Why is APS-C usually called 1.5× crop when the math gives about 1.53×?
Asked 4/10/2018
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I’m calculating crop factor relative to 35mm full frame using the sensor diagonal. For an APS-C sensor listed as 23.6 × 15.6 mm, I get a crop factor of about 1.53× rather than the commonly stated 1.5×. Is that correct, and is 1.5× just a rounded convention? I’m building a focal length/field-of-view calculator, so I’m wondering whether I should use 1.53× (or model-specific values) instead of the simplified 1.5× figure.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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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?
Originally by user15871. Source · Licensed CC BY-SA 4.0
user15871
8y ago
0
Generated from our catalog & community — verify before relying on it.
Yes—your result is essentially correct. Crop factor is based on the ratio of the 35mm/full-frame diagonal to the sensor’s diagonal, and many “APS-C” sensors are not exactly the same size across brands or even across camera models.
In practice, “1.5×” is a rounded, conventional shorthand for many APS-C cameras. Actual values can be around 1.52×–1.53× for many Nikon APS-C sensors, with some models slightly smaller and closer to 1.55×–1.56×. Canon APS-C is commonly called 1.6×, though the exact value can also vary slightly.
For a calculator, using the actual sensor dimensions for each camera model is the most accurate approach. But for general discussion, one decimal place is usually enough because other real-world variables are less precise anyway—for example, a lens’s marked focal length is nominal and can vary, especially with focus distance.
So: use exact sensor dimensions if accuracy matters in your tool; use “1.5×” as a practical rounded label in ordinary conversation.
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