Question for the techies here.

[Ron Evers]

I shoot with a micro four thirds (m4/3) camera which has a 2x crop factor. I was in a camera shop buying extension tubes for my Canon FD 50mm 1:2 macro & the man behind the counter said "that is a very nice lens, do you realize you get greater than the 1:2 magnification on your crop body?" This lens uses a 25mm tube to convert it to a 1:1 macro. I bought a 15 & 25mm tube.

A smaller field of view is the only consequence that I can see. What say you, regarding the greater magnification on the crop body with a macro lens. I think it's bull but I am no lens expert.

 

[Overread]

Magnification is always a fixed ratio between the size of the reflected image on the sensor and the size of the object in real life. As such magnification itself never changes as the sensor/film size changes - it will always remain the same.

However if you take a photo from, say, a 35mm film/sensor and compare it to a sensor/film half the size and then print both to the same dimensions the photo taken with the smaller sensor will appear to be more magnified. Even though the "magnification" hasn't changed, when the photos are compared at a like for like dimension smaller sensors appear to get more.

 

[Ron Evers]

Magnification is always a fixed ratio between the size of the reflected image on the sensor and the size of the object in real life. As such magnification itself never changes as the sensor/film size changes - it will always remain the same.

However if you take a photo from, say, a 35mm film/sensor and compare it to a sensor/film half the size and then print both to the same dimensions the photo taken with the smaller sensor will appear to be more magnified. Even though the "magnification" hasn't changed, when the photos are compared at a like for like dimension smaller sensors appear to get more.

My thought exactly.

However, I was second guessing myself due to the fact it is a macro lens. A normal 50mm lens would need a 50mm tube to convert it to a 1:1 macro but this 1:2 lens only requires a 25mm tube to convert it to a 1:1. That I do not understand.

 

[Overread]

I think its because the more correct formula for extension tubes is something like:

(length of the extension tubes in mm - divided by - focal length of the lens ) Added to the native magnification of the lens itself = magnification :1


so for your lens its:
(25mm / 50mm ) + 0.5 = 1:1

Where 25mm is the tube length
50mm is the lens focal length
and 0.5 is the numbered version of the magnification of 1:2

I think the addition of the native magnification often gets dropped because its a much smaller value than the gained magnification when using more normal lenses with extension tubes (which is a more common usage - whilst those that do use them on tubes tend to just work out the magnification from the photos of a ruler rather than the maths).

 

[Ron Evers]

Thanks.