Pentaxophile wrote:
Payment received, lens in transit.

Dangermouse wrote:
I'm not quite sure what you mean - are you talking about picking a lens in good condition from a shop's used stock?

Take a small LED torch and your camera with you.

First inspect the lens. Is it clean? Are there dents in the filter ring or lens barrel? Minor paint rubbing and "shine" on plastic parts is normal but dents suggest the lens has been carelessly handled in the past and may be best avoided. Don't be put off buying a lens with paint wear if the price is right, so long as the optics and functions are perfect. It can be a useful bargaining tool though if you're haggling!

Next look at the glass - check for scratches and rubs on the front and rear element. Open the aperture as far as possible and shine the torch into the lens to look for fungus. You will always find some dust in older lenses, but it doesn't usually cause trouble.

Check the focus and aperture rings move correctly. The focus should be smooth and not overly stiff, the aperture should click precisely. Close the aperture right down and flick the lever on the back of the lens mount to check that it opens and closes freely. Check for oil on the blades which will make the mechanism sluggish and will cause exposure problems. On an "A" lens push the button and set the aperture to A, a small pin should pop up from the flange of the lens mount (like the contacts on your kit lens). This is how the lens communicates its aperture range to the camera. The pin should retract when you move the aperture ring from A.

Mount the lens on your camera and check that it will focus properly, if it has been poorly serviced in the past then it may not be able to focus on infinity. It's easy enough to fix but again, either a deal-breaker or a bargaining tool depending on price and whether you can sort it out. The lenses you're after are common enough that you can wait for a perfect example. Try a couple of test shots to make sure it works on the camera.

Pentaxophile wrote:
Just stick it on your camera and take some photos surely? You can do controlled tests with the camera on a tripod using the same settings but what's the point! 'The better lens' depends on a lot of different criteria, and what you want to use it for.

You'll soon work out which is more worthwhile to *you*! These lenses are quite cheap and you can always sell them if you end up not using them.

Dangermouse wrote:
Yep, for use on a DSLR A is preferable to M.

The only less-desirable thing on the A lens is the plastic aperture ring, but on a DSLR you set this to A and leave it alone thereafter so it won't bother you at all.

The focusing on both should be ultra-smooth, so have no worries there.

To give an idea of prices, I've paid anywhere between £10 and £30 for an M 50mm f1.7 (the latter included a tidy chrome ME Super, the former needed internal cleaning to remove mould). The only A version I have of the f1.7 cost £42 alone, at the time this was relatively cheap as it had been poorly listed.

The f2 variants are cheaper, but not quite as good as the f1.7. There isn't much in it though, especially when you stop down a bit. I have shot with a fungus-ridden 50mm f2 I bought for £5 and cleaned the mould out of, the resulting images were perfectly acceptable.