 1.5x crop but from my calculations an aps-c sensor with the dimensions "23.6x15.6" will give you a 1.53x crop.

Is this correct, or is it standard practice to round to only the first place with crop factor? I'm working on a super accurate focal length calculator so it seems this should be right.

My math is as follows:

(√((23.6^2)+(15.6^2))) / 43.2666153056

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    "I'm working on a super accurate focal length calculator so it seems this should be right." Crop factor has no effect on focal length. – Philip Kendall Apr 10 '18 at 21:19
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    What's the point calculating a super precise crop factor while focal length isn't that precise? – Gerhardh Apr 10 '18 at 22:07
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    Things that "everybody knows" are frequently incorrect. – Caleb Apr 10 '18 at 22:19
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    "Everybody" also knows that a typical 50mm lens can be anything from 43-57mm in actual focal length, too. And that is just when it is focused at infinity. Since most still camera lenses "breath" when focused at shorter distances, all bets are even more off then. And that "70-300mm" zoom you have is really more likely a 77-78mm to 285-292mm zoom when focused at infinity. You and most everyone else doesn't want to know the real focal lengths when you focus closer with many of them, either. – Michael C Apr 11 '18 at 2:34
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    @Agent_L Film sizes weren't exactly cut and dried either. The rough edges, caused by the uneven edges of felt seals, meant that prints from film were usually cropped slightly from the actual film format dimensions. For more, please see: Why aren't the 35mm film scans I got back from a lab at a 3:2 aspect ratio? – Michael C Apr 11 '18 at 18:29
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You are correct.

Like most manufacturers, Nikon APS-C sensor sizes vary depending on the camera model. Nikon crop factor can be 1.52, 1.53, 1.55, or 1.56

A list of the most common crop factors for APS-C, from the Wikipedia article on APS-C:

A crop factor (sometimes referred to as a "focal length multiplier", even though the actual focal length is the same) can be used to calculate the field of view in 35 mm terms from the actual focal length. The most common multiplier ratios:

  • 1.7× — Sigma DP1, Sigma DP2, Sigma SD15, Sigma SD14, Sigma SD10, Sigma SD9, Canon EOS DCS 3†
  • 1.6× — Canon EOS 7D†, 7D Mark II, 50D†, 60D†, 70D†, 77D (9000D), 80D, 550D (T2i)†, 600D (T3i/X5)†, 650D (T4i/X6i)†, 700D (T5i/X7i)†, 750D (T6i/X8i), 760D (T6s/8000D), 800D (T7i/X9i) 1100D (T3/X50)†, 1200D (T5/X70)†, 1300D (T6/X80); Canon EOS M†, M2† (sold in Asia only), M3 (not sold in North America until Oct 2015), M5, M6, M10
  • 1.56× — Nikon D3100†
  • 1.55× — Sony Alpha 58, Nikon D3200†
  • 1.54× — Pentax K20D,†[5] Samsung NX5†, Samsung NX10†, Samsung NX11†, Samsung NX100†, Sony NEX-5†
  • 1.53× — Pentax K-3, K-S1, K-S2, K-5 II†, K-5†, K10D†, K200D†, Ricoh GR, Nikon D3300†, Nikon D5300†, Nikon D5500, Nikon Coolpix A, Sony NEX-7,† Sony NEX-6, Sony α6000†, Sony α6300, Sony Alpha 77, Sony Alpha 77 II, Sony Alpha 65, Sony Alpha 57, Sony Alpha 37, Sony Alpha 580, Sony Alpha 500, all Samsung NX except NX5, NX10, NX11, NX100
  • 1.52× — All Nikon DX format DSLR cameras except D3100, D3200, D3300, D5300, and D5500; Pentax K-30†, K-01, K-50, K-500;† all Fujifilm interchangeable lens X-system cameras and Fujifilm X100 series; Sony Alpha 100; Sigma SD1, Sigma SD1 Merrill, Sigma DP1 Merrill, Sigma DP2 Merrill
  • 1.35× — Sigma SD Quattro H
  • 1.3ׇ — Canon EOS-1D Mark IV†, 1D Mark III†, 1D Mark II† (and Mark II N), EOS-1D†, Kodak DCS 460†, Kodak DCS 560†, Kodak DCS 660†, Kodak DCS 760†, Leica M8, M8.2

Notes:

† Discontinued
‡ A 1.3× Focal Length Multiplier is also known as APS-H. Actual multiplier factor is 1.255× for the 1D Digital, 1.28× for the Canon EOS-1D Mark III and 1.29× for the Canon EOS-1D Mark IV. Leica M8 is 1.33×

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    Canon "1.6X" APS-C sensors range from 1.6X to 1.61X depending upon the exact model. – Michael C Apr 11 '18 at 2:27
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    Mostly just a screen dump from another web site is not appropriate here. – Olin Lathrop Apr 27 '18 at 11:41
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Nikon 1.5X APS-C sensors in their current lineup are actually 1.52-1.53X depending on the exact measurements of the various different sensors in different models. Some older, discontinued models in the D3x00 series are slightly smaller at 1.55-1.56X.

The difference between 1.52X and 1.53X is 0.65789 percent.

The difference between 1.5X and 1.53X is a mere 2 percent.

The difference between Nikon 1.5X APS-C sensors (actually 1.52-1.53X depending on the exact measurements of various different sensors in different models) and Canon 1.6X APS-C sensors (actually 1.6-1.61X depending on the specific sensor) is only around 5 percent.

The difference between stated/marketed focal lengths of lenses and the actual focal length of the same lens can be as high as 10% or more when the lens is focused at infinity.

There are some fairly expensive lenses that breathe so heavily that the field of view can vary by 30% or more when focused at the minimum focus distance. Take for instance, the Nikon AF-S 70-200mm f/2.8G VR II. At maximum zoom (marked 200mm) and minimum focus distance the field of view projected onto a full frame sensor is equivalent to a focal length of only about 134mm! That's only 67% of the stated focal length!

Can you see where this is going?

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It doesn't really matter, but two decimal digits is more precision than is practically useful.

... or is it standard practice to round to only the first place with crop factor?

This is good practice, if not always standard practice.

That's because in practice, it doesn't matter. Few other things are going to be so precise that going to two decimal places makes a noticeable difference. A lens that is nominally "50mm" only has that focal length when focused at infinity — and may be actually not 50mm exactly even then.

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    forget the exact quote...it's something along the lines of the most accurate you can be is equal to the least accurate measure. IMO, crop factors are only important because of decades of 35mm shooting and the more prevalent use of focal lengths to communicate differences in lenses over fields of view. – Hueco Apr 10 '18 at 23:29
  • @Corey Sure. And that's my point: this isn't the least accurate measure. Even if you really care for some reason, it doesn't matter. – mattdm Apr 11 '18 at 0:07
  • Yep. Right there with ya. – Hueco Apr 11 '18 at 0:36

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