There's been a lot of tests comparing these lenses with other lenses, but I was hardpressed to find any that compared them directly to each other. I know one is a 2.8 constant zoom and the other is a 3.5-4.5 zoom but they are similar in their range and similar in price. I can see someone wondering if their old trusty 18-70mm is as good as the 'off brand' Tamron.
I got a hold of a demo Tamron 17-50 and spent a day making a challenge. As far as physically, these two lenses are very similar in size. The tamron seems weightier and very solidly put together. It is made of plastic, but then again, so is the 18-70 Nikon. I have no problems with either of the lenses zoom rigidity or smoothness. The Tamron has a longer lens hood and the AF to MF switch seems to be a little weaker than the Nikon's but I wouldn't worry about it. The Tamron also has a zoom lock which I'm assuming is for transport even though there is no lens creep that I can see. (I have a sigma lens that I'd like this on though)
Now, what everyone cares about. The images.. All of these were done on a rock solid tripod with a remote shutter. I didn't correct lens distortion and these are jpgs straight out of the camera. These are also 100% crops. The focal point was the A.K. Dewdney book.
To be fair, the Nikon lens has been through the ringer as far as usage and as mentioned, the Tamron is a demo model. And, if you decide you want to buy one of these, use my link to amazon or adorama.. It's free for you, and it makes it so I can make more of these tests.
Up first, I decided I would pit the Tamron at 17mm 2.8 vs the Nikon at 18mm 3.5 .. Both camera's wide open end.
Image #1 is the Tamron at 17mm & 2.8 ..
The second image is the Nikon at 18mm and 3.5
After I saw both of these in the back of the camera, I thought there must have been some mistake. Some camera twitch. Some weird thing that I wasn't even thinking about. So, later that day, I ran the Nikon shot again.. with identical result.
All I can say is, holy crap. The Tamron at its focal point is light years sharper than the Nikon. The Tamron was half a stop wider open too! I wonder what the corner shots are going to look like! Let's see..
this is bottom left of the photo. The focus is still on that Dewdney book as above.
Tamron 2.8 at 17mm
the nikon at 3.5 and 18mm
Once again, you shouldn't even need the full size image to see the results. The Tamron, even at f2.8 is hands down better than the Nikon 17-80mm at f3.5.
Do we even need to see the f8 photos? I am kinda curious just to see how the nikon recovers.. I did shoot the Tamron at 3.5 just to compare apples to apples as much as possible, but really, the test above proves that it doesnt really matter.
The tamron f8 at 17mm
the nikon at f8 and 18mm
The nikon recovered quite well at f8.. In fact, there's not a whole lot in between them. Maybe the Tamron is a bit more contrasty, maybe not.
How about the corners you ask? well here you go!
The corner look at the f8 photo for the Tamron
The corner crop of the Nikon at f8
the look similar to me. The Tamron is slightly better, but its barely noticable.
the f16 and f22 were nearly identical in quality and all they really confirmed was that I needed to clean my sensor!