# How does a macro lens compare to a telephoto one?

Started Dec 17, 2012 | Discussions thread
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 Re: How does a macro lens compare to a telephoto one? In reply to dpyy, Dec 18, 2012

dpyy wrote:

nelsonal wrote:

A macro lens focuses close allowing very small objects to fill the image frame. A long lens (most are telephoto but I have a few that aren't) has a narrow field of view that allows distant objects to fill the frame. Imagine an elephant, a long lens would allow the elephant to fill the image at a fairly long distance. A macro lens would allow the frame to be filled with the elephants eye. A long macro lens will fill the frame with the eye from a greater distance than a shorter macro lens (other lenses won't be able to focus on the eye when it's at a distance close enough to fill the frame).

Since macro is usually a fixed magnfication, the various focal lengths (30mm to 200mm) alllow variance in the "working distance" between the lens and the subject at macro magnfications. Longer lenses allow more space (at the same magnfication) which means you can be further from say a wasp who might decide the strange object that just approached to 3" was a threat.

So if I understood you correctly I can still put the elephants eye onto the frame with my 55-200 telephoto lens it's just that it won't be able to focus on things that small?

Hi dpyy,

To understand the reproduction ratio better, you should start with the physics of optics. Have you come across the very famous lens equation 1/f=1/a+1/b and the ray diagram during your secondary school physic class? By drawing three rays you can determine the size, position and image form by the optics.

As long as the object (subject) is further away from the lens by 2 focal length distance, the image form will be smaller than the object.

When the object (subject) is 2 focal length distance from the lens, the image will have the same size as the object. This is the so called "reproduction ratio of 1". This image is 2 focal length distance away from the lens as well, but at the opposite side.

When the object (subject) is less than 2 focal length distance from the lens, the image form will be larger than the object.

A macro lens that can produce a reproduction ratio of 1 is a lens that can focus an object ( subject) 2 focal distance away from the lens to an image plane (CCD) which is 2 focal distance away at the other side of the optics. This is the only formula how most macro lens works.

Non-macro lens (reproduction lesser than 1) including the telephoto you mention is "longsighted" or technically called "Hyperopia". When the object (subject) move too close to the lens, the image form by the lens is behind the CCD. To collect this, the extension tube come in to help the lens to see object close. Extension tube is a cheaper option to Macro photographing. However, it can't guarantee a reproduction ratio of 1 (or more) and it is still depending on the construction of the lens.

Hence, you can fill up the frame with the elephants eye using a telephoto lens because the elephants eye is bigger than your camera CCD. Similarly, you can't fill up the frame with a fly's eye using a telephoto lens as a fly's eye is smaller than your CCD. You need the reproduction ratio.

To fill up the frame with a fly's eye, you can do 2 things.

1. Find a MACRO lens that have a reproduction ratio of 1 and a CCD that is the size of the fly's eye.

or

2. If you can't find a CCD that is same size with the fly's eye, look for a MACRO lens that have a reproduction ratio of >1 and hope that the magnification fit. :-P.