Sid,
It's simple.
The 45mm lens can capture light from a wide angle of landscape. Actually anything that is within 25 degrees to each side of a point directly ahead.
The width of landscape is normally limited by that total 50 degrees of capture of that lens.
So, how to increase the width of landscape seen by the 45mm lens?
O.K. we could step back, but there's a road!
Another way is to use a lens with a wider angle of capture, for example a 24 mm lens. Here the angle is 84 degrees. A lot better. However, all this extra information has to be recorded on your one small CMOS chip!
So, if there is a lot of detail in the landscape, that information can't all fit into the single little chip! (Why? lens and pixel limitations).
So, details of a flower or a single leaf are going to be short-changed. How can we alter that? The camera, after all, only has one single sensor!
Now, if we have a 45mm shift lens with the same capture angle of just 51 degees, we can get a wider piece of landscape by making it work as if it had several sensors alligned sided by side, each capturing an adjacent section of the scene.
It works this way. The lens is larger than usual and produces an image circle with the [COLOR=BLUE[whole[/COLOR] landscape focused on the plane which contains, at its center, the single camera sensor.
In fact, much of your great landscape image is shining to either side of the single CMOS/CCD sensor.
Now, here's where the shift comes in. If the lens is shifted to one side, the part of the image to each side can now be made to land on the sensor.
So three images can be taken; one central, one shifting the barrel of the lens to the left and one shifting the barrel of the lens to the right.
(for the advanced user, to prevent errors of parallax, the camera needs to be shifted, not the lens!
We "achieve" this by counter-shifting the camera on the tripod by the same number of mm that the lens was shifted, but in the opposite direction. We'll explain that more fully in a later article.)
When these 3 images are taken (with care to use the same light meter settings since we switched to manual!), they can be imported into photoshop, dropped into a wide empty "New" image frame as three layers.
The overlapping parts are alligned and the image can be flatenned.
Without going into details, by not using straight join lines, one can completely hide any trace that the image was derived from 3.
Now, if you would compare this "panoramic" image with that produced by an ordinary 24mm lens, you would find that there is much more detail although the width of landscape covered is similar.
So this is how a lens' "focal length coverage" can correspond to a lens of about half that particular focal length. In this case from 45mm to 24mm.
This effort allows one to essentialy "double" the MP of your camera for landscape! You get the same image, but it can be enlarged more aggressively for perfect prints.
Asher