There are many things we do to an image after taking the picture that can affect the Depth of Field (DoF).
- Any time you crop an image and display the crop at the same size as the original you are altering the Depth of Field because you have increased the enlargement of the captured image.
- Any time you increase or decrease the display size at the same viewing distance you alter the DoF.
- Anytime you change the viewing distance of the same photo you alter the DoF.
Understanding what DoF is and what it is not is important here.
In a way, depth-of-field is an illusion. There is only one plane of sharpest focus. Everything in front of or behind the point of focus is out of focus to one degree or another. What we call DoF is the area where things look, to our eyes, like they are in focus. This is based on the ability of the human eye to resolve certain minute differences at a particular distance. If the slightly out-of-focus blur is smaller than our eye's capability to resolve the detail then it appears to be in focus.
When you magnify a portion of an image by making it larger or moving closer to it you allow your eye to see details that before were too close together to be seen by your eyes as separate pieces of the image. There is no magic barrier beyond which everything is equally blurry and inside of which everything is equally in focus!
Since things are gradually blurrier in a photo the further they are from the point of focus, as one gradually increases the magnification at which the image is viewed the perceived depth of field gets shallower as the near and far points where one's eyes can discriminate fine details move closer to the point of focus.
Perhaps a lot of the confusion stems from the unstated assumptions used by many Depth of Field calculators. They make assumptions about viewing size, viewing distance, the viewer's vision, and the size of the image sensor or photographic negative. If most users of DoF tables and calculators are not aware of the assumptions in place it's easy to understand how the idea that DoF is an intrinsic quality of an image apart from viewing conditions has gained so much traction.
For many years during the 20th century the assumptions were an 8x10" print viewed at a distance of 10" by a person with 20/20 vision. Some DOF calculations, such as those used by lens manufacturer Zeiss, assumed the viewer had 20/15 vision! As is stated above, any time any of those variable are changed the perception of DoF in the same image is altered.
In the current environment, most of those assumptions are no longer applicable. We routinely view images at sizes ranging from near postage stamp size on our portable electronics to the various sizes of our computer monitors to large screen televisions viewed from fairly close distances, to billboard sized banners not always viewed at typical billboard sized viewing distances.
When we view them on our computers we even change the DoF depending on whether we choose to view the entire image scaled to fit our screen or whether we choose to view one piece of the image zoomed in to "100%" where each pixel of the image is depicted by a pixel of the screen we are using. If part of a 24 MP image is viewed at 100% on a 23" HD monitor with 1920x1080 resolution that's the equivalent of seeing the entire image displayed at 60x40 inches! If we view a 50 MP image on the same monitor we're looking at a small piece of a 125x83 inch display size of the 50MP image! Since we've more than doubled the amount of enlargement applied to an image viewed from the same distance we've also effectively halved the DoF of the larger resolution image compared to the lower resolution one if they were both shot under the same conditions: sensor size, focal length, aperture, and subject/focus distance.
One online DoF calculator that does allow changes to viewing conditions is found at Cambridge in Color. To get the additional options please click on the "show advanced" button.
For further reading here at Photography.stack exchange, please see:
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