|Side-by-side comparison courtesy of dpreview|
There are 4 feasible "super zoom" lenses.
The most recent are the Sigma 18-250 HSM and the Tamron 18-270 PZD.
There are some reviews, but no real head-to-head testso it's seriously difficult to compare the alternatives; to make things worse, the Tamron PZD model replaced an analogous (but fundamentally different) screw-drive lens with a similar name, so many reviews actually describe the old model.
I will quote below some paragraphs that I found interesting, with some notes in color and some highlights.
DPreview review of the Sigma
It's therefore a lens which takes a slightly different approach than usual to the 'all-in-one' superzoom concept, trading off absolute sharpness against other desirable optical characteristics, namely relatively low distortion and chromatic aberration.
In terms of operation and ergonomics, the 18-250mm scores pretty highly. The HSM autofocus is fast, silent and positive
Many potential buyers will of course be interested in how the Sigma compares to its most obvious direct rival, the Tamron 18-270mm F3.5-6.3 Di-II VC. As usual, the answer to this question is not entirely simple, with the two lenses having a different array of strengths and weaknesses. In a fashion that's almost stereotypical between the two brands, the Tamron certainly appears to be consistently sharper (note: consistently probably means at different focal lengths, not in different zones of a single picture), and has a rather more effective stabilization system (note: the Sony version has no VC). The Sigma, in contrast, has arguably a more balanced overall design approach; alongside its low distortion, the autofocus is far faster and quieter (note: here the reviewer is talking about the old non-PZD), and the build quality and ergonomics are better too.
Sharpness results are extremely mixed. Typically for its class, the Sigma 18-250mm is strongest in the mid-range (24-50mm), but then has a fairly catastrophic dip in sharpness at longer focal lengths before rallying a bit at full telephoto. At almost all focal lengths (except 50mm), the corners are distinctly softer than the center of the frame. At its best (24mm F5.6) the lens returns excellent results across most of the frame; at its worst (135mm F5.6) it's very soft indeed. Overall, like most superzooms, optimum sharpness is normally obtained a stop or two down from maximum aperture.
Lateral chromatic aberration is a mixed bag - there's pronounced red/cyan fringing at wideangle, and even worse green/magenta fringing at full tele, but scarcely any in the middle of the zoom range. To be fair, few superzooms do any better, and some are worse.
Falloff is generally low and unlikely ever to be photographically problematic.
Sigma has managed to keep distortion unusually low for a superzoom. Barrel distortion at wideangle measures 1.9%, however this is a complex 'wave' distortion with substantial corner re-correction, of a type that is relatively difficult to correct in software if desired. (...) Overall (...) this is rather less extreme than we've seen on other super zooms. (note: this does not mean that distortion is low, it's simply better than the competitors)
Lenstip review of the Sigma
Sigma 18-250 mm f/3.5-6.3 DC OS HSM
- very universal focal lengths range
- very good image quality in the frame centre at all focal lengths
- good image quality on the edge of the frame in the 18-50 mm range
- quite well corrected coma
- slight astigmatism
- distortion noticeably lower than that of its rivals
- good quality of anti-reflection coatings
- silent and relatively efficient autofocus
- weak image quality on the edge of the frame at 120-250 mm focal lengths
- high chromatic aberration at the maximum focal length
- weak performance against bright light
SlrGear review of the Sigma
The 18-250mm provided sharp test results, even when used wide open, to about 80mm. Wide open in the telephoto range, things start to get a bit soft, especially in the corners:
- At the wide end, the lens performs very well. In the mid-range - 24-50mm - the lens showed its best performance.
Chromatic aberration performance is fairly good for a lens in this category, showing some significant fringing in the wide-angle and telephoto ranges only. Here's another indication the lens is designed for mid-range performance: at the 50mm and 80mm settings, CA is almost non-existent.
- On the telephoto end, the lens shows its shortcomings. Wide open, even by 80mm - just a third of the way through its entire focal range - it's showing significant corner softness wide open. Results at 120mm and 250mm are similar, with particularly soft corners at 120mm
The complex array of lens elements that allows such a vast range of focal lengths in one lens leads to some dramatic results for distortion (note: see my previous comment about distortion)
The 18-250mm OS employs Sigma's HSM technology, which allows for fast and quiet autofocusing. Indeed, the lens was able to focus between infinity, close-focus and back to infinity in around one second.
A very comparable lens from Tamron, the 18-270mm offers a slight edge on telephoto performance, but not enough to sway most users. Optically, this lens tested very well, and to compare the two takes a keen eye to spot the differences. In the end, I'd give the edge to the Tamron; just slightly sharper, especially in the telephoto end, and slightly better with CA (note: this is contradicted by other reviews)
However, distortion is much greater in the wide end, where it's better-controlled by the Sigma.
SlrGear review of the Tamron PZD
Tamron handles chromatic aberration fairly well with this lens, especially in the mid-range.
The Tamron 18-270mm uses a new autofocus motor, the PZD (Piezo Drive). The new motor is substantially less noisy compared to the older drive, and it's substantially faster - but it won't be setting any records in the speed department. It takes about a second to go through the focusing range.
The PiezoDrive focusing is similar to Nikon AF-S/Silent Wave or Canon USM, or Sony SSM, but not identical and on Sony models it can contrast-detect autofocus reliably. Sigma’s HSM hunts
The Tamron lens has visibly higher detail contrast than the Sigma, and in the centre of its field produces a very sharp image. The edge of the image lets it down, however, rather badly. The detail is soft at longer focal lengths unless stopped well down (ƒ/11 or so) and red-green chromatic fringes are serious enough to spoil JPEGs. They are not even very well corrected by using Adobe Lens Profile to process from raw (there is no Sony profile but Nikon, using similar sensors, can be selected).
At full aperture and 270mm the performance is markedly inferior to the Sigma at 250mm wide open. The lens has better multicoating but poor field flatness, which creates the softening to the edges and corners.
Some user opinions from dpreview forums:
- Tamron was much lighter than the Sigma
- Reviews seemed to vary, and actually balance out to the point that it seemed like the two lenses were really about the same for image quality
- a LOT of reports of issues with Sigma lenses, but almost none of issues with Tamron lenses. These reports include misaligned elements, mechanical failures, compatibility issues, etc.
- The Tamron is quiet when focusing. Not silent - I have not had a chance to do any video to see if the noise will be captured.
Tried both Sigma 18-250 and Tamron 18-270. not a clear winner/loser either in my unscientific, quick evaluation. I went with Tamron mainly because it was lighter (part of my whole reasoning for the SLT was to cut weight when hiking/traveling) and quieter. I do both still and video, and I was unprepared for how much noise is generated and transmitted to video by the focus motor. The bokeh with the Sigma seemed much better to me, but my particular unit was stiff to zoom and heavy, as well as noisier when focusing. But with the light bodies of the SLTs, the heavy lenses feel...awkward.
Another factor the Sony 18-250 has in its favor over the Tamron 18-270 PZD and the Sigma 18-250 OS HSM (in the context of the A77, that is), is that it has the "old", screw-drive, body-driven AF. The Sigma and Tamron being discussed here both use in-lens micromotors for AF, and therefore the DMF feature of the A77, as well as its rear AF/MF button, won't work with those two lenses. The only way to switch between AF and MF with these particular lenses is by using the switch on the side of the lens. To me, that gives a sizable advantage to the Sony 18-250 (or to any screw-driven lens, for that matter), for which the DMF mode and AF/MF button work as designed. Note: The preceding comments and conclusions would be different in the context of, say, an A55, A580, etc -- since those cameras don't feature a DMF mode or a rear AF/MF button, anyway.
One thing that is not often mentioned is the Sigma lens OS. I tested the camera stabilization and the lens stabilization. It wasn't a scientific test but I determined there was no difference in effectiveness. However, and this has been my most pleasant discovery about Sigma, the lens OS provides a much smoother view through the EVF. Absolutely rock solid at 250mm. The difference is striking and I always keep my camera "steady shot" off and the lens OS on. If you get the Sigma, you are in for a pleasant surprise.