What's the difference between a lens labeled "Macro" and one that isn't?

Started Aug 3, 2012 | Discussions thread
photonius
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Re: What's the difference between a lens labeled "Macro" and one that isn't
In reply to Peter Galbavy, Aug 3, 2012

Peter Galbavy wrote:

Other have eluded to the fact that a Macro lens can focus closer. This isn't quite everything. Most Macro lenses will also employ a "zoom" mechanism even on a prime where the image circle will be expanded when focusing closer than a certain point - usually indicated on the focus ring. This means that the aperture will not, effectively, be as wide at all focal distances. For example both my Sigma 150mm Macro and my Canon 100mm IS Macro are labelled f/2.8 but that is only true up to the point the lens is acting normally.

Hmm, I think you are mixing two things here.

First, all lenses in principle, no matter whether macro or not, will loose light as you focus closer. Many lenses extend when focusing closer, and as the lens elements move away from the sensor you loose light according to the inverse square law, this is a continuous effect. If you go to a macro magnification of 1:1, you loose 2 stops of light. A 100mm lens would have an extra extension of 100mm to go to 1:1. Because of the doubling of the distance, the circle of light has doubled, and light loss in term of area is 4 fold, i.e. 2 stops. In between you will have continuous intermediate values.

Second, some modern lens designs with internal focusing actually change the focal length as you focus closer to achiever closer focusing distances, e.g. the Canon 60mm macro, or the 100mm macro. So, I forgot the exact numbers, but the Canon 100mm is more like a 70mm lens at 1:1. Because the lens has a shorter focal length, it doesn't have to extend as much as the "simple" lens in the first example above, so an "internal focusing" becomes possible. Nevertheless, you still have the same light loss rules as above, as the image circle on the sensor increases, you loose more and more light.

Some of the confusion may arise from the fact that apparently Nikon and Canon deal very differently with this light loss. For canon, if the aperture is set to f2.8, it will stay at f2.8 even at 1:1, the automatic exposure of course calculates it all correctly. In the Nikon case (don't have one), it apparently would change what aperture is indicated as you focus closer.

Macro lenses are also labeled with a ratio which indicate how close to real lilfe the image size projected onto your sensor (or film in ye olde days) is. A 1:1 Macro means that at the closest focus the image on your sensor will be the same size as the physical object you are taking a shot of. A 5:1 Macro will actually magnify the image 5x on your sensor. A 1:2 will only project an image 1/2 the size of the real world onto your sensor etc.

Would love to post links but this is all from my aged memory, sorry.

rgds,
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Peter Galbavy

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