Do you need to refocus a camera lens after a 15°C temperature drop for astrophotography?
Asked 12/9/2013
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When shooting stars at infinity focus, how much can a normal camera lens shift focus as the temperature drops during the night? For example, if focus is set around 5°C and the temperature later falls to -10°C, is there a practical way to predict the shift in advance? I'm especially wondering whether a taped focus mark made earlier would still be reliable, or whether a 15°C drop is enough that refocusing is typically needed on a small DSLR-style camera with a 14mm lens.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
12y ago
2 Answers
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From my reading, the amount materials within a "handheld" lens expand / contract over, for example, 15C is so minimal it's not worth thinking about.
However this really becomes an issue on large telescopes, both refracting and reflecting (even more so).
Why?
As a (rough) example, let's imagine a large reflecting telescope which has a body length of 3m, and is constructed from tubular aluminium. Pure aluminium pure has a linear thermal expansion rate of 0.0000222m/m/K, which means it gets longer / shorter by 0.0222mm per degree Kelvin (or C) per meter of its length.
Therefore the telescope would shorten by 0.0222mm × 3m × 15C = 0.999mm as the temperature drops 15C. This coupled with the magnification at the secondary mirror leads to a dramatic focus shift.
Originally by user9999. Source · Licensed CC BY-SA 4.0
user9999
12y ago
0
Generated from our catalog & community — verify before relying on it.
For a normal handheld camera lens, a 15°C temperature drop is usually too small to worry about in any predictable, calculation-friendly way. Thermal expansion and refractive-index changes do affect focus, but in camera lenses the effect is generally very small compared with large telescopes, where long structures can shift focus dramatically.
In practice, there isn’t a simple universal formula you can apply to your 14mm lens, because the shift depends on the lens design, materials, and how temperature changes propagate through the optics over time. A taped focus mark can still be useful as a starting point, but it may not remain perfectly accurate if conditions change.
The best guidance is practical: test your own lens. If you want to know whether the shift matters, compare image sharpness or MTF at different temperatures with the same setup. For astrophotography at infinity, it’s wise to check focus again after the lens has cooled, especially if temperatures continue dropping. So: for small camera lenses, temperature-related focus shift is real but often modest; for critical star sharpness, recheck focus rather than trying to calculate it precisely.
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