I can only speak from my experience with EOS-1D Mk IV bodies. With them, I found every lens I use must be adjusted to get the sharpest focus possible. Each lens required a different amount of micro adjustment to gain the best sharpness, some as little as -2 (closer to camera) with a 28-105L and as much as -14 with a 50, 1.2L! Other lenses I have fall somewhere in between these two. Checking the same lenses on the other 1D4 body required about the same adjustment. Several lenses looked very sharp initially. However, on closer look , especially at long focal lengths or close distances when using a precise focusing point (eye or very small subject), it was clear the focus point of the lens was off (sharper behind the intended point of focus). Taking the small amount of time to fine tune each lens to the body made all the difference in the world! I don't use the fancy equipment recommended by some others, not that they don't work, it 's just that I have found a method that works for me! I set the focus point (one shot) to the center one, set the shutter speed to 500, select the maximum aperture on the lens, set Auto ISO, and turn off the IS. Using a steady rest, I focus on a small subject on the ground, outdoors in full sunlight (seed, pebble, or other very small subject). I closely place other small objects along a line closer and farther away from the center subject. I try to place the subject just beyond the minimum focus distance of each lens. For zoom lenses or telephoto lenses, I will use the longest focal length. The camera is at a 45 degree angle above the subject to give depth to the shot. I shoot a couple of frames and then check the focus, make the adjustment and try again until the focus is sharpest possible. This usually takes less than 10 minutes total time. Again, I have found this method works for me. Bottom line, no lens I have is its sharpest with "0" adjustment! Hope this helps!