Understanding magnification

[KirbyKrieger]

I can't seem to wrap my mind around magnification.

Can anyone recommend to me a good analytical photography guide/textbook/site on this?

Here are some specific questions I can't figure out how to answer.

Why is the magnification for zoom lenses not a range?  As I currently understand it (mostly from the Cambridge in Colour page on Macro Camera Lenses), the magnification of a lens is determined by only the focal length and the minimum focusing distance, in which case there should be a range of magnifications for zoom lenses.  Yet each lens lists just one.  In particular, I am puzzled by the magnification listed for the Sony 70-400G (my most used lens, on a full-frame Sony a850).  It is listed as 1:3.7.  Using the calculator on the CinC page, the magnification for a 400mm lens with a min focusing distance of 1.6 m should be 1:1.

How can I determine which lens will give me the most information (highest resolution, I think) for a given subject size?  (I use "subject" here to mean the entire scene, including everything in it.)  Again according to the CinC site, this is determined strictly by magnification and sensor size.  Since I am at present only interested in results for one camera (with, of course, one sensor), the magnification should tell me all I need to know.  For a full-frame camera, 1:1 will allow me to focus on and capture a 24 x 36 mm subject, while 1:4 allows me to focus on a 96 x 144 mm subject (effectively 1/4 the information density per subject area).  I conclude from that that the macro lens will give me more information for subjects the size of the sensor or smaller.  If the subject is 96 x 144 mm, will the macro lens give me the same amount of information as a lens with a magnification of 1:4?  What are the other important factors to consider in order to maximize the amount of information one captures relative to the subject size?  Is there any practical difference between, say, the sharp 100/2.8 Macro and the also sharp 70-200G/2.8 at 100mm capturing a subject 100 x 166 mm?

Thanks in advance for teaching me how to think about these things.   

[mouse]

the magnification of a lens is determined by only the focal length and the minimum focusing distance, in which case there should be a range of magnifications for zoom lenses.  Yet each lens lists just one.

Quite right.  However the spec sheet for zoom lenses usually gives only the maximum magnification available with that lens; determined at the maximum focal length and the minimum focus disance.

Don't confuse maximum magnification with maximum resolution.  They rarely turn out to be the same thing in any given lens.

[bradleygibson]

Zoom lens designs do not necessarily have a consistent minimum focusing distance throughout their focusing range.  As Mouse stated, the magnification number quoted is the greatest magnification you can achieve with that particular lens.  Since you can achieve any factor less than that, it may not make much sense to specify a magnification range  (the lower limit is effectively zero if you can focus at infinity).

[EricV]

You are correct that magnification is the basic factor to consider.  Start with your image requirements, then calculate the needed magnification.  For example, if you want to photograph a subject which is 96x144mm, using a camera with a sensor which is 24x36mm, then you need a lens magnification of 1/4. 

Any lens which can achieve the required magnification will capture the subject on the full sensor frame, giving the best detail. Any lens which cannot achieve the required magnification will capture the subject on only part of the sensor.  You will then have to crop the image to print just the subject, effectively wasting sensor resolution.  Unless the second lens is much better optically than the first lens, you will end up with less detail in the final photograph. 

Macro lenses are optimized to perform well at large magnifications, so they should have a small optical advantage in that sitiuation.  For small magnifications, a macro lens will not produce better results than a normal lens. 

The lens focal length and the corresponding focus distance are secondary considerations, which affect image perspective and working distance.  For some subjects, these considerations may be important.  However, the primary concern for capturing detail should simply be to fill the frame with the subject.

[KirbyKrieger]

EricV -- many thanks.  Your reply is what I was looking for in all regards.  Appreciate the specific and clear answer.

I'd been partly thrown off balance by the behavior of my Sony 100/f2.8 Macro, which changes the magnification while focusing.  There are even marks on the lens which tell the user what the current magnification is.  I now "understand" that macro lenses actually change the focal length of the lens (they zoom) -- in effect, many macro lenses are regular lenses with built-in extension tubes.  I had been trying to understand the macro lens as a prime, and I didn't have the experience to know that my assumption that primes have a fixed focal length could be wrong.

Again, thanks.   

[pegelli]

I'd been partly thrown off balance by the behavior of my Sony 100/f2.8 Macro, which changes the magnification while focusing.  There are even marks on the lens which tell the user what the current magnification is.  I now "understand" that macro lenses actually change the focal length of the lens (they zoom) -- in effect, many macro lenses are regular lenses with built-in extension tubes.  I had been trying to understand the macro lens as a prime, and I didn't have the experience to know that my assumption that primes have a fixed focal length could be wrong.

Kirby, I wouldn't call it "zooming" because at every distance there is still only one focal length (and corrosponding field of view and magnification).
The way I see it is that focussing of a lens to shorter distances than infinity can be achieved by a combination of two mechanisms:
1) moving the lens further away from the film/sensor
2) leaving the lens distance the same, but move optical elements such that the focal length shortens
The Sony 100/2.8 macro does both of these, and at full extension (1:1 magnification) the focal lenth is actually 87.5 mm. The Sony 50 mm macro only uses the first method. Most 3rd party zoom lenses like the Tamron 90/2.8 or Sigma 105/2.8 work the same way as the Sony 100/2.8, allthough their use of method 2 is stronger. There's an easy way to calculate the focal length at 1:1, just take the sensor-object distance at 1:1 from the lens spec-sheet and devide that by 4.