The Pentax 645D uses the Pentax 645AF2 mount with a 0.79× crop factor (about 43.8 × 32.9 mm sensor). Below: equivalent focal lengths, maximum shutter speeds for sharp stars, a hyperfocal distance chart and how to check the shutter count.
Equivalent focal length & field of view
| Lens focal length | Full-frame equivalent | Diagonal FOV | Horizontal FOV |
|---|---|---|---|
| 14 mm | 11 mm | 125.9° | 114.8° |
| 16 mm | 13 mm | 119.4° | 107.7° |
| 18 mm | 14 mm | 113.4° | 101.2° |
| 20 mm | 16 mm | 107.7° | 95.2° |
| 24 mm | 19 mm | 97.5° | 84.8° |
| 28 mm | 22 mm | 88.7° | 76.1° |
| 35 mm | 28 mm | 76.1° | 64.1° |
| 50 mm | 40 mm | 57.4° | 47.3° |
| 85 mm | 67 mm | 35.7° | 28.9° |
| 135 mm | 107 mm | 22.9° | 18.4° |
| 200 mm | 158 mm | 15.6° | 12.5° |
| 300 mm | 237 mm | 10.4° | 8.4° |
| 400 mm | 316 mm | 7.8° | 6.3° |
| 600 mm | 474 mm | 5.2° | 4.2° |
Multiply by the 0.79× crop factor for the full-frame equivalent field of view.
Milky Way exposure: NPF and 500 rule
| Focal length | NPF @ f/1.8 | NPF @ f/2.8 | Strict NPF @ f/2.8 | 500 rule |
|---|---|---|---|---|
| 14 mm | 17.1 s | 19.6 s | 10.6 s | 45.2 s |
| 16 mm | 15.0 s | 17.2 s | 9.3 s | 39.6 s |
| 18 mm | 13.3 s | 15.3 s | 8.3 s | 35.2 s |
| 20 mm | 12.0 s | 13.8 s | 7.4 s | 31.6 s |
| 24 mm | 10.0 s | 11.5 s | 6.2 s | 26.4 s |
| 28 mm | 8.6 s | 9.8 s | 5.3 s | 22.6 s |
| 35 mm | 6.9 s | 7.9 s | 4.3 s | 18.1 s |
| 50 mm | 4.8 s | 5.5 s | 3.0 s | 12.7 s |
NPF = (35 × f-number + 30 × pixel pitch) ÷ focal length, with a pixel pitch of 5.9 µm (typical for this format — exact resolution not in our database). The strict version (Michaud's detailed NPF for the galactic core) gives pin-point stars at 100% view; the 500 rule is a loose rule of thumb that shows trailing on modern sensors.
Hyperfocal distance chart
| Focal length | f/2.8 | f/5.6 | f/8 | f/11 | f/16 |
|---|---|---|---|---|---|
| 16 mm | 2.4 m | 1.2 m | 0.9 m | 0.6 m | 0.4 m |
| 18 mm | 3.1 m | 1.5 m | 1.1 m | 0.8 m | 0.6 m |
| 20 mm | 3.8 m | 1.9 m | 1.3 m | 1.0 m | 0.7 m |
| 24 mm | 5.4 m | 2.7 m | 1.9 m | 1.4 m | 1.0 m |
| 28 mm | 7.4 m | 3.7 m | 2.6 m | 1.9 m | 1.3 m |
| 35 mm | 12 m | 5.8 m | 4.1 m | 3.0 m | 2.0 m |
| 50 mm | 24 m | 12 m | 8.3 m | 6.0 m | 4.2 m |
Focus at the hyperfocal distance and everything from half that distance to infinity is acceptably sharp (circle of confusion 0.038 mm). Use the hyperfocal calculator for other settings.
How to check the Pentax 645D shutter count
Pentax and Ricoh DSLRs store the count in MakerNotes; ExifTool decodes it with `exiftool -ShutterCount image.dng`.
ExifTool tag: ShutterCount. Always test an original, unedited file straight from the card.
Where to buy
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