Apertia · Library

How does sensor aspect ratio change the part of a scene that fits?

At 2 m, a 50 mm lens on full frame covers about 1367 × 911 mm while 25 mm on Micro Four Thirds covers 1349 × 1014 mm; compare width and height together.

Same distance, different rectangle

Apertia's sensor presets keep the real width and height of each format: 36 × 24 mm for full frame (3:2), 17.3 × 13 mm for Micro Four Thirds (about 4:3), 23.5 × 15.6 mm for APS-C. At a finite focus distance the field-of-view tool divides each sensor dimension by the thin-lens magnification v ÷ u, so the scene width and scene height scale by the same factor and keep the sensor's proportions.

That is why matching one dimension is not enough. Two setups that fit the same width can differ noticeably in height when their aspect ratios differ, and a subject that is taller than it is wide meets the vertical limit first.

Worked example

The 36 × 24 mm model gives approximately 1367 × 911 mm of coverage in this setup. A subject that fits within the width can still exceed the height.

Width before rotation or crop (mm): 1367.051975
Height before rotation or crop (mm): 911.367983

Fit a subject in both directions

Measure the subject's width and height, and the distance from the sensor plane to it.
Choose the sensor preset and the actual focal length, then enter that distance as the focus distance.
Compare the subject's width with the scene width and its height with the scene height.
If you shoot in portrait orientation, swap the two scene dimensions before comparing.
If you will crop to another ratio later, recalculate the cropped height or width from the scene values.

Worked example: 3:2 against 4:3

Both setups are focused at 2000 mm. A 50 mm lens on full frame gives roughly 1367 mm by 911 mm. A 25 mm lens on Micro Four Thirds, which the app describes with the same 50 mm full-frame equivalent, gives about 1349 mm by 1014 mm. The widths differ by less than 20 mm, but the 4:3 frame is about 100 mm taller. A poster 1000 mm high fits the Micro Four Thirds frame and does not fit the full-frame one.

Full frame, 50 mm, 2000 mm: 1367 × 911 mm (ratio 1.5)
Micro Four Thirds, 25 mm, 2000 mm: 1349 × 1014 mm (ratio 1.331)

Common mistakes

Comparing equivalent focal lengths and assuming the framing is identical; equivalence matches the diagonal, not both sides.
Checking only the width of a tall subject such as a doorway or a standing person.
Forgetting that a 16:9 video mode records a smaller height than the still-photo preset.
Using the infinity angle of view for a close subject instead of the scene size at the focus distance.

Limits of the rectangle

The scene size is a thin-lens estimate for a flat subject square to the camera. Distortion, focus breathing and camera-mode crops are not included; the Nikon focal-length guide and the DOFMaster equations below describe the angle and distance relations it relies on.