Originally posted by donlass Are you saying, I must be 4 inches from the subject in order to focus? Not 3 inches not 4.5? 4 inches?
With a Raynox 250 attached to any lens, any FL, you could not focus on an object 4.5 inches away. To be in focus, the subject would have to be four inches away or less, but I'm uncertain how much less is possible, but not much, maybe an inch, maybe much less than an inch closer than four inches
The 250 Raynox, when attached to a lens of any focal length, or a zoom lens set to any focal length, with the lens set to a correct infinity focus*, causes the lens to focus on an object that is four inches from the front of the lens (not from the position of the camera's sensor). If the focus ring is turned the lens will focus closer, but not by a great deal. Focusing using the focus ring may be complicated because the lens may extend forward, which will also change the focus distance. If it is a zoom lens, focused @ infinity, with a Raynox 250 attached, an object four inches from the end of the lens should be in focus regardless of the focal length to which the zoom is adjusted. However, if you have (as an example) a 55~300mm Pentax on your camera, set to 55mm, focused @ infinity, with a Raynox 250, and there is an insect four inches from the filter threads of the lens, it will be in focus, but if you zoom out even a little, to 75mm, the insect will go out of focus because the lens will extend so the insect will now be less than 4 inches from the filter threads. You
might be able to re-obtain focus by pressing the shutter release and allowing the AF to hunt, but the AF might not be able to refocus. The surest way to obtain focus when the subject-to-lens distance is very short is to move the entire camera + lens, camera set to MF. The problems of focusing at such short distances are the reason why focusing rails are made, and why most high-quality bellows have some kind of rack and pinion focusing mechanism in addition to the bellows extension.
*Some lenses can focus past a "hard" infinity stop.
---------- Post added 07-04-17 at 09:37 PM ----------
RECOMMENDATION FOR FIRST TIME RAYNOX USER. Sit down at a table with camera, lens you intend to use with the Raynox, and some small object - - a coin. little figurine, six-inch ruler. Mount the Raynox, set the camera to MF, using the lens' focus ring set the lens to infinity. Now aim at your selected object through the viewfinder and move the entire camera + lens back and forth until the object is in focus. Somewhat better: turn on the camera, turn of live view, turn on focus peaking (if the camera has this feature) and again, move the camera + lens until your selected object is in focus. Look at and take note of the distance from the lens' front edge (filter ring) to the object when it is in focus. Now turn the lens' focus ring to the closest possible focus distance and repeat the procedure to see what the minimum focus distance is with the Raynox. This will give you a mental picture of how close the lens must be to a subject when you're actually trying to take a picture.
Another way to run this test: Mount the camera + lens + Raynox on a tripod, a little table-top works especially well. Take the object selected for the test and MOVE THE OBJECT fore-aft in front of the lens until it is in focus. Again, two trials, one with the lens manually set to infinity focus, another trial with the lens set to its closest focus distance. There are additional ways to run these tests, for example, if you have a table-top tripod, you can slide the camera + lens + Raynox on the table top to focus on some object such as the cover of a book that has been placed upright on the table.