I have always wondered, why is it that a lens can only focus from so far away from the subject?
I know some lenses have a kind of macro switch which basically allows them to focus closer but why are they unable to in the first place?
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A given lens's optical formula can shift groups of elements in the lens around only so much to achieve focus. If you wear corrective glasses or have a magnifying glass, you can simply see how this works: you need the corrective/magnifying glass some specific distance from your eye for it to be able to focus on your subject. Move the corrective or magnifying glass back and forth to subjects of varying distance and you'll find there's a close-up distance where you just can't get the glass any closer to your eye and achieve focus -- it just doesn't work.
Some reasons off the top of my head:
Lenses are full of flaws, designers try very hard to reduce these flaws using different glasses and groupings of elements. This correction varies by focus distance and will be optimal at just one distance, most people shoot not-macro therefore most lenses are corrected for far away and not up close.
Lenses at their not-optimal distance suffer in performance and the easiest way for a manufacturer to keep their lens from looking like a real dog is to just not let you focus where it's not corrected well. Side note: most people similarly do not recommend using a special purpose macro lens corrected for up close for general photography, same reasons.
To focus close the lens must be moved farther away from the film. This extra extension makes a lens heavier, more complex inside, and more expensive. Unless you're building a special purpose lens or trying to market to a niche avoiding those three gets a manufacturer the best bang for the buck.
Last thought: lenses that focus from infinity down to 1:1 require a large focus range and a lot of spinning to reach focus. Having this full range available slows autofocus way down when it has to seek. The "macro switch" you mention is a hack way to limit this huge range for normal shooting. It also adds complexity and cost to the build.
As others have wrote - it is difficult to make lenses that can focus at very different distances. Designer of the lenses have to choose possible distances because it may affect size, weight, complexity and finally cost of the lenses.
In fact there are also lenses than can focus at very close distances but CAN'T at far distances. The example is this: Canon MP-E 65 mm f/2.8 1-5X Macro Photo http://en.wikipedia.org/wiki/Canon_MP-E_65mm_f/2.8_1%E2%80%935x_Macro_lens
And about the switch. To see how this kind of switch works look at, I think, most advanced one. One of the lenses has three ranges:
The third option is easy to use in any condition to the extent that you can forget about the switch! Thus it seems to be the best option.
But if you operate only in a particular range you can switch to the first or the second option. The reason for this is speed of autofocus which has smaller range to "scan" while finding a focus. So it may take a twinkling of an eye if the switch is in optimal position or very long several seconds if the switch is in the third option.
The lenses I have used as an example is Olympus M.Zuiko 60mm (look for switch in the middle of the page): http://www.lenstip.com/356.3-Lens_review-Olympus_M.Zuiko_Digital_60_mm_f_2.8_ED_Macro_Build_quality.html