Please excuse me if I am telling you stuff you already know but I thought that this might be useful for an audience that doesn't know somw of the fundamentals about photography.
The aperture size is quite important when used in macro photography. The smaller the aperture size (the higher the f-stop) then the greater the depth of field
. This is the extend of the depth that is in focus.
I saw a great example that can be used to illustrate this in the "Get me some relief" thread
and I have attached this image below. I hope
that the member (Dominicus) doesn't mind. There is nothing wrong with the image but it does serve as a great illustration.
This coin was photographed with the coin tilted back rather than square on to try and show off the relief. The focal point on the coin can be seen as the centre of the area
of the coin that is in focus. I have added a small white dot (roughly in the centre of the image) at the place that I think is the focal point as assessed by the camera. As you scan towards the top of the coin you can see that the coin become more blurred because it is "out of focus" and the same wwhen you scan from the centre towards the bottom of the coin. The area
that you can see that is acceptably in focus is called the depth of field
The illustrated coin was photographed with a shutter speed of 1/100th of a second at F2.8.
The extent of this depth of field
is a function of the aperture. The larger the aperture (smaller f-stop) then the smaller the depth of field
i.e. the less area
of this image that would be within acceptable focus and thus a smaller aperture allows you to increase this depth of field
. The problem with this is that the smaller the aperture the more time is needed to get the correct amount of light into the camero for a correct exposure and thus the shutter speed gets larger with a smaller aperture. The shutter speed for any given aperture is a function of the amount of light and determined by the meter reading.
This now leads us to another potenital problem. As we make the aperture smaller to increase the depth of field
and the shutter speed has increased i.e. the shutter is open for longer then we need the camera to be stable to maintain a sharp
image. Any shutter speed slower than 1/90th of a second is open to introducing camera shake and even at these speeds it can impact on an image. With a macro image of a coin where we are trying to bring out as much detail in focus as possible any shake in the camera is liable to introduce blur. The image may well be in focus but because the lean is moving around the image blurs. As a result of this we need to use something such as a tripod or stand to keep the camera stable. Even having the camera on a tripod doesn't eliminate vibration however as the act
of pressing the button on the camera can cause the camera and tripod to brirate which can introduce blur. The solution to this is to use some form of remote shutter release or time delay release for the camera to avoid any chance of vibration.
I apologise for the lengthy email but once I started it just seemed to go on and on.... (sums me up!)