1- Why does the Sony Alpha DSLR system doesn't need stabilized lenses?
The Sony Alpha DSLRs use a different approach to stabilize an image than the one used by other camera manufacturers.
In simple terms, the Alpha use sensor stabilization, which means that when the camera senses movement, the sensor is kept as still as possible by motors inside the camera. This helps to reduce the blur seen in pictures when the camera is hand held.
The difference between this system and stabilized lenses is that all the lenses you purchase will have access to the sensor stabilization at no extra cost, making them lighter and cheaper in some cases. DSLR systems that require stabilized lenses require you to purchase each lens with stabilization or vibration reducing motors, which can make them heavier and some times more expensive.
2.- What is a bayonet mount and why Minolta and not Sony developed it?
In photography, the interface used to make a camera and a lens fit and talk to each other is called lens mount. There are screw driven mounts (like the one used in lenses you can attach to P&S cameras), friction lock and bayonet mounts.
A bayonet mount is the most used type of lens mount in DSLR cameras due to the fact that they align precisely the electronic and mechanical components of the lens. It's called bayonet because it's based on the military weapon of the same name. When soldiers needed to attach a bayonet to their rifles, they could do so quickly because of the mounting system. This also applies to photography since DSLRs allow you to change lenses much faster than a screw driven mount.
The Alpha mount was originally the Minolta AF mount, which was the first lens mount to auto focus, this mount was introduced in 1985. When Minolta had to close down operations, Sony purchased most of Minolta's assets, including lens designs, the flash mount and the lens mount. Sony rebranded the Minolta AF mount in to Alpha mount and added further improvements, such as addition of lens contacts in the mount for better camera-lens communication and the use of lenses with the flash ADI function.
All the Minolta AF lenses fit the current Alphas because they all work on the same mount. Minolta lenses older than 1985 can't be used on the Alphas unless you do some custom work on the mount to make it fit.
Lens information explained
This section will explain the items listed in the section Reading a lens of Part 1.
Maker: This states who made the lens, Sony, Minolta, Tamron, Sigma, Carl Zeiss, etc.
Focal length: A lens can zoom or not, but they all got focal length. In simple terms, focal length is how much space there is between the sensor and the lens. The less distance, means a wider angle of view, the more distance, it means the lens can reach far in the distance.
Example: The Sony 18-200mm lens tells you it has a variable focal length. It's shortest distance is 18mm and the longest end is 200mm. At 18mm the lens will have a wide angle field of view whereas at 200 it will have a very narrow view. This is an all purpose lens, which means that can be used to take portraits, landscapes or telephoto shots because of the wide distance range it covers, however this makes it bigger and heavier than other lenses, like the 18-70mm.
Example 2: The Sony 50mm is a fixed focal length lens (lenses with fixed focal lengths are called primes), this means there is 50mm between the sensor and the lens. The field of view is fixed, and to zoom or have a wider angle, you need to move back or forth physically. This focal length is often used for portraits.
Example 3: The Sony 11-18mm is a super wide angle lens with a zoom range. The widest distance is 11mm and the longest distance it will go is 18mm. This lens is designed to take wide angle shots but you can zoom in or out to allow more or less of the scene to be captured. It can't take shots farther than 18mm because it's not designed for that.
Example 4: The Sony 500mm lens is a super telephoto prime lens. You can't move its focal length and everything you will see with this lens will be at 500mm, which will make far things really close to the view. You cant shoot nearby subjects unless they are within the focusing distance of this lens.
Example 5: The Sony 18-70mm is a zoom lens, usually sold as part of a kit. It has a wide angle distance of 18mm and a telephoto distance of 70mm. This lens doesn't have a long reach like the Sony 18-200 or 18-250. It is designed as an all purpose lens which covers enough distance for portraits or landscapes without being too heavy. Since it's usually the first lens most Alpha users get, it teaches them how to pick what focal length to shoot at. Eventually the user will need more or less focal length for different shots and that's when they move to other kind of lenses with different focal lengths.
So when you see a lens with 1 focal length number on it, it means its a prime lens and the zoom changes as you move, if you see a lens with 2 focal length numbers on it, it displays how short and far it will go.
Maximum constant aperture or variable depending on focal length: A lens can or can not keep a constant aperture at a given focal length. It depends on it's construction, lenses that can keep a constant aperture are usually a lot bigger and heavier than one that has a variable aperture.
But what does this mean?
All lenses have a maximum aperture number, such as 1.4, 2.8, 3.5, 4, 8, etc. Having a big aperture increases the amount of light that reaches the sensor and reduces depth of field (how much of the subject is in focus). Depending on the lens construction, the lens can hold on to the maximum aperture even if you zoom or not.
Example: The Sony 70-200 f/2.8 lens its a telephoto lens that can keep the aperture at 2.8 in all the focal length range, be it 70, 100, 150, or 200mm. This is really helpful because if you're shooting under low light and you need to zoom and keep the f number at 2.8, the lens will be able to keep it, giving you the necessary light to work with. The downside of this lens is that is bigger and heavier than other telephoto lenses.
Example 2: The Sony 75-300mm f/4.5-5.6 its another telephoto lens, but this one has a variable aperture. At 75mm, the lens biggest aperture will be f/4.5, but if you zoom at 300mm, the lens' maximum aperture changes to f/5.6. Why does it happen? The lens construction doesn't allow the lens to keep the aperture constant, therefore it has to change when the focal length changes. The plus point of this is that lens size is not too big and heavy, but you have to sacrifice light if you zoom in.
Example 3: The Sony 18-250mm f/3.5-6.3 its a zoom lens with variable aperture. At 18mm the maximum aperture will be f/3.5 and at 250mm it will be f/6.3. As you move from 18 to 250mm, the aperture will change to f/4, 4.5, 5.6 depending on the focal length until you reach f/6.3 at 250mm.
So when you see a lens with just one f-number, it means its aperture it's constant at any focal length (if its a zoom/telephoto lens), when you see a lens with 2 f-numbers, the first means the maximum aperture for the shortest focal length, and the second means the maximum aperture at the longest focal length.
If your photography requires you to work with low light most of the time, a lens with a constant aperture may be better for you than a variable one. The downside of these lenses are their size, weight and price. They tend to be big, heavy and expensive. Third party lens makers however also offer constant aperture lenses at more affordable prices and lighter too.
On the other hand, if what you need are lenses you can carry around, a variable aperture lens is the way to go. You will need to play with shutter speed and ISO to keep the exposure as needed to compensate for aperture changes with focal length.
Focusing motors: Higher grade lenses have focusing motors built into them, unlike normal lenses that depend on the camera's focusing motor located at the mount.
The advantage of these motors is that they make focusing a lot faster and silent.
These motors have all sorts of names depending on the lens maker. In the Sony/Minolta lens range this motor is called SSM which stands for Super Sonic Motor. This motor works at ultrasonic vibrations and makes focusing a lot more precise.
Example: The Sony 70-300 f/4.5-5-6 G SSM is a lens that carries this motor.
Usually this motor is found in high grade lenses, which makes the lens expensive.
Third party lens makers also use this kind of ultra sonic motors, but name them differently. An example is Sigma's H(yper)S(onic)M(otor). Carl Zeiss also uses the SSM motor in their lenses for the Alpha mount.
Higher grade lenses: In the Alpha system, there are lenses that have better quality over the rest of the lenses that are compatible with this mount. By better quality I mean better glass that reduces chromatic aberrations, distortion, have SSM, have more options to customize the lens (like a focus holder button or focus range limiter). Minolta used the G letter to identify these lenses from the rest, and Sony kept it.
Whenever you see a Sony/Minolta lens with a G on it like the Sony 70-300 f/4.5-5-6 G SSM, it means its the best quality you can find in a lens made by Sony/Minolta.
There are also lenses made by Carl Zeiss. These lenses are also high grade lenses, the build quality, glass and coatings used are of top quality as well. They also use SSM.
All Carl Zeiss lenses for the Alpha mount have the ZA designation in the body. Ziess makes lenses for other camera manufacturers as well, but the ones for the Alpha mount are labeled as ZA. The most demanded lens from Zeiss is the Vario Sonnar T* 24-70mm SSM f/2.8 ZA.
That is a zoom lens with a 24-70mm focal range and constant f/2.8 aperture. Uses SSM and has digital coatings (T* designation), the Vario Sonnar designation means its a big lens and the ZA means its a Zeiss lens made for the Alpha mount.
The Carl Zeiss lenses and the G series lenses are the top of the line for the Alpha DSLR system.
Lenses designed for APS-C sensors: Due to the difference in size sensors, image cropping, focal length multiplier issues, etc. manufacturers faced the need to develop lenses that covered the APS-C sized sensor in order to create the desired effect the lens should create, such as wide angle view or effective zoom range for that sensor. A 20mm wide angle lens for full frame is 30mm in APS-C, and the wide angle effect is not visible as it should be.
These specially APS-C size lenses carry the DT designation in order to prevent them from being confused with full frame lenses.
DT stands for Digital Technology. Even though full frame sensors are now available for DSLRs, in the beginning of the digital age, most DSLR makers used the APS-C size and it became a standard size. All lenses that carry the DT designation are only to be used in APS-C sensors. You can use them in full frame, but vignetting will be created.
Lenses like the DT 11-18mm f/4.5-5.6, DT 18-70mm f//3.5-5.6, DT 18-200mm f/3.5-6.3, and the Vario-Sonnar T* DT 16-80mm f/3.5-4.5 ZA are all made specifically for APS-C sensors.
Tamron designates its APS-C size lenses as Di-II, Sigma labels them as DC. Carl Zeiss also uses the DT denomination.
If you need a wide angle lens or a fisheye lens and you use a camera with an APS-C sensor, make sure you look for the DT designation in Sony lenses or the equivalent for third party makers in order to get the effective focal length you need, otherwise you will end up with a different focal length due to cropping.
Type of coatings: A lens focuses light rays in a single point, making it a filter between light and the sensor.
When light hits a lens, it creates all sorts of aberrations, such as chromatic (when subjects have colors they don't have in reality), flare (when light is diffused across the lens surface and make streaks of light appear all over the photo) or ghosting (when a virtual image is formed separate or overlapping the real subject), etc.
In order to address these issues, coatings are applied to lenses. If you grab a lens and look at the front element, you will see there are colors on it, those are the coatings.
Some makers like to label the lens to display the type of coating used on it. Zeiss for example marks their lenses with a T* denomination. There are a lot of denominations for digital coatings and they vary from maker to maker.
The coatings also protect the front lens element, since its the piece of glass exposed to the environment.
The quality of coatings or amount of them used on a lens varies depending on the quality of the lens. Usually higher grade lenses have better coatings than the normal lenses.
Lens kind denomination: Usually a lens doesn't mark if its a telephoto, wide angle, zoom, etc. But sometimes lens makers do mark the lens with the kind it is.
Examples like the 50mm f/2.8 Macro, 135mm f/2.8 Smooth Transition Focus (STF), 500mm f/8 Reflex are example of lenses that carry their kind printed into them. This works to distinguish them from other lenses with similar focal lengths. This way you wont confuse the 50mm f/2.8 Macro with the 50mm f/1.4.
In the case of the 500mm f/8 Reflex, the marking is necessary for two reasons: this lens is not made like all the lenses are, this uses a mirror as part of its elements, which makes it a kind of small telescope and it creates a donut shape of the stuff not in focus, second is because there are other 500mm lenses available, but not constructed like this one, so the marking Reflex helps to differentiate this one from other lenses with 500mm focal lengths.
It's worth mentioning that the Sony/Minolta 500mm f/8 Reflex is the first and only reflex lens to autofocus in the world.
Brief term glossary
SSM: SuperSonicMotor, focusing ultra sonic motor found in G and ZA series lenses.
G: G lens Series, professional high grade lenses.
DT: Digital Technology, denomination to indicate lenses designed especifically for APS-C sensors.
ZA: Zeiss Alpha, Carl Zeiss lenses made for the Alpha mount.
End of Part 2.