If you have a Canon Rebel or APS-C sensor Nikon body, check out DxOMark’s review of the Sigma 18-35/1.8 zoom lens. Nikon and Canon haven’t put too much effort into designing lenses for the bodies that they are actually selling, i.e., the small sensor bodies. Sigma now shows up with a 29-56mm equivalent lens with a constant f/1.8 aperture that outperforms prime (fixed focal length) lenses.
This is a surprising result to me because I’d always thought that a lens designed for the full 24x36mm frame would do an awesome job on an APS-C camera since the sensor is underneath the sharpest center portion of the image circle. Apparently a lens specifically designed for the job does a lot better, even when it has to zoom. (In retrospect this makes sense; in the film days we didn’t see a lot of folks using Zeiss and Schneider medium format lenses, designed to cover a 6x6cm frame, being used on 35mm film bodies.)
(Amazon
[Separate question: What is it about a lens that makes it suitable or unsuitable for use with contrast-detection autofocus, as opposed to the phase-detection autofocus that has been conventional on SLRs? A variety of lenses, including this one, are available only for conventional SLRs and not the APS-C-sized mirrorless systems, such as Sony NEX. A quick Web search reveals people asserting that it is because the lens design is not compatible with contrast-detecting autofocus, but I would think any f/1.8 lens would be a fine feed to a contrast evaluation system (shallow depth of field causes images to snap in and out of focus).]
The new (heavy and expensive0 Canon 24-70mm f/2.8L II outperforms pretty much every Canon f/2.8 prime as well. Sigma has been on a roll for the last few years – their 35mm f/1.4 and 50mm f/1.4 lenses are the best available for Canon and Nikon mount. If only their quality control were less spotty…
Perhaps too narrow a depth of field makes it harder for the CDAF to figure out where the gradient of contrast lies, and when they try to hunt for focus lock, the steps in the hunt are so wide they pass over the focus point without realizing it.
> What is it about a lens that makes it suitable or unsuitable for use with contrast-detection autofocus, as opposed to the phase-detection autofocus that has been conventional on SLRs?
It is about the lenses ability to mechanically move the focusing elements quickly and responsively. A PDAF sensor can measure the difference between the current focus position and the correct one and send a single (or few) commands to the lens motors telling it how much to move. A CDAF system needs the lens to move during focusing to compute where the final focus position will fall – i.e. it makes one measurement, moves the lens, makes another, computes a best-guess, moves the lens there, measures, fine tunes, etc. Thus the lag time from the sensor to the lens physically moving must be made as small as possible.
The smaller distance between sensor & lens, the smaller size of the lens probably make it much easier to get better results with the smaller sensor. That’s why Leica lenses did so well. If Ken Rockwell’s reviews are any guide, the 1.6x lenses have become dominant, with the 18-135 EF-S getting rave reviews.
My guess is that the lens wouldn’t sell as well because it’s designed to be larger than needed on a mirrorless camera because it needs to clear the mirror box on the DSLR. Of course this is only a guess.
I haven’t noticed any issues with CDAF and different lenses on the Nikon DSLRs when using LiveView. They all seem to focus quickly and accurately (even the older non AFS ones).
The lens sounds good, but I find the zoom range to be one of the less useful ones to have a zoom for. This is based on owing the the Nikon 18-35mm lens on a couple of cropped sensors. It seemed to never be wide enough or long enough; covering a wide to moderate long range (say 18-70) makes for a much more useful zoom lens. But if it’s not a compromise over the prime lenses I guess it’s excellent and can just be treated as a pair of primes, and not as a walk around zoom.
As and aside, the resolution on the Nikon 18-35 was tolerable on the D100, but not good on the D300. I can’t imagine how bad it would be on the newer 24MP cameras. It’s amazing how far lens design has come in 15 years.