Micro 4/3 crop factor confusion

[JkLpl]

Hi
I have a Lumix G3 which is micro 4/3 and decided to try fitting my old 35mm kit Tamron 80/210 zoom lens to see the results. I bought a cheap and cheerful convertor and gave it a go.
Contrary to what I expected I found the image size exactly the same as my Lumix 45/200 lens at the same settings although 2 stops faster. I expected the magnification factor to kick in and for my old lens to be something like a 140/320?
Can anyone explain why this is.

[DanK]

Because the two lenses are about the same focal length.

[xpatUSA]

This might confuse more than it helps but it's certainly accurate:

http://www.falklumo.com/lumolabs/art...nce/index.html

Keeping it perhaps more simple and direct than Lumo's ramblings, the micro 4/3 standard calls for a 17.3x13mm sensor. That's a diagonal of about of 21.6mm. Now, the resulting scene coverage comes from the angle of view which in turn comes from the focal length and the sensor diagonal (sorry, angles are involved - albeit simple trigonometry). If the sensor diagonal is constant (which it is) then it follows that the scene coverage from each lens depends what focal length you set it to. So, if you set, for example, 200mm on each lens then the scene coverages will indeed be the same - more or less.

There's something called "effective focal length" which is why I said "more or less" . . .

I assume that, by "magnification factor", you were referring to the so-called "crop factor"?

[JkLpl]

This might confuse more than it helps but it's certainly accurate:

http://www.falklumo.com/lumolabs/art...nce/index.html

Keeping it perhaps more simple and direct than Lumo's ramblings, the micro 4/3 standard calls for a 17.3x13mm sensor. That's a diagonal of about of 21.6mm. Now, the resulting scene coverage comes from the angle of view which in turn comes from the focal length and the sensor diagonal (sorry, angles are involved - albeit simple trigonometry). If the sensor diagonal is constant (which it is) then it follows that the scene coverage from each lens depends what focal length you set it to. So, if you set, for example, 200mm on each lens then the scene coverages will indeed be the same - more or less.

There's something called "effective focal length" which is why I said "more or less" . . .

I assume that, by "magnification factor", you were referring to the so-called "crop factor"?

Yep. I'm afraid I didn't have a good understanding and have to do some research . I thought when it was quoted as saying there is a 1.5 nactor it was related to the focal length.
Thanks

[Mike Buckley]

I thought when it was quoted as saying there is a 1.5 nactor it was related to the focal length.

You're correct that it's related to the focal length in the sense that a 50mm lens mounted on a camera with a 1.5 crop factor produces the same results as a 75mm lens mounted on a full-frame camera. In your case, you're mounting two lenses on the same camera. So, when both lenses are set to the same focal length, the magnification of the image will be the same whether both lenses are mounted on a full-frame camera or both lenses are mounted on a camera with a 1.5 crop factor.

[dem]

The focal length is the property of the lens itself and it is not affected by whichever camera you put behind the lens. What changes though is the angle of view (AOV) that the same lens can produce on a small sensor (narrow AOV) and on a large sensor (wide AOV).

Most (all?) manufactures mark their interchangeable lenses with the actual focal length in mm. So it does not matter if you are using a native lens or an adapted lens that used to be attached to a full frame DSLR - the AOV will be the same at a given focal length on your M43 camera. A 200 mm lens of any origin will give you the same AOV on your M43 camera.

If you spent 30 year shooting 35 mm film and wonder what kind of lens would give this exact AOV on a full frame camera, the answer is 200 x 2.0 = 400 mm, where 2.0 is the crop factor for an M43 camera. That's why a 200 mm lens attached to an m43 camera can be referred to as 400 mm "full frame equivalent", or "135 format equivalent", or "35 mm equivalent", same thing. There is no 400 mm lens anywhere in sight, it is just a hypothetical reference point.

If you put the adapted 200 mm lens back on the full frame camera, this combo will give you the AOV of an 200 mm lens, obviously, which will be about twice as wide as the AOV you saw on the M43 camera.