Today's topic is the close-up, as in macro-photography. I am not going to illustrate this as it is rather technical and I want to simplify it for you. Let us say that we want photograph a small insect. If you were using a slr 35mm film camera you might want to fill as much of the image area as possible or at the very least shoot it the same size as in real life. The standard lens for a 35mm has a focal-length of 50mm (this corresponds to the diagonal measurement of the film plane). To obtain an image of 1:1 ratio ie same size you will need to use extension tubes that will add another 50mm to the distance between the effective centre of the lens and the image plane. If you achieve this you would find that point of sharpest focus is about 100mm away. The point of sharpest focus at the widest aperture is very shallow about 1 or 2 mm. To get sharp focus therefore you need to be able to rack the camera in and out with quite a lot of accuracy. It is extremely unlikely that you could use autofocus effectively as there is no point in moving the focussing ring on the camera as all it will do is make your slightly out of focus image smaller or larger as by try to focus using the cameras focus moves the lens further or nearer the image plane.
Now then, if you are trying to do this with a zoom lens you might find it difficult as the construction of a zoom lens uses negative or extra elements to change the focal length. Most lenses of this type use a telephoto design to reduce overall size. The most likely outcome is that the subject ends up so close to the front of the lens that it is impossible. As an example of this the Canon 300D was made with the reflex mirror small enough to site the lens nearer the image plane - but still it is what is called retrofocus (ie the focussing distance is greater than the effective focal length). To achieve optimum close up focussing the focal length of a lens needs to be greater than the physical body depth plus the distance to the effective centre of the lens array. Even if you can achieve real close up with a zoom or retrofocus lens the field of sharp focus will not be flat. Lens manufacurers go to a lot of trouble to make "macro" lenses. However, they are worth it as an investment. As a general rule I prefer single focal length lenses over zoom lenses as I have found them to both sharper and clearer than zoom lenses. If you are thinking of buying new lenses always read the reviews and not just from one source. Never trust a salesman to give unbiased advice. There is a lot more to be said on this subject. I will try to illustrate it next time. Any questions - please ask.