The Compression Effect
When you photograph a distant aircraft against the Moon with a long telephoto lens, your distance to the aircraft and your focal length determine how large the Moon appears relative to the plane. This has nothing to do with the Moon's actual size or whether the image is real.
The angular size of an object depends on the ratio of its actual width to your distance from it. A longer lens simply magnifies what's already at a large angle from your position. If you're 1.5 km from a 747 and the Moon happens to pass behind it as viewed from your location, the magnification and apparent size are set by that geometry, not by focal length alone.
Interactive: Angular Size by Distance
Adjust your distance from a 737 (fuselage width 35.8 m) and see how its angular size changes. The Moon's angular diameter is about 0.52 degrees.
At 1.5 km, a 737 spans about 1.37 degrees, making the Moon (0.52°) appear roughly 2.6 times smaller. Move closer and the aircraft grows larger. The focal length you choose determines magnification uniformly across the frame, but does not change these ratios.
Try It Yourself
The numbers above describe the geometry; this tool lets you drive it. Pick an aircraft, set its altitude and the Moon's position in the sky, and change the focal length to see exactly how the ratio in your viewfinder changes. Switch to the Crowd & Launch tab to see why a telephoto shot of a rocket launch from a mile or more away can still put a crowd of spectators and a 200-foot rocket in the same frame at a believable scale, the exact situation behind the New Glenn photo below, with the rocket lifting off on a loop so you can watch that relationship change in real time.
Interactive simulator built by John Kraus (Skyline).
Depth of Field and Hyperfocal Distance
The second objection is about focus: "How can anything 240,000 miles away be in sharp focus at the same time as something 1 km away?" The answer is hyperfocal distance. With a long telephoto lens stopped down, the zone of acceptable sharpness extends far into the distance.
At a given focal length and aperture, there is a nearest distance such that everything from that distance to infinity appears sharp. This is the hyperfocal distance. With a 600 mm lens at f/8 on a full-frame sensor, the hyperfocal distance is about 1.5 km. Everything from 1.5 km forward is in focus.
f = focal length (mm)
N = f-stop (aperture)
c = circle of confusion (≈ 0.03 mm for full-frame)
| Focal Length | Aperture | Hyperfocal (FF) | Hyperfocal (APS-C) |
|---|---|---|---|
| 400 mm | f/5.6 | 953 m | 1,429 m |
| 600 mm | f/8 | 1,501 m | 2,251 m |
| 800 mm | f/11 | 1,940 m | 2,910 m |
| 1200 mm | f/13 | 3,694 m | 5,540 m |
Shoot an aircraft transit at 1.5 km away with a 600 mm lens at f/8. Focus on the aircraft. Everything from roughly 750 m to infinity reads as acceptably sharp. The Moon is infinitely distant, so it falls well within that zone.
Examples
Real lunar and solar transits, shot with this in mind. Each was taken at a known distance with documented exposure settings.
View the full gallery and technical details at Photos of Stuff.
Further Reading
Lens compression is sometimes confused with other optical phenomena. Here is what it is and is not:
Lens compression is real. It is the optical fact that apparent size ratios between near and far objects depend on your distance to the near object and the object's actual size. It is not a property of the lens; it is a property of your position and perspective.
Background blur (bokeh) is real. A longer focal length at the same subject distance gives shallower depth of field, producing more background blur. This is often conflated with compression and is a separate optical effect.
Lens distortion is real. Wide lenses show pincushion or barrel distortion; longer lenses are optically "straighter" and show less distortion.
What compression is not: compression does not mean the lens is flattening the image, bending space, or causing objects to appear closer together relative to their actual arrangement. If a building and a mountain are farther apart in space than your photograph suggests, it is because you are viewing them from a very specific angle where they overlap as seen from your location, not because the lens has compressed them.