Originally posted by kh1234567890 It is also useful to lock the exposure. And no need to turn the camera upside down, just get the distant subject in all four corners of the frame in four shots.
Yes you are totally correct you only need to take one picture instead of turning the camera upside down for a second. However I prefer the upside down camera approach for a couple reasons; for example:
•1 The sun may be at the right of the frame and cause loss of contrast on that side, (even though it doesn't make a noticeable flare on the photograph) from this loss of contrast on the right side, the left side may appear sharper even though the lens is not decentered.
•2 The photographer may be mistaken that the objects on the left and right side are on the same focus plane, thus making one side appear sharper depending on which side the focus is weighted more correctly.
•3 Even if the objects on both sides are at true infinity for the lens and therefore on the same focus plane the object on one side might still be closer say 400 m vs. 480 m. Even though they're on the same focus plane the objects that are 400 m away may still appear sharper than the objects 480 m away just by virtue that more pixel real estate may be allotted to rendering detail on the nearer objects. And if the objects are at something like 1 mi. vs. 1.5 mi. atmospheric haze can become a problem.
•4 Additionally the light could be hitting the objects on one side differently than the other, (such as highlighting) affecting perceived sharpness, or the objects themselves might have different properties, such as a fur coat resting on a park bench vs. a stop sign, obviously with the stop sign having clear defined edges it is much more likely to appear sharp than a fur coat at equal distance.
I always recommend taking the same picture with the camera flipped as I believe this way is a more, "apples to apples comparison" and much more unlikely to create a false positive for the reasons mentioned above. [I know in your post you say, "distant subject" which should eliminate problem 2 but not problem 1]
Of course, again, you're totally correct that theoretically flipping the camera is unnecessary. And of course you're also correct about locking the exposure for even though it is unlikely that the camera would choose to change exposure for taking the same shot a few seconds later it is still possible and it is possible that that difference in exposure could affect perceived sharpness and that the photographer might not catch that the photos were exposed differently. And of course there are some other things we could add to this such as it may also be beneficial to stay out of auto-pic mode, (as it may choose to render each shot differently) etc.
This is why I say to, " repeat the process a few times to confirm conclusion" just in case something like this might have happened, (or if the photographer happens to be such a noob he can't recognize one of the two photos happens to be blurry from camera shake and misdiagnosis it as decentering).
However I wanted to be able to stick all the instructions on a little photo and still be readable so I simplified the instructions slightly but not to the extent that they either cannot diagnose decentering or are likely to arrive at a false positive.