You are running a risk and like all gambles sometimes you lose. you can cut back dramatic on the loss factor if you know what your looking for and at. The biggest problem with second hand lenses is that you tend to buy them over e-bay and have to trust what thy say in the auction. If you buy them at a camera swap meet people tend to know the value of them. if you buy it in a camera shop it will be over priced but may have some sought of limited warranty.
There are a few things you need to look out for when buying second hand lenses.
Lenses are vulnerable to fungal infections which can if left unchecked damage the surface of the lens. A very wide range of funguses are responsible, these include the families Phycomycetes, Ascomycetes and Fungi Imperfecti, to name a few.
There are a number of myths often repeated about fungal infection in lenses, such as the belief that they are very contagious, and that once infected the lens is ruined beyond any hope of repair. The fact is that these myths are very largely untrue. While it is certainly true that in certain circumstances a lens can be so badly affected by fungus that it is no longer economic to attempt the repair, in most cases the lens can be ‘cured’ and returned to service without any fear of transmitting the infection.
Some fungi secrete acids, and other substances which will attack the coatings on the lens surface. In extreme cases this etching can penetrate quite deeply into the coating. Some of these secretions are waste by-products of their biology, but often it is part of their way of collecting nutrients. Rectifying damage of this type involves removing the damaged coatings and re-coating the lens. This is rarely economic.
Fortunately, these very corrosive fungi are not the most common fungi to attack lenses, though the fact that many fungi are difficult to clean off the lens surface leads many to believe the opposite is true. Fungus growing into the lens cement is more problematic, as the elements have to be separated in order to clean them, and then re-cemented. Usually a job for a professional.
If a lens has mechanical or physical problems you may not notice it straight away. Even the smallest drop could cause minor problems internally e.g. a slight miss alignment in the internal lens elements, the zoom or focus ring may become sticky. At first they may seem insignificant problems but my develop into serious problems like no being able to use the full zoom range of a lens or very soft focus. A long time ago I was in a photography forum, one of the members posted that he had just dropped his very expensive G lens and that is wouldn’t focus properly any more. Members of the forum put him onto his local repair service. 3 months latter the same person was selling the same G lens online, there was no mention of it ever being dropped or repaired.
Moving parts wear and age. Over the years cogs, contacts and other moving parts will wear a bit, a very minor amount is O.K. But once things like focus gears start to wear down or get stripped repair can outweigh the cost of a new lens.
Scratches Cracks and Dust
I have 1 lens that has dust in it and I hate looking at it, but it isn’t visible in any photos so I shouldn’t worry about it. My problem is that the new chip sets are so sensitive that eventually it will show up (guess I’ll get it cleaned then). Scratches on the glass can be a real problem, especially if its the rear element.
Minor scratching on the front element may cause a softness in your photos but a large hunk of missing glass (like in the photo above) will show up in every photo. In either case you can forget about the lens. If a lens has been dropped it my have cracks in the internal lens groups. If the crack is on the very out side of the glass element you may have luck, and the crack may not be visible in your photos (especially if its a full frame lens on a crop format camera)
Brands, Mounts and Full Frame Vs Crop
Sony aren’t the only people who make lenses for Sony DSLR’s. In fact the Sony mount came from Minolta (latter Konica Minolta). Minolta sold millions of AF (Auto focus) lenses that that work with Sony DSLR cameras, Minolta also sold millions of MF (Manual focus) lenses that wont fir your camera with out an adaptor.
Sigma, Tamron and Tokina are the big 3rd party names that make lenses for the Sony/Minolta mount. But several others have either made or still make lenses they include names like Cosina, Phoenix, Soligor, to name a few, most of these brands a re re-badged lenses sold under a variety of names. There are a few problems when using 3rd party lenses. Some of the cheaper brands have very bad quality control and should only be bough as a last resort.
Several older lenses from Sigma aren’t compatible with DSLR’s and need to be re-chipped so the communication between the camera and lens works properly. Sigma used to offer this as a free service but as far as I know it now costs approx €25.00 (if the chips are still available). There have also been reports that the AF motor in the new Sony DSLR’s is to strong for some old Sigma 75-300mm APO lenses, an old zoom in that range isn’t a lens I would recommend any way but keep it in mind when lens shopping.
Sony have 2 different sizes of chip set in their cameras crop format (APS-C) and Full foemat (35mm)
If you own a full frame camera (e.g. Alpha 900 and 850) you will only want to buy full frame lenses i.e Lenses that can cover the full image circle of a 35mm film negative. The good news is all the old film lenses are the right size. If you own a crop format camera (e.g. Alpha 700, Alpha 350 etc) you can use lenses designed for full format as well as lenses designed for the smaller APS-C chip set. There are a few advantages using full format lenses on a crop format camera.
• APS-C only uses the centre of a full frame lens, the centre always tends to be the sharpest part (sweet spot) so you are already “cropping” out any problems the lens may present around its edges.
•If you upgrade to a full format camera you already have lenses you can use.
• You have a wider choice of lenses available i.e you can chose from crop format lenses or full format lenses when lens shopping.
Crop format lenses are specificity designed for the smaller APS-C image circle, your full format camera will do one of two things.
• If the lens sends information to the camera that it is a crop format then the camera will only use the centre part of its chip i.e. it will become a APS-C chip set when this lens is used. Not that great as you 24mpx camera is now running at about 11mpx. you can turn this function off i the camera but then you will get the next problem.
•If the lens doesn’t sent information to the camera that it is a crop format then the camera will continue use the full surface of the chip set causing a massive vignette.
IMHO the only real reason to buy a crop format lens is if you want ultra wide angles shots on an APS-C camera