How do I know whan a Lens is a macro lens

I have been lucky enough to get a K20 off a member here, so I have been scouring ebay and other places for cheap old glass.
I want a macro lens, as I want real close ups of plants and insects, water droplets etc, but Im realising that unless it has 'macro' in the name, I have no idea if it is a macro lens or not..
Wikipedia has helped with the Pentax naming scheme, but Im still a bit stumped..

Is there anything in the focal length or other numbers that will help me to identify whether a lens is a macro lens?

Anyone have any recomendations for good old macro lenses, Im guessing old as I cant really afford to go much above £80. Buying the camera cleaned me out pretty much, though Im a very happy man now

Many lenses with 'Macro' in the name are not true macro lenses which have 1:1 magnification ratio meaning the subject will appear life size in the image.

True macro lenses tend to be very expensive, both the current Pentax offerings are just short of £400.

For your budget you might consider a second hand 50mm prime such as the Pentax A version and a set of extension tubes.

Not sure about the 1:1 part many earlier macro lenses only could do 1:2 and still some are available, like the Carl Zeiss macro's.
I think the biggest defining difference is that Macro lenses are specially made to preform the best at close focus and they also have a very flat field of focus because of that, therefore only Prime lenses can be true macro lenses.
Zoom lenses with macro function only means you can focus close with them but they don't come close to what a real prime does/can in therms of quality, the zoom lenses aren't made for that.

Focal length change the prespective of the macro.
50mm give a wider "immersive" view than a 100mm which more isolate the subject.
Also the 1:1 distance with 50mm is closer then with 100mm. With 100mm it's around 30cm, 50mm around 20cm so with bugs a 100mm would be better.

Pentax Users might expect to be looking for a lens marked as having a PK mount of some type, rather than Nikon, Olympus or Minolta!

The older pentax macro lenses (50mm or 100mm) are usually f4 lenses with a maximum magnification of 1:2 (half life size) and a special extension ring increases that to 1:1

Later macro lense (A, F, FA, D-FA)are usually f2.8 and focus right down to 1:1

A macro lens is (theoretically) a lens that (at least) allows you to get a real size image of an object on the sensor.

So a subject of 1 cm diameter will be 1 cm on the sensor (or bigger)

Others state that macro already starts earlier (e.g. 50%).

If a zoom says macro, it's often not able to achieve 1:1 (not 'real' macro)

You can consider the use of an adapter (Raynox is a name that is mentioned quite often) instead of a dedicated lens; not sure about prices.

Sorry John, forgot to clarify that.

Paul

Available 2nd hand are special 2X Macro Converters. These act as a normal two times converter and also have an adjustable macro setting. Used with a 28mm lens it becomes 56mm macro lens. A 50 mm lens becomes a 100mm macro etc etc.
I have the Teleplus version of this macro converter and am pleased with the image quality when using Pentax primes.

I have heard of the Raynox and they seem to be from 50-100 depending on model..

Have just seen this on ebay
M42_Vivitar

Bit bigger than Id like, but has good reviews..
I know I would need to get a M42 adapter, but it would be a handy thing to have for older lenses..

Or there is this?
convertor

Maybe I should mention that I only have the 18-55mm kit lens!

The Vivitar you linked to is a Zoom lens having some macro'ish capability. All true Macro lenses are primes having a fixed focal length and are (IMHO) the only real option for decent macro work.
Macro zooms tend to simply adjust the internal elements to allow for slightly closer focussing at shorter distances than they otherwise can and you lose focus at infinity. The loss of infinity focus is not a real issue but a true macro lens can focus from very close to infinity without a problem.

All other options to achieve macro images can produce good results with patience but a dedicated macro lens is best. One of the Dixons group was selling the SMC Pentax D-FA 100mm Macro lens recently for under £300, a real bargain.

The cheapest 'entry level' option to macro photography is a standard 50mm prime lens and a set of extension tubes. I would try that first because there is no point investing several hundred pounds in a macro lens if you eventually discover you haven't got the patience for it, and you need that by the bucket load.

Other options include reverse mounting a wide angle lens, say 28mm, on your camera for which you will need a reversing adapter (male K mount to male filter thread to match the chosen lens).

My least favourite option is to stack two lenses, one reverse mounted to the other, for which you will need stacking rings (dual male threaded). EG: A 50mm lens stacked onto a 200mm lens which is mounted to the camera will result in magnification of approx 4X. It borders on micro-photography though, rather than macro-photography.

You say that you want close up's of plants etc. That is not macro photography. Quality close up shots can be obtained with prime lens. I use 200mm and 300mm. I do have the Pentax 100mm 2.8 but it is limited with depth of focus for 'close up' work.
You need to be very clear in what you want to photograph. If you want ultra detail of small subjects, then macro is your aim. If you want real detail of, say lillies, the close up is your aim.
Suggest you try to find someone with a true macro lens and try before committing yourself.

The 18-55 can focus pretty close, just as close as most zoom "macros"
Set the lens on 55mm and the focus on the closets settings and move your camera/body till you get the focus right.
It should be able to do 1:2,9 that way, good enough for plants and for big insects maybe???

OK, I obviously need to narrow it down a bit, having gone the the complete quagmire that was choosing a camera, I am slowly sinking under the weight of choosing lenses. Half the problem is that as a noob, I just have no idea what the various numbers actually mean in real world pictures.
I know 50-200 or 55-300 is a zoom
I know a prime is fixed length
But I have no idea what differnce there is between a 50mm prime and a 90mm prime when it comes to the actual photo.
Im aware that I have alot to learn!

Now I have a K20D, I want to play with water drops, cliched yes, done before - lots, but it is something I have always enjoyed looking at and I want to have a go...
Im guessing that I need a macro as it is close up photo? Or a prime with and extension.

I also want to get a 2nd hand prime as new lenses are out of my price range, this is because I want to be able to get some nice DoF shots.
I do need a prime for that - right?

If I can get a prime that will allow me to do both, then that would be great. Im looking to build up (slowly) a nice collection of 2nd hand glass. That is part of the reason why I chose Pentax over Canikon, for the availability of good 2nd hand glass. But looking on ebay, the choice is massive and I don't know what is a good lens. I know most lenses will fit my 20D, but Im getting confused now.

50mm f/1.7 is good lens for stacking and because it's a f/1.7 that means the aperture is relatively larger so the DOF is small.
Also the lens is very cheap secondhand

Here is a nice tool to see what the focal length does so you can see the difference between 50mm and 90mm http://www.tamron.eu/en/lenses/focal-length-comparison.html

Here is a read about lenses, it can be a bit technical but i hope it helps
http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

Most of us have been there. It is a large learning curve you face. This Forum will offer you a lot of advice. Until you put it to the test to discover if it is for you then you will not gain the most valuable lesson..that is..experience.
If you have a camera club near you then this will be your fastest route to experience. Members are only too willing to help and advise and offer practical support. You will find that the members will encompass most of the photographic disciplines. Camera clubs are not for everyone but for beginners they are invaluable for avoiding the costly mistakes that we all make at some time.
Being in London you have many camera shops to visit. Some will advise and even let you try lens. Give it a go.
regards thoramay