The distance from the lens to film/digital sensor is a variable based on subject distance. When the camera lens focuses on a far distant object (infinity – a far as the eye can see symbol ∞) the distance lens to focal plane is at its shortest. As we focus on subjects that are closer than infinity, the distance lens to focal plane is elongated. We focus by causing the lens to rack away or towards film or digital sensor. If we close focus on an insect and realize a life-size image (unity or magnification 1) the lens will have moved forward one complete focal length. In other words at magnification 1 “unity” a 50mm lens will be positioned 100mm from film/sensor.
Many camera and lens makers stop the forward movement when the subject is about 1 yard (1 meter) distant. This is because standard camera lenses are optimized to image distant subjects and compromised when tasked to image nearby objects. Additionally, the lens stops (f-numbers) assigned to a lens become inaccurate when tasked to work subjects under the 1 meter focus distance. This inaccuracy becomes more severe as you move in closer. At unity, the error is two complete f-stops (4X dimmer). Because of this, close up photography is challenging.
The modern solution is to replace ordinary lenses with a macro lens. This is a specialized lens that is optimized to work at unity and slightly compromised when tasked to work distant subjects. Best of all, the macro overcomes the f-number error. In other words, your best bet for close-up work is a macro lens.
That being said, you can wet your feet by mounting a close-up lens. These are lenses akin to eyeglasses that help us gray-hairs read. They come in all different powers. These are positive magnifiers so they are labeled +1, +2, +3, +4, +5 etc. The common reading glasses found at drugstores are kinsman to the photo close-up. You can go to the drugstore with your camera and hold a +3 before your lens and practice close focusing. The magnifications provided will be same however photo grade close-up lenses are often two lenses sandwiched to mediate color aberrations. The photo grade close-up gets unfair criticism. Good ones are quite good. In any event they are an inexpensive way to enter into the realm of close focusing.
The language of the close-up lens is cryptic. The unit is the diopter. A +1 has a focal length of 1 meter 39.37 inches. If you mount a +1, with the camera set to ∞ (infinity) objects come to a focus at 1 meter from the front of the lens. You can continue with the usual camera focusing to 20 inches (500mm). Mount a +2, the focal length is 500mm you can continue to get down to 13 inches/333mm. Mount a +3 and the range is 13 thru 10 inches (330 thru 250mm). Mount a + 4 and the range is 250 thru 200mm). Despite with others say here, OK to try a close-up.
To fort the lack of the lens to move far enough away from film/digital seniors, we can mount a spacer between lens and camera body. If a 50mm lens is in use and we mount a 50mm spacer, the camera will now allow you to focus at unity. The downside is; the mechanical linkage lens to camera body is broken and you must now do all the lens adjustments manually. Some advanced spacers (rings/tubes) maintain the electric and mechanical connections, these are expensive but best. Let me add that the learning curve using tubs makes this type of close-up work challenging.
Most close-up subject are flat like postage stamps or coins. Insects and leaves have depth but not much. The ordinary camera lens is optimized to work in a world with depth; these are compromised when the subject is flat. To mitigate we can turn the lens backwards (reverse the lens). This helps because the backside of the lens is optimized to project an image onto flat film or digital sensor. A lens revering ring can be added to a tube, this improves imaging of flat and shallow subjects. Again the learning curve is demanding
Your best bet is close-up lenses; they do not induce f-number errors. They work but not a good at rings/tubes. Best is a ring/tube with reversing ring.
Best of all is the macro lens. It’s design take all the agony of the tubs/rings away. Save your money and buy a macro as soon as you can.
