Does subject distance affect focus when using a 35mm lens at f/1.8?
Asked 11/26/2015
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I understand the basics of depth of field, but I’m confused about whether there is a maximum distance I should be from my subject when shooting with a 35mm lens at f/1.8. If I move farther away, will the subject itself eventually go out of focus, or does aperture only affect how much is acceptably sharp in front of and behind the focus point?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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I think your understanding of Depth-Of-Field (DOF) needs some clarification:
We can sharply focus our camera on a subject regardless of camera to subject distance (super close distances fall under special circumstances). If our subject is a person we likely focus on the eyes. If no DOF existed, the eyes would be sharp and clear though the nose and ears would be out-of-focus. The fact is; a zone exits before and after the point focused upon that images with reasonable focus. This is the span we call depth-of-field.
The span of acceptable focus is not split down the middle; it extends 2/3 to the rear and 1/3 back towards the camera. For example: Subject at 8 feet, aperture f/11, zone of DOF 6 feet thru 15 feet.
We can expand the length of the zone of DOF by:
a. Using tiny diameter apertures like f/11, or f/16 or f/22
b. Causing the subject to camera distance to increase
c. Mounting a short focus (wide-angle) lens
We can diminish the length of the zone of DOF by:
a. Using large diameter apertures like f/1/4 or f/1.8 or f/2
b. Placing the subject close to the camera
c. Mounting a longer than normal focal length lens
Related to DOF is a topic called hyperfocal distance: We can consult tables and charts that give the span of DOF for every distance and aperture setting. Likely these same tables will give the hyperfocal distance settings. The hyper focal distance is commonly used by landscape photographers and for those occasions when focusing the camera is a too time consuming or bothersome.
Setting the camera to its hyperfocal distance maximizes DOF. Say you find a table that tells you to set your camera at the hyperfocal setting ; focus at the 11 feet mark, set the aperture at f/16. Now your DOF will span 3 feet to infinity ∞ (as far as the eye can see).
Originally by user44949. Source · Licensed CC BY-SA 4.0
user44949
10y ago
0
Generated from our catalog & community — verify before relying on it.
There isn’t a “furthest distance” where a properly focused subject suddenly goes out of focus just because you’re using f/1.8. If you focus on the subject, that subject is in focus at that distance.
What aperture changes is depth of field: how much area in front of and behind your focus point looks acceptably sharp. At f/1.8, that zone is narrower; at higher f-numbers like f/8 or f/11, it becomes wider.
Depth of field depends mainly on aperture, focal length, and subject distance, and also somewhat on sensor size and how large the image is viewed or printed. So if you move farther away, the depth of field changes, but the point you focused on remains in focus.
If you want to know exactly how much will be sharp for a given setup, use a depth-of-field calculator. For example, with a 35mm lens, the area that appears sharp may extend only a limited distance beyond your subject at f/1.8, while stopping down increases that range.
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