Magnesium bodies

Started Mar 26, 2012 | Discussions thread
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lenses mount to plastic subframes
In reply to Leonard Migliore, Mar 29, 2012

Leonard Migliore wrote:

One big difference between plastic and metal is stiffness. This is typically quantified by the material's elastic modulus. Magnesium, which is about 40% denser than polycarbonate, has an elastic modulus 16 times larger. So a magnesium structure that's the same weight as a polycarbonate structure will deflect 1/12 as much. This is good for stuff like optics that you would like to keep in one place.

This might all be good and well, but what many people don't realize is that the lens mount of many of these magnesium camera bodies actually mount onto plastic subframes made of stiff engineering plastic!

When people look the lens mount of magnesium alloy bodies like the Canon 10D-50D, 7D, or 5D-5D MKII, they assume that the lens mount is attached directly to the magnesium alloy body. But if you remove the shiny metal lens mount, you'll find that it actually attaches to a subframe made of stiff engineering plastic:

It's this subframe of engineering plastic that is really keeping everything in place. The lens mount attaches to this plastic substructure. The tripod mounting socket attaches directly to this plastic substructure. The surrounding magnesium shell is mainly there for show: cosmetics, feel, and marketing. Also, many people also may not realize that the bottom plate of Canon's magnesium 10D-50D, 7D, and 5D-5D MKII bodies are also plastic. If you have any of these bodies, just turn them over, tap on the bottom plate, and you'll see for yourself.

Also, some bodies, such as the Nikon D7000 exemplify this "magnesium for show" principle even more. Many people know the D7000 as having a magnesium body, but the reality is that only the top and back plate are magnesium. Everything else is plastic.

The reality is that whether you're using a plastic-body DSLR or a magnesium-body DSLR, you're not going to see much of any practical difference in their capacity to carry a lens and keep it in place. Over the course of years, I've owned the Canon Rebel XT, 10D, 20D, 40D, 5D, and 60D. They all support lenses just fine, even heavy ones. And the really big lenses have tripod collars to support their weight. I currently own the 40D, 5D, and 60D, and when I mount my 100-400L IS on any of these bodies, they all support the lens just fine. No difference.

Ironically, someone who owns a Nikon D7000 might brag that his camera is stiff because it has a magnesium body, but he may not realize that the magnesium in his D7000 is limited to cosmetic plates on the top and back of the camera only. It's really the D7000's tough, stiff, engineering plastic subframe that is doing all the work.

It's only the high-priced bodies like Canon's 1-series or the Nikon D800 that have subframes that are magnesium. Just don't expect that you're getting the same thing with lower-priced magnesium bodies.

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