DSLR and mirrorless sensors for astrophotography: sizes, pixel pitch and crop factor

Interchangeable-lens cameras are sold on megapixels and sensor format, so their spec sheets are built for photographers rather than for plate-scale arithmetic. Every maker here publishes two things we can use: a sensor size in millimetres and a recorded image size in pixels. Pixel pitch is the missing third number for most of them.

What each maker publishes

Canon is the exception. Its specification sheets list a “pixel unit”: the EOS R6 Mark II sheet gives approximately 6.00 µm square next to a 36.0 × 24.0 mm sensor, and 6000 pixels × 6.00 µm is exactly that width. Nikon’s Z 8 page gives a 35.9 × 23.9 mm sensor and an 8256 × 5504 image but no pitch, and the same is true of the Sony and Fujifilm pages in our data. For those bodies the table derives pitch and marks it ‡:

derived pitch (µm) = sensor width (mm) ÷ pixels across × 1000

That gives 4.35 µm for the Nikon Z 8 and D850, 5.94 µm for the Z6III, Z f and Z5II, 8.40 µm for the Sony Alpha 7S III, 3.76 µm for the Alpha 7R V and 3.04 µm for the Fujifilm X-T5. A derived value assumes the recorded image spans the whole published sensor width, which is close but not guaranteed. Canon’s own values are rounded too: the EOS R7 and 90D list 3.2 µm, and 6960 × 3.2 µm is 22.27 mm against a published 22.3 mm.

Crop factor from the diagonal

crop factor = 43.27 mm ÷ sensor diagonal (mm)

where 43.27 mm is the diagonal of 36 × 24 mm. Canon’s 22.3 × 14.8 mm APS-C bodies come to 1.62, the EOS 850D’s 22.3 × 14.9 mm to 1.61, and the Fujifilm X-T5’s 23.5 × 15.7 mm to 1.53. The “full-frame” sensors are not all identical: Nikon’s 35.9 × 23.9 mm is 1.003 and the Sony Alpha 7S III’s 35.6 × 23.8 mm is 1.010. Those differences are far smaller than the spread between APS-C brands.

Wide lenses need the exact formula

The field table below uses 2 × arctan(size ÷ 2f), which assumes a rectilinear lens. The shortcut “size ÷ focal length in radians” fails badly at short focal lengths: for 36 mm at 14 mm it gives 147.3°, while the exact value is 104.3°. By 135 mm they differ by less than 1 %. Fisheye lenses use a different projection and do not follow either. The field of view formula guide explains the difference.

Why pitch matters for untracked shots

Field depends on sensor size, but trailing is measured in pixels. Stars on the celestial equator drift about 15.041″ per second (360° per sidereal day). On a 24 mm lens, a 10-second exposure lets a star drift 150.4″. That is 2.1 pixels on the Alpha 7S III (72.19″/px) but 4.7 pixels on the Alpha 7R V (32.31″/px), two full-frame bodies with the same field. The 500 rule gives 500 ÷ 24 = 20.8 s for both because it ignores pitch; the NPF rule includes it. The untracked exposure calculator runs both, and NPF rule vs 500 rule compares them.

The Canon EOS Ra

The EOS Ra is the one body here with a filter Canon specifies for hydrogen-alpha. Its specification sheet lists the low-pass filter’s transmission sensitivity for hydrogen-alpha at 656.28 nm, 6720 × 4480 recorded pixels and a 5.36 µm pixel unit, and it allows roughly 30× magnification for manual focusing. Its status is unclear: we found no Canon page stating that it is current or discontinued, so we record its status as unknown rather than guessing.