what's the difference between these 2 lenses?

The first one looks mislabeled. If you look at the picture it's a 28-200 or something.

 

According to the titles, the first lens has Internal Focus while the second does not. IF implies that the lens length will not change when focusing.

 

If you are looking for a macro lens, I know that a lot of people strongly advise against zoom macros in favor of primes, especially the inexpensive models.

 

Prime lens: fixed focal length; ex. 100mm
Zoom lens: adjustable focal length; ex. 70-200mm
Macro lens: lens that can focus up close, up to 1:1 ratio and sometimes better.

 

Most digital camera have sensors that are smaller than a 35mm negative, so the light from the lens gets cropped by the sensor's small size. In effect, what is a 50mm lens on a film camera becomes a 65mm lens, or a 75mm lens, and 80mm lens, or even a 100mm lens depending on the small size of the sensor. Some digital cameras are full-frame, meaning a 50mm lens is a 50mm lens.

If you're going Nikon, you're dealing with a 1.5 crop factor. 50mm = 75mm.

Now, what DC and APS or whatever means is that these lenses are designed to work with the smaller sensors of digital cameras, so they will ONLY work with digital cameras with small sensors (on a full-frame digital or film they will be cropped at the corners.

The advantage of lenses made for digital cameras is they are cheaper to make, so you get more lens for your buck. The disadvantage is that if you buy a full-frame digital camera in the future, these lenses will not work.

 

APS is perfect for the d80. APS is the sensor size of the N70 digital camera and afaik the n80 has the same size sensor.
The new 17-70mm from sigma (new now with better focus and macro) is a GREAT lens. I decided to get the 10-20mm though because I didn't need a walk-around lens, I wanted a real wide lens. The 17-70 is a GREAT substitute for the kit lens, better than the kit lens in many people's opinions.

 

Yes, but since this is a very very new lens and they only make one type of this lens, I'm guessing Amazon is actually referring to the same lens, although it may not seem like it.

The "DC" means that this lens can only be used on the smaller sized APS sensors. The lens was designed for smaller sensors so that they could make the lens physically smaller, which is fantastic.

Of course there's a negative side to this. Firstly, if Nikon were to ever come out with a full frame (ie: 35 mm film sized) sensor, and you bought that camera, the Sigma 17-70 mm lens you're looking at would be useless, because it wouldn't work. Actually, it WOULD work, but you'd only get a photo in a part of the frame, and a pitch-black corners because the lens is too small to spread the light over the entire sensor. Call it "vignetting" if you want. I will only buy lenses designed for 35 mm film AND digital sensors so that when Nikon goes with a full-frame sensor, my lenses can still be used. I made an exception when I bought my Tokina 12-24 mm f/4, but that will be very easy to sell.

Secondly, corner sharpness is incredible on digital DSLRs with the smaller APS/APS-C sized sensors when coupled with a larger lens designed to be used on digital and 35 mm film cameras. Why? Because the centre of the lens is the sharpest. Since the lens is, in a way, too big for the sensor, you're using mostly the centre of the lens, and getting sharp sharp photos as a result, and with less vignetting.

 

www.sigma4less.com has very good prices on some lenses

 

That's what i thought. Ont the sigma website there is only ONE 17-70mm lens.

The 18-50 _has_ just been updated to MACRO capability though.

In any case. I'd still buy the Nikon 18-70 over the Sigma 17-70. That apparent f/2.8 advantage over the f/3.5 of the Nikon will drop off very quickly as you zoom in. The Nikon also has a silent-wave motor. This allows instant manual focussing override, and extremely fast silent focusing. bhphoto sell the 18-70 for around $300 so the price is better too.

 

In this case, APS means something else.

A few years before the boom of digital, Kodak released a film format called the Advanced Photo System (APS). Each frame was smaller than a regular 35mm frame. Since most digital SLR cropped sensors are roughly the same size as the APS film frame, they are called APS sized sensors. The "-C" stands for "classic" and refers to the traditional aspect ratio of 35mm film (3:2), hence APS-C.