There seems to be a bit of confusion among newer SONY Digital forum members concerning GRADES OF LENSES and that of Lens-mount designations. If you are new at this (and YOU know who you are ...), there really are a number of questions that need to be answered and you need to be clear about BEFORE you go "lens shopping." All of those letters in the lens names mean something on the lens boxes and this thread is offered to help hash that out, so we are all talking the same SHORTHAND (or coded-jargon), as it were. LOL
(This is a rough draft ... so you "experienced folks" cut me some slack, will ya? Also, some of this information is somewhat general in nature and can be applicable to other manufacturer camera bodies. in other words ... "please, do read on" ...)
First things first:
SONY DSLR (aka Minolta SLR/DSLR) Lens-mounts come in roughly three types. (For a brief history, use this link -> Minolta AF Mount)
Full-Frame Digital Sensor
APS-C Digital Sensor
Normally, these lenses are not "digitally coated" ... and were developed before 2003.
The "digital coatings" are used to reduce and hopefully eliminate light reflections that occur inside the camera body. These reflections occur when the light initially comes through the lens ... hits the shiny digital sensor in the back of the camera, reflects back to the rear element of the mounted lens and then reflects off, scattered, back to the sensor (at the speed of light!). It can cause all sorts of ghostly and ghastly effects, if the light source is strong enough (basically sunlight and flash response).
Full Frame Digital lenses
Okay, this where the digital-distinction begins ... so please read through this carefully.
"Film lenses" and "FF Digital lenses" share a common basic design ... they BOTH have a larger rear element (the last piece of optical glass, in the lens, where the aforementioned digital coatings are done) than the APS-C lenses. That rear element has to present the image circle to cover the entire size of the Full-Frame sensor (which is equivalent to the size of a 35mm-film frame).
APS-C lenses are designed to work on APS-C sensors (the SONY Full-Frame camera (currently the α900) is designed to include a "CROP" mode with reduces the FF-sensor to the same image area as the APS-C sensor.) and although they are "graduated" (lenses in mm) in same way as the Full-Frame Lenses, their response on the APS-C sensor is actually a bit different.
Because the APS-C sensor is smaller, the image circle created by the APS-C lens is smaller to match up with it. SO, if you place an APS-C lens on a Full Frame camera … the APS-C image circle is not large enough to encompass the entire FF sensor and you get a resultant black halo or vignette around your image. If you put the α900 in “Crop” mode, you basically trim-off this vignette and are left with a solid edge around your image.
Originally termed the "DCF" digital cropping factor, different camera manufacturers had different DCFs with their APS-C line of camera bodies. It is roughly a 1.5 - 1.6x multiplier to the lens you mount. SONY is 1.5x
Basically, what this means, if you look in the image below ... on a Full Frame camera you get a 50mm image, with a 50mmm lens. On an APC-S Sensor camera body, the very same 50mm image is "cropped" down to what a 75mm lens would see on a Full-Frame camera. An APS-C Sensor simply samples less of the FF- Image Circle.
If you have never known a full-frame digital or 35mm-film camera, you would not know about this difference. In fact, using the APS-C camera … you can safely assume that What you see, is what you get (WYSIWYG). Now there is a slight reduction in the viewfinder, so it is possible that the result image you see is going to be about 5% more than what you actually saw. Some of the intro-DSLR cameras … it can be quite a bit more.
I digress. Back to the mounts and the various manufacturers:
Different Manufacturers Designations
SONY differentiates their APS-C lenses from the Full Frame lenses by the use of the “DT” designation. “DT” means for use on the APS-C sensor
TAMRON differentiates their APS-C lenses from the Full Frame lenses by the use of the “Di-II” designation. “Di” means digitally optimized for use on all SONY/Minolta APS-C, Full-Frame & 35mm-film camera bodies. If there is NO “Di” or “Di-II” designation in the lens’ name, the lens was designed for use on a 35mm-film camera and normally is not rated or suggested for use on a digital sensor body.
SIGMA differentiates their APS-C lenses from the Full Frame lenses by the use of the “DC” designation. “DG” means digitally optimized for use on all SONY/Minolta APS-C, Full-Frame & 35mm-film camera bodies. If there is NO “DG” or “DC” designation in the lens’ name, the lens was designed for use on a 35mm-film camera and normally is not rated or suggested for use on a digital sensor body.
Tokina (Currently Tokina does not have the SONY mount in their line-up of lenses, but if they did, it would probably go like this …)
Tokina differentiates their APS-C lenses from the Full Frame lenses by the use of the “DX” designation. “FX” means digitally optimized for use on all SONY/Minolta APS-C, Full-Frame & 35mm-film camera bodies. If there is NO “FX” or “DX” designation in the lens’ name, the lens was designed for use on a 35mm-film camera and normally is could still be used on a digital sensor body, understanding that it was designed for film use, not digital.
Overall, do not use the “Di”, “Di-II”, “DC”, “DG”, “DX” or “FX” designations as deciding some type of lens quality. It has nothing to do with that part of the lens, but merely how the lens works with different film bodies.
You can usually detect lens quality by the retail price you pay for it. Any lens that exceeds the $1000 price point, it is usually considered “professional-level” glass and will be about the best you can obtain in that class lens. They can call it "G" or CZ or whatever, but that dollar amount is usually unmistakable.
Zoom lenses roughly under $400 price point are consider entry-level lenses and usually have substantial drawbacks for “indoor” use. They, more than likely, will require a flash unit of some substance to render an image faster than 1/30th-second shutter speed.
TAMRON SP AF 17-50mm f/2.8 XR Di-II LD Aspherical (IF) <- designed for use on an APS-C Sensor body
compared to a ...
TAMRON SP AF 28-75mm f/2.8 XR Di LD Aspherical (IF) <- designed for use on an FF Sensor/35mm-film body & the APS-C sensor ... but, when mounted on a FF camera, the resultant images are proportionally the same as the 17-50mm lens is mounted on the APS-C.