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Among the Sony lenses there are some types with a DT abbreviation and some are without it. Those having DT called DT-series.

What does DT mean? What is the difference between DT-series lenses and the rest of Sony?

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Look here: photo.stackexchange.com/questions/496/… –  jrista Aug 1 '10 at 14:31
    
Thank you! Maybe, if this question is viable as stand-alone, you could make your tip as an answer? Or is it better to delete the question? –  rem Aug 1 '10 at 14:40
    
I think this question should stand - I've said why in detail at meta.photo.stackexchange.com/questions/199/… –  Hamish Downer Aug 1 '10 at 15:11

4 Answers 4

up vote 5 down vote accepted

DT (Digital Technology - source) means that the lens is designed to work with a cropped sensor camera. It should not be used with a full frame sensor or 35mm film. If you use it with a full frame sensor then some of the image would be black, particularly towards the corners, as the light projected by the lens would not cover the sensor.

Having a smaller area to project light on to means that lenses can be smaller, lighter and are generally cheaper. However if you later upgraded your DSLR body to one with a full frame sensor you would have to replace the lens.

Similar questions:

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Thanks, your info is really helpful –  rem Aug 1 '10 at 19:12

Regarding: However, they will not be able to take full advantage of a full-frame sensor.

The first fact is 95% of the DSLR users use APS-C sized cameras and the second fact is with few exceptions (Sony DT lenses, for example), almost all DSLR lenses are designed exclusively for the need of full-frame cameras. As such, those lenses cause the 95% DSLR users mismatch problems of all sorts. The simple fact is the 95% DSLR users are in dire need for APS-C lenses (such as DT lenses and few others offered by Nikon and Canon, etc.). The majority of those 95% users will never have a need for full-frames, let alone preparing for a so called upgrade. My point is the 95% should be the majority and not the minority and certainly do not deserve the 5% lense treatment by the camera and lens manufacturers and especially do not need the minority piont of 'not be able to take advantage of a full-frame sersor.

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1  
Another fact is that majority of DSLR users never shell out money for an extra lens, or they'll only buy the cheapest prime, so they wouldn't really care. Among those who do invest in optics, expectation of upgrading to full frame one day is higher than average. Especially if they look for quality specialty lenses, not merely a variable aperture tele zoom to extend reach. –  Imre Apr 2 '12 at 11:32
    
This does not appear to answer the question. Perhaps it would fit better as a comment, though it is also wrong since standard lenses work equally well on both FF and APS-C bodies, where as APS-C only lenses can't reach as much of the market, particularly the portion that buys lenses. –  AJ Henderson Sep 30 '13 at 14:00

TL;DR answer: DT lenses have a smaller image circle designed to match APS-C sensors, which are smaller than full-frame sensors. This makes it possible to design high-quality lenses that are smaller, lighter, and less expensive than an equivalent full-frame lens. However, they will not be able to take full advantage of a full-frame sensor.


A DT lens is designed so that the image circle projected by the lens covers the 16x24mm APS-C area, but not necessarily the 24x36mm area required by a 35mm full-frame camera. In the case of zoom lenses, the image circle may cover full frame at certain focal lengths, but this is not true for all zooms. DT prime lenses either do not cover full frame at all, or would exhibit serious optical compromises such as extreme softness in the frame corners on a full-frame DSLR.

When the image circle only needs to cover the APS-C area, several advantages can be realized. It makes it much easier to design lenses with shorter focal lengths, allowing lenses that are truly wide-angle on APS-C. An 18mm lens is expensive and difficult to design and manufacture if it needs to cover full frame, but can easily be done if it only needs to cover APS-C. The smaller image circle allows for smaller and therefore lighter lens designs, especially for lenses with focal lengths up to medium telephoto. It enables superzoom lenses like those made by Tamron to be compact and lightweight while retaining reasonable image quality. It allows fast lenses to be made more easily as well. It also makes the lens significantly less expensive to manufacture. These advantages hold for all manufacturers of APS-C cameras and lenses, not just Sony. In fact, Canon exploits the fact that APS-C camera mirrors are smaller with the EF-S mount, an extension of the EF mount that allows the rear element to be placed farther back than in EF lenses, making it even easier to design ultra-wide lenses (such as the remarkable EF-S 10-22mm lens).

Of course, it means that you won't be able to take advantage of the full-frame sensor in high-end models. On Canon, an EF-S lens physically cannot be mounted on full-frame or APS-H bodies with an EF mount and can only be used on APS-C bodies with an EF-S mount (which can take full-frame EF lenses as well); the rear element may extend past the lens mount far enough to collide with a full-frame or APS-H mirror if an EF-S lens could be mounted on such a body. On Sony, the image area will be limited to that of the APS-C crop area of the sensor leading to a loss of resolution, and exposure metering accuracy may be compromised, because darkened or black frame corners (vignetting) can confuse the light meter and cause overexposure.

According to Sony (you'll see this on all Sony DT lens product pages):

Advisory when using DT lenses with full-frame (35mm equivalent) DSLR cameras

Sony α (alpha) lenses with the Digital Technology (DT) designation are ideal for use on APS-C size DSLR cameras and are not recommended for use with full frame DSLR camera bodies (such as the DSLR-A900). Although DT lenses can be used with full frame cameras, they can only realize a maximum of 11MP image size (when using the DSLR-A900), and are not guaranteed to provide accurate Auto Exposure performance.

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The above poster is generally right, however most Sony Full frame DSLR's have a mode that allows you to use a DT lens with the full frame. What it will do is crop out the black edges that Hamish is talking about, and half the resolution (megapixels).

So the DT lens can still be usable if you upgrade your camera, but keep in mind the half megapixel thing. eg. If you have a 24MP fullframe Sony Alpha, your images can only be a maximum of 12MP.

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