I started dabbling in photography with a basic Sony P&S. At the time, I was thrilled with its 8x zoom range – from wide angle landscapes I could zoom into a car in the distance. Very flexible, but the camera was quite low quality like most point & shoots.. When I finally transitioned to a DSLR, the Nikon D3100 it came with a 18-55mm kit lens. Although I wasn’t too thrilled with the roughly 3x zoom range at the time, it took some splendid images. Soon, I bought the Tamron 70-300mm to cover the long range, which meant I was carrying around two lenses almost all the time.
So recently when I upgraded my camera, I opted for the 18-40mm which has a roughly 8x zoom, covers a wide range of focal lengths and has got good reviews all around. Unfortunately my experience with this lens wasn’t quite as good as the other reviewers.
I find two critical faults – the image quality at wide focal lengths is poor and the lens doesn’t achieve good focus at wide focal lengths.
Lets start with the first. This is the scene I used for my tests:
A busy scene, with lots of foliage and detail
Since most lenses aren’t very good fully wide open, I decided to take these photos at an aperture of f/8, which is usually around the sweet spot. All these are 100% crops from the top right hand corner of the image, which exhibited the poorest image quality.
18mm: Terrible distortion, there is chromatic aberration as well
24mm: Just as bad
35mm: Although the spatial distortion is better, there is still a tinge of chromatica aberration
50mm: Now images start becoming usable
85mm: Quite good
140mm: These leaves are out of focus, but dot distorted
All focal lengths below 50mm exhibit high distortion – note that its only in this part of the image, the rest of the image doesn’t have such poor quality. Its like a linear scale, as you move up the focal range, from 18mm to 140mm, the overall image quality improves.
The second critical flaw is with the focus at wide angles. At 18mm, when the camera says you have focus, you really don’t. Look at the image below, it is a wide angle shot of a beach. It looks alright at web resolutions.
But crop to 100% at the center of the frame and you see its not sharp at all. This image, despite its beautiful colours, is one I can’t print.
This isn’t a lens I can use wide open with any confidence. I cant trust the image quality enough to print them or export at high resolutions. So, this lens in effect has a focal length range of 50-140mm – quite disappointing.
