Can we use a macro lens as a telephoto lens? - 1

I'm really enthusiastic about bird and wild life photography but it will need a telephoto lens and telephoto lenses are very expensive, so i came to know about a lens SIGMA 70-300MM F4-5.6 DG MACRO LENS, Tamron Lens AF 70-300mm F/4-5.6 Di LD, but those are macro lens so can we use macro as telephoto and how will be the result? I have nikon d3100 camera, please guyz suggest me any gud, and inexpensive telephoto lens

Of course you can. Macro lenses have the ability to focus very close to a subject, but that does not change how they behave when aiming at a subject that is further away. You should know that those might claim to be macro lenses, but neither will give you the true 1:1 of a real macro lens.

Neither of those lenses are macro lenses. They have a "macro" feature stamped on them as a marketing ploy. It just means that they focus a little closer than other lenses in their class. A true macro lens is defined as one where the subject and it's image on the film/sensor are the same size. To get that, you need to be within centimeters of your subject. Your lenses will not come close to that. Since it sounds like you really want a telephoto, don't be dissuaded by the lenses being labeled "macro". The 300mm end will be useful for wild life photos. However, you still might find that you must use some guile to get closer to your subjects.

Yes, you can use them for both.
The definition of Macro is an image that is 1:1. The size of the image on the sensor is equal to the size of the subject. It gets a bit more complicated than that. For some understanding read the wikipedia here: http://en.wikipedia.org/...hotography

The biggest catch between macro and telephoto is that the lens will focus slower on a macro lens. Otherwise? They are great for both.

If this is a hobby those lenses MIGHT be good. There's a catch…
Is this the sigma lens you are looking at? Http://www.adorama.com/US%20%20%20%20433029.html It is an older version sigma lens and sigma does not guarantee that it will work with newer technology cameras. I have a few of sigma's older lenses that do not work with my digital cameras. Might want to do some research on it and people who have experience with your camera and that lens.

And is this one the Tamron version? Http://www.adorama.com/US%20%20%20%20435945.html
Tamron's older technology tends to be more forward compatible, but again you might want to talk with someone who has experience with those two lenses and your camera.

In general Sigma and Tamron are going to be a little lower in quality than a Nikkor lens, however they have made some amazing advances in recent years and the difference would only be noticeable to a pro who is rather discerning (these lenses do not have the newer glass quality.) In which case a pro wouldn't be using the Nikkor version of those lenses either but would opt for the 70-200 f/2.8. Sigma's version of that lens is amazing and fast on the focus but not a macro-the minimum focus distance does not produce a 1:1 image. Sigma makes an "OS" version of that lens which is equal to Nikkors VR-it is an amazing lens for about a grand less than the Nikkor.
Tamron's version is an awesome lens and is macro capable. THey do not have an "OS" "VR" "IS" version. It's focus is SLOW if you are shooting sports or fast moving/changing subjects. It is an awesome Macro. They are all 3 of the off brand SIGNIFICANTLY more money than the two you are looking at. Tamron starting at $800 and Sigma starting at $1000. Somehow I do not think that is what you are looking for!

Those two lenses being a quite a bit older technology are also going to be lesser quality glass than the newer lenses. You will get awesome photos from them-don't get me wrong. However if you are wanting QUALITY I would definitely NOT buy them. As an average and affordable lens, they are a good option.

The two you are looking at-some things to really think about:
the tamron's minimum focus distance is 37.4CM or 14.7inches at 70mm. As you zoom that minimum distance increases to about 3 feet or more. Not much good for macro work at 300mm.
The sigma's minimum focus distance in macro mode is a constant 37.4inches/59.1cm. It will produce a macro (1:1) but if you are hoping for the zoom to get you some amazing increase in size, you aren't going to get it from either lens.