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Angle of view by focal length

A regular wide-angle lens (rectilinear, the kind that keeps straight lines straight) covers a horizontal angle of 2 × arctan(sensor width ÷ (2 × focal length)). A full-frame sensor is 36mm wide, so a 16mm lens covers 96.7° across a landscape frame. The same angles on a crop sensor take a shorter lens; the last three columns are the focal length that gives the same horizontal angle on each.

Sensor sizes used: full frame 36 × 24mm, APS-C 23.5 × 15.6mm (Sony, Nikon and Fujifilm), Canon APS-C 22.3 × 14.9mm, Micro Four Thirds 17.3 × 13mm. Exact sizes vary by a few tenths of a millimeter between bodies, which moves the angles by well under a degree. Most interiors are shot between 16 and 24mm on full frame, as our guide to camera and lens choice explains, and the calculator works for any focal length in between.

Why a corner shot never gets both walls end to end

Stand exactly in the corner of a square room and the two walls running away from you are 90° apart, so the lens needs a horizontal angle of view of at least 90° to show both of them all the way to the next corners: 18mm on full frame, about 11.8mm on a Sony, Nikon or Fujifilm APS-C body and 8.7mm on Micro Four Thirds. Two things push that higher in a real room. The tripod sits a foot or so out of the corner, which puts the ends of those walls further off to each side, and in a room that isn't square, aiming at the far corner leaves more of one wall out than the other. In a 12 × 14 ft room with the lens 1 ft out, a 16mm full-frame lens shows all of the 12 ft wall but cuts the nearest 1.3 ft of the 14 ft one; getting both end to end would take 110°, about 12.6mm.

That's normal, and going wider to fix it usually looks worse than the cut. The corner shot sells the room because it shows two walls receding to the far corner, which is why our guide to photographing small rooms shoots across the longest diagonal. The near ends of the walls are the least interesting part of the room, and the other corner shot picks them up.

Equivalent focal length on a crop sensor

The "full-frame equivalent" printed in reviews multiplies the focal length by the crop factor, which compares sensor diagonals: about 1.53× for Sony, Nikon and Fujifilm APS-C, 1.61× for Canon and 2× for Micro Four Thirds. For a 3:2 sensor that matches the horizontal angle too. Micro Four Thirds frames are 4:3, narrower for their diagonal, so a 7mm lens is called a 14mm equivalent but covers 102.0° across, against 104.3° for a real 14mm on full frame. The calculator uses each sensor's actual width for the angles and shows the diagonal equivalent alongside.

Why wide lenses stretch the edges

A rectilinear lens keeps walls and door frames straight by stretching everything toward the edges of the frame. At an angle of x off center, a round object comes out 1 ÷ cos(x) times as wide as it is tall: 1.25× at the side edges of a 24mm full-frame frame, 1.51× at 16mm and 1.63× at 14mm, and more again in the corners of the frame. That's the pulled-sideways armchair and the egg-shaped dining table in an ultra-wide listing photo. Lens profile corrections in Lightroom or your editor remove barrel distortion, the bowing of straight lines; they can't remove this stretch, because it's what straight lines cost. It's why the guidance is to stay at 16mm or longer on full frame (about 10.5mm on APS-C) unless the room really leaves nowhere to back up, and to keep furniture out of the outer edges when you do go wide.