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I'm considering switching from DSLR to mirrorless and have been researching the micro four thirds (M43/MFT) and Sony's new Alpha 7 (A7). While Sony hasn't released many lenses for this new system, many are saying the lenses will be much larger and heavier due to the sensor being larger than MFT, despite the Sony being the same size as popular MFT cameras (Olympus OM-D E-M1). Lens size and weight is important to me since I'm wanting roughly APS-C quality or better in a much more portable system.


How are people making the assumption that the full frame sensor of the Sony A7 will net larger heavier lenses to be equivalent to MFT versions? Does this pertain to width, length, or both? Would it be possible for Sony to eventually release smaller lenses similar to the Olympus MFT system without cropping the full frame sensor?

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Micro four-thirds and full frame are two ends of a spectrum in terms of sensor size, if you're wanting "roughly APS-C quality" then there are systems with APS-C sensors which will provide exactly that, e.g. Fuji's X series and Sony's NEX 3,5,6,7 (not A7/A7R). –  Matt Grum Dec 26 '13 at 22:25

2 Answers 2

up vote 7 down vote accepted

All other things being equal, yes.

There are two primary reasons why this is so.

  • To maintain the same amount of field intensity of light over a larger area, a lens used with a larger sensor has to be able to collect more total light than a lens used with a smaller sensor. This means a larger entrance pupil, which usually works out to require a larger front element.

  • The larger the sensor, the longer the focal length of a lens needs to be to give the same Field of View (FoV), and the larger an image circle it needs to project. On a compact camera with a small 1/1.6" sensor, a 50mm lens gives the same FoV as a 215mm telephoto lens does on a 35mm film camera. On an APS-C camera, a 50mm lens results in an FoV that is in the beginning of the telephoto range for a 35mm camera, 75-80mm. On a 35mm camera a 50mm lens is considered normal, because the focal length is very near the registration distance of the camera and because that focal length is roughly equal to the diagonal measurement of the plane that records the image. A medium format camera will give an FoV using a 50mm lens that is about equal to a 32mm lens paired with a 35mm camera. On a Large format 4X5 camera a 50mm lens gives an FoV about equal to a 13mm lens on a 35mm camera.
    a) Especially with longer focal lengths, the physics involved are hard to overcome. Although the strict technical definition of a telephoto lens is one that is designed so that the distance from the image plane to the front of the lens is shorter than the focal length of the lens, there is only so much one can do in reducing the size of a lens with a long focal length. This is especially true if the cost to produce such a lens is considered.
    b) Just as a 50mm lens used on a 4x5 camera must project a light circle considerably larger than the light circle cast by a 50mm lens designed for a 35mm camera, lenses designed for smaller sensors can project smaller light circles. This allows the lenses to be smaller in diameter. It also allows them to be made with less quantities of some of the most expensive materials used in a lens: the corrector element at the rear of the front group.

So a 50mm lens designed for a small compact can be both shorter and smaller in diameter than a 215mm lens that gives the same FoV on a 35mm camera. Even in terms of Micro Four-Thirds sized sensors, only a 100mm lens that throws a light circle slightly larger than 22mm is needed to provide the same FoV as a 200mm lens throwing a light circle at least 44mm in diameter on a full frame camera. An APS-C camera would need a 125-133mm lens with a 27-29mm light circle, depending on whether it is a Canon APS-C or Nikon/Pentax/Samsung/Sony.

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The size of a lens is governed by the complexity of the design and the amount of light that needs to pass through it. For a given focal length with a cropped sensor, the amount of information that has to be brought in to the lens to produce the image circle can be much smaller because the entire full frame image circle doesn't have to be filled. If you are only using 60% of the sensor space, you only need 60% of the image circle and only need about 46% of the lens glass.

This is why a point and shoot can have a lens that is the equivalent of a 19mm to 450mm lens with a fairly quick aperture and only take up an inch or two where as the full frame DSLR version would take over a foot and a be bigger around than the point and shoot camera body is. It only needs to be able to resolve a very thin slice of the middle of the image circle that the full frame lens would have to be able to resolve. This means simpler, lighter, smaller and cheaper lenses, but it also means greatly reduced quality as diffraction plays a bigger role and the traditional sensor size issues all come to bear.

There are technologies like difractive optics that could reduce the sizes some, but that adds complexity and cost in exchange for size. It isn't possible to say that they won't ever make smaller lenses, but there is a good reason why the lenses are bigger and why the larger sensor size and slightly larger lenses are worth the size and cost if you are looking for the best quality you can get.

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This may sound like a nitpick, but I think it's actually quite important to realize how much reducing the sensor size helps creating compact lenses: if using 60% of the sensor area you need only ~46% of glass by volume/mass due to the square-cube law. –  JohannesD Dec 26 '13 at 21:16
@JohannesD - That's not a nitpick at all, that's great information. I'll update the answer accordingly. –  AJ Henderson Dec 26 '13 at 21:27

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