Pros: Extremely compact for storage, inexpensive, very sharp images.
Cons: Large torque required to operate zoom, slow focus response
The Bottom Line: The Sigma 28-300mm zoom does work, but is functionally slow, and definitely not usable for everyday photography, and has limited use for fast action.
Pirich's Full Review: Sigma 28-300mm f/3.5-6.3 DG Macro For Canon
The Sigma 28mm-300mm zoom lens was originally sold for the Konica-Minolta Maxxum series Digital SLRs. When Sony took over this product line in 2006, they kept the same lens interface as the original Minolta Maxxum cameras for the new Sony 100alpha camera. This gave the new camera a complete line of mature accessories Sony could add to, rather than attempting to introduce new standards which would be difficult to succeed with (given their love of introducing new types of proprietary media and devices, this is a refreshingly mature action by Sony).
Background
For me, one of the drivers for getting a lens with a wide zoom range is for dynamic photography sessions where the subject changes distance and apparent size quickly, so swapping lenses would not be an option. In particular, I had high hopes for the Sigma 28mm-300mm zoom both for shooting aircraft flying over Tucson. I also expected its ability to zoom to a 300mm focal length would make it easier to get photos of my daughter without it being obvious her picture was being taken.
By necessity, any zoom lens is going to be an attempt to optimize a range of features to have a usable compromise. And, a larger zoom range means a more extreme set of conditions to try to reach a compromise with. One of the most difficult areas is the low end of the focal ratio- the wider the iris can open up, the more visible any optical faults become, so a wide zoom often will stop reducing in focal ratio around f/4 or so simply because the glass would have to be free of distortion AND false color out to an extreme degree. And if you look at zoom lenses able to get down to f/2 or lower, they are quite expensive as a result.
Description
The Sigma 28-300mm zoom is quite a bit larger than the Minolta/Sony 18-70mm zoom, which I have been using as a general purpose lens. The lens is finsihed in a flat black finish with a glitter effect. The barrel has small straightd ridges on the grips for the zoom and the manual focusing. Like all Maxxum and Alpha compatible lenses, this one has a screwdriver slot-like feature in the mounting ring for driving the focuser as well as a series of electrical contacts inside the ring. When attached to the camera, it can either be used as an auto-focus or manual focus lens via the camera controls.
The lens comes with a matching lens shade sized to lock on to the front outside the filter thread, so a filter may be left on, and the lens cap can be used with the lens shade on, although unlike the Minolta lens, this one has just a basic lens cap without the second set of "Pinch" type tabs for removing it, so it more difficult to remove with the lens shade on.
The zoom on the lens causes a three-section sleeve to extend from the front with the objective lens. The focus is part of the main barrel and the end of the lens does not rotate. The mechnism also has a lock slide to keep the zoom from moving- for example, if you put the camera with lens nose-down into a camera bag with the lens shade on without locking the zoom, the lens will extend when the camera is pulled out of the bag.
Usage
The combination of camera and lens is still relatively compact while the lens is at the near end of its zoom range. The torque needed to extend the lens to full zoom or anywhere in between is substantially more than the 18mm-70mm Minolta lens. In fact, I would have to say the torque is so high, at first it seems to be too great to support dynamic situations where a photo must be framed quickly, even though the desired level of zoom is somewhere in this lens' range. The large difference in length turns the camera and lens combination into a much larger piece of equipment, so with the unusual amount of force needed to operate it make this lens a piece of equipment to master and get used to.
The payoff is the glass in it is pretty good, and across the zoom range the frustration of mastering it is rewarded with crisp images free from false color. But there are some definite work-arounds going on to make this lense successful. For example, the lens has a strange interaction with the camera's autofocus mechanism which causes a bit of hesitation I haven't seen elsewhere in an autofocus SLR. Simply put, it will act like it can't find focus, even whe it seems to be as plain as day. Part of what is happening here is many of the interesting points in zoom to frame a photo with seem to have poor contrast. On the other hand, using it in bright daylight seems to reduce this problem some, but I found myself switching the camera to manual focus, then using the thumb focus switch to do a momentary focus and then shoot on that focus.
So, one of the big events for photography in Tucson every year is the annual certification for pilots in mixed formations of WWII and modern aircraft. The Air Force practices for air shows over Tucson every March, so it is possible to photograph planes like old P-51s, P-47s, and P-38s learning to fly in formation with jets like F-4s, F-16s, A-10s, F-15s, and F-22s. Since they fly around over the city, at some points you will be able to photograph them as they practice making and breaking formations, turns, and other maneuvers. So, I used the Sigma 28-300mm zoom for taking photos.
Trying to photograph moving subjects in fully auto-focus mode has proved to be very difficult. For example, if you are tracking a formation and the camera crosses a nearby telephone wire, the focus will drive to focus there, and as they emerge, will take several seconds to recover to photograph the aircraft. In this case, the photo taken when they are nearby is lost. So, the method of getting a usable focus either on a mid-distance object, or on one pass using the momentary auto focus button with the camera on manual focus otherwise eliminates the confusion caused by crossing nearby objects. Here are two of the photographs taken of formations flying above Tucson (the only processing has been cropping to the subjects):
The second photo with three aircraft is significant in two ways: First, the image is the first one where I managed to change zoom in real time without losing track of the subject. This is where the large torque needed to operate the zoom reared its head- if you are pressed with time and zoomed all the way in to 300mm while tracking the subject, deciding you need to pull back to get more in the frame can be a disaster. What is more, you don't have to be watching jets flying to have this happen- trying to photograph an 18 month old child yields a lot of the same results. The only benefit here is if the camera is in focus at the given range and the subject is not changing distance significantly, then you can change zoom without changing focus. So, in the aircraft example, the fact I had it on manual focus meant the focusing mechanism did not try to refocus as I zoomed out to 200mm. In the case of photographing kids, I can't say this lens passes the test- the smaller and easier to operate 18-70mm zoom takes the contest with its instantaneous response and low operating force works far better.
Conclusion
As can be seen in the aircraft photos, the Sigma 28-300 mm lens produces sharp focus and images free from false color. The main difficulties with this camera come from the force and time needed to operate its zoom and focusing mechanisms. Yes, it is compatible with the Minolta heritage Sony DSLRs, but the limitation for this lens it it behaves more like a fixed focal length lens since it takes so much force to move the lens, and if the subject has moved to a new focal point, auto-focus will end up being slow. And because of this, I can only recommend this for scenes which are going to be repetitive in scale or change slowly, say a ball game from one vantage point or landscape photography. The images you get will be tack sharp and have perfect color, but the mechanics of the lens force me to rate it lower than the glass alone would have suggested.
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