Originally posted by macTak While there is no IQ degradation from extension tubes versus a supplemental add on such as a close-up filter, I would point out for the OP that most regular lenses are not as well corrected for what becomes such close focusing, and perform more poorly than a dedicated macro lens designed for such work (the better a lens you use with extension tubes the better; the kit zoom would not be a great choice, say the Pentax 50mm f1.7 prime an excellent choice.
And that is why I recommended macro or enlarger lenses on extension, not standard camera lenses. Of standard lenses, one low-cost favorite is the Industar-50/3.5, which can still be had for ~US25. On 50mm extension it goes to 1:1; on 100mm extension, to 2:1. With tube sets at ~US$8 each, shipped, a good macro setup need cost only ~US$40. I think you'll find a number of Industar-50 shooters here.
@OP,
Here's how it works: Every lens sees two visual fields, the subject (input) and image (output). (A visual field is the entirety that a lens sees or projects.) The image field (output) MUST be flat and sharp, to project the image onto the flat camera frame (film or sensor), else it is utterly worthless. Designing a lens so the subject field (input) is flat is rather tricky and costly, thus many standard lenses make-do with a somewhat curved subject field. Macro and enlarger lenses are designed with flat subject fields, for edge-to-edge sharpness; portrait lenses don't require that, and cheap lenses don't even bother. That's how the optics business goes.
Now, reverse a standard lens. The flat-field side now sees the subject, and the curved-field side projects the image onto the frame. Ah, but being so close to the frame, the field curvature is too insignificant to matter, unless it's a REALLY lousy lens. So now you get edge-to-edge sharpness (pretty much) on both sizes of the lens. Thus can almost any lens be turned into a fine macro lens! Glorioski! Que milagro!
But optics giveth and optics taketh away. Reversing a lens means your working distance is the lens
register, the normal distance from lens mount to frame. With Pentax-compatible lenses, that's ~45.5mm, under two inches. And reversal alone provides sharpness and close-focus, not magnification. To magnify, the reversed lens still needs some extension behind it. Also, some lenses just don't work well reversed: lenses without aperture rings, and many zooms, and especially AF zooms (like the kit lens). And, as with simple tubes and bellows, you lose any aperture automation, so using flash is tricky. Lens reversal is good in studio settings with controlled light, and not so good for field work.
And that's why I recommend enlarger lenses on bellows (cheap and flexible), or a Raynox on a lens you already have (cheap and easy).