Derrel
Mr. Rain Cloud
- Joined
- Jul 23, 2009
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- USA
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- Can others edit my Photos
- Photos OK to edit
A couple of comments would be that the focus distance calculated from the film/sensor plane, and not the front of the lens. Second, in macro shooting depth of field is not a function of lens focal length, but is almost entirely based upon image magnification: using a 60mm macro lens and a 180mm macro lens and achieving the same,exact size images, I was able to prove that strange fact to myself some years ago.So, to re-state, at macro ranges, the focused distance has hardly any impact whatsoever,and depth of field is entirely a function of image magnification; that, however changes at some point. also, and this is weird, but at macro magnifications, and indeed throughout most of the close-up range, depth of field is almost the same in front of the focus point and behind the focus point; this firmly goes against the old simple "rule" that DOF is 1/3 in front of the focused point and 2/3 behind the focused point. A thoughtful examination of that old,simple "rule" will tell us something, and that something is in this next paragraph.
Under "close to normal" focusing distances, let's say roughly two feet to 15 feet, as in your tiger and travel coffee cup experiment, depth of field is "reasonably shallow" when the focusing distances are short. However, one depth of field principle that your experiment does not show is that as camera-to-subject distances move beyond about 15 feet to 20 feet, the depth of field on a small-sensor or small-format camera approach the hyperfocal distance EXTREMELY rapidly, and depth of field increases at an amazing,almost exponential rate. And the depth of field at these longer camera-to-subject distances extends roughly one third in front of the focus point, and 2/3 of the total depth of field lies BEHIND the focusing point. At longer focusing distances, longer than this indoor-kitchen table-to kitchen wall range, depth of field increases EXTREMELY RAPIDLY BEHIND THE FOCUS POINT. In other words, depth of field is not linear: it kind of "snowballs" as camera to subject distance increases.
So, depth of field is actually not described accurately by your indoor experiment. What your experiment shows is close-range depth of field behavior that is nether macro-range, nor medium to long range. If, for example, your stuffed animal had been positioned at 22 feet from the camera, the background would be rendered much,much more clearly with both the 18mm setting and also the 55mm setting. So when you wrote, "No need for theoretical calculations. Just look at the pictures." that comment does not actually describe the true behavior of depth of field; your testing scenario is missing two of the major "different" behaviors of depth of field, because your do not have DOF scenario 1) an extremely close situation, nor do you have Scenario 3) a situation where the focused distance approaches within 50% of the hyperfocal focusing distance.
A good article on this subject can be found buried somewhere within the many pages in the Bob Atkins web pages.
Under "close to normal" focusing distances, let's say roughly two feet to 15 feet, as in your tiger and travel coffee cup experiment, depth of field is "reasonably shallow" when the focusing distances are short. However, one depth of field principle that your experiment does not show is that as camera-to-subject distances move beyond about 15 feet to 20 feet, the depth of field on a small-sensor or small-format camera approach the hyperfocal distance EXTREMELY rapidly, and depth of field increases at an amazing,almost exponential rate. And the depth of field at these longer camera-to-subject distances extends roughly one third in front of the focus point, and 2/3 of the total depth of field lies BEHIND the focusing point. At longer focusing distances, longer than this indoor-kitchen table-to kitchen wall range, depth of field increases EXTREMELY RAPIDLY BEHIND THE FOCUS POINT. In other words, depth of field is not linear: it kind of "snowballs" as camera to subject distance increases.
So, depth of field is actually not described accurately by your indoor experiment. What your experiment shows is close-range depth of field behavior that is nether macro-range, nor medium to long range. If, for example, your stuffed animal had been positioned at 22 feet from the camera, the background would be rendered much,much more clearly with both the 18mm setting and also the 55mm setting. So when you wrote, "No need for theoretical calculations. Just look at the pictures." that comment does not actually describe the true behavior of depth of field; your testing scenario is missing two of the major "different" behaviors of depth of field, because your do not have DOF scenario 1) an extremely close situation, nor do you have Scenario 3) a situation where the focused distance approaches within 50% of the hyperfocal focusing distance.
A good article on this subject can be found buried somewhere within the many pages in the Bob Atkins web pages.