Why does aperture matter less for depth of field at long subject distances?
Asked 2/18/2020
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Using a Sony RX100 III at its 24mm equivalent view, I noticed that when focusing on subjects roughly 50–100 meters away, changing aperture from wide open to stopped down seems to have much less effect on depth of field than it does at close distances. Is that expected? Why does depth of field increase so much as subject distance gets larger, and what are the trade-offs of leaving the aperture wide open in this situation?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
2 Answers
5
That observation is true.
The nearer to the lens, the smaller the depth of field.
If you use an online DoF calculator like e.g. https://www.dofmaster.com/dofjs.html you can easily see this.
Example: full frame, 50mm f1.4
at 1m -> DoF appr 0.02m
at 3m -> DoF appr 0.19m
at 10m -> DoF appr 2.16m
at 50m -> DoF appr 75.40m
at 95m -> DoF infinite (behind)
So, once you have reached a certain distance, the aperture will not give you more DoF behind the focal point, but it might influence the near limit where the DoF starts. You can use this to your advantage to calculate the best DoF for landscape photography.
You should also be aware in this context, that the aperture has an effect on the sharpness. There is no fixed aperture of the best performance of a lens, so you need to test that for each lens. Many lenses have their peak performance around f3.2 to f5.6, but your mileage may vary greatly. If you chose a very small aperture like beyond f16, you start getting refraction problems, which decreases lens performance. So it might be worth checking for the sweet spot of each lens. Or in your case, the sweet spot of the fixed lens.
Originally by user88965. Source · Licensed CC BY-SA 4.0
user88965
6y ago
0
Generated from our catalog & community — verify before relying on it.
Yes — that’s expected. Depth of field grows rapidly as subject distance increases, so at long distances a much larger range appears acceptably sharp even if you open the aperture. Eventually, when focused far enough away, the far depth-of-field limit can extend to infinity, so stopping down further mainly affects the near limit rather than adding much more “behind” the focus point.
This behavior is tied to the circle of confusion: blur becomes less noticeable as distant subjects project smaller blur circles relative to the final image.
So if your subject is far away, aperture often has less visible effect on depth of field than it does for close-up subjects.
The trade-offs of shooting wide open are mainly:
- less depth of field on nearer subjects
- possible changes in lens sharpness, since lenses are not always at their sharpest wide open
In other words, for distant scenes you may be able to use a wide aperture without losing much practical depth of field, but image sharpness can still change depending on the lens.
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