Originally posted by Nick Siebers Like throwing a 50mm on a bellows - the size of the aperture is the same, but you need a lot more light or longer exposure to make up for the light lost to extension.
It's not the extension, but the magnification. A subtle point, but read on...
Quote: One of the positive aspects of the "close-up filters" is that they do not reduce light in this way. (The others are that they are easy to carry, and easy to put on. Downside - mostly they suck :-)
A simple uncorrected +diopter closeup meniscus does indeed suck. Corrected closeup glass like the Raynox gems don't suck so much. The best such 'adapter' is a stacked reversed lens. Put a 105mm lens on-camera. Add a thread-reversal ring. Screw a 35mm lens onto that, reversed. Magnification is PRIMARY / SECONDARY, or 105/35 = 3x. And even though no extension has been added, you STILL lose over 4 f-stops of light. AP=NP*(M+1) so if the primary and secondary are both at f/2.8 (nominal aperture, NP), the apparent aperture (AP) is 2.8*4 = 11.2, or ~f/11.
Quote: Reversing a lens is also useful for good macro with somewhat less light loss, but I don't know the math.
Reversing a lens alone provides a fixed close working distance and a flatter image field, not magnification. With a single lens, magnification comes from extension. A reversed lens with a deep front inset will magnify more than a reversed lens with a shallow inset, because the inset acts as extension. And because a reversed Pentax lens has a working distance of around 45mm (under two inches) you're necessarily VERY CLOSE to a subject, and so fill the frame with its image.