How to get my macro lens to focus closer?
Alan: A neat and inexpensive way to get closer
A neat and inexpensive way to get closer: Mount a close-up supplemental lens. These are simple devices akin to reading eyeglasses. These lenses have a metal rim with threads that screw on matching female threads, commonly found on the front lip of camera lenses. Even if no such screw threads are present, holders and clamps are available to mount this supplemental lens. These close-up lenses are also mistakenly called close-up filters . They are so similar to the common reading eyeglass at the drugstore, you can go to where they are displayed and hand-hold one of the lenses before your camera. If you do, you will be amazed; such a lens drastically changes how close you can focus. Reading eyeglasses and supplemental close-up lenses are labeled using a unit of measure favored by opticians call “diopter”. You will discover that reading glasses come in assorted strengths, such as 0.5 or 1 or 1.5 or 2 etc. If you mount a +3 before your camera, with the camera set at the infinity ( ∞ ) position, your camera will now focus at a maximum distance of 333mm (13 inches) plus you can adjust the focus ring and get even closer, perhaps 250mm (10 inches). Table of revised focus distance with mounted supplemental of various powers. 1 diopter = 1000mm = 40 inches 1.5 diopter = 666mm = 26 inches 2 diopter = 500mm = 19 ½ inches 3 diopter = 333mm = 13 inches 4 diopter = 250mm = 10 inches 5 diopter = 200mm = 8 inches. Formula 1/diopter power X 1000 thus for 6 diopter = 1/6 X 1000 = 166mm ÷ 25.4 = 6 ½ inches. Again, the close-up supplemental lens changes the close focus to a revised object distance. Likely the revised range will span a diopter so with a +4 mounted, you can image over the range of 10 inches to 8 inches. The drugstore eyeglass test will give you a feel for what the close-up can do as to various powers. Sorry to report the drugstore eyeglass will yield poor quality. You only use this test to get a feel. Once you decide what power to buy, you purchase photo grade close-up supplemental. Some will foo-foo their use, but likely the quality will be satisfactory for your needs. A good photo grade close-up is a sandwich of two lenses. This dual arrangement mitigates color errors (aberration). Personally, I find photo grade supplemental close-up lenses OK to use.
Don't confuse minimum focusing distance with lens magnification. The reason behind buying a 105mm macro over a 60mm one is BECAUSE of the longer focusing distance. But both lenses produce the same coverage of the image. So when you say you want the lens to focus closer, are you saying that you want to capture a smaller area as oppose to having to be physically closer? Both the 105mm and 60mm are 1:1 macro lenses, which means that they both cover the same area. Take a photo of a dollar bill, for example, and the images will be identical even though the 105mm was further away. Using an extension tube will make all lenses, including your 105mm macro, very near sighted. In fact, the magnification will be beyond 1:1 to something like 2:1 or even 3:1 depending on the amount of extension. So when you put an extension ring on the lens, you'll have to get much, much closer to the subject so that it'll be within the focusing area of the lens. Once you put the ET on the macro lens, you can't focus to infinity anymore. Your range of minimum-to-maximum focusing is greatly diminished.
Each lens has a minimum focusing distance. You should have considered that when you bought your macro lens. Using extension tubes with your 105mm Micro may have pushed the focusing plane to the back of your head. The solution to your problem is to buy a shorter focal length macro lens that has a focusing distance that meets your requirements or but a standard/normal lens and pair it with extension tubes or a bellows.