Aperture – Magnification – Depth Of Field
These three concepts are extremely important for the macro photographer, but they are not always fully understood by everyone.
This is often a source of confusion. Why is 16 smaller than 4? Well, the numbers are actually fractions, you often see them expressed as f/16 and f/4. The f stands for focal length of your lens, so if you have a 100 mm lens, f/2 would be 100/2 (100 divided by 2) or 50 mm. This is the actual opening size of the aperture. So you see, f/4 would be 100/4, a 25 mm aperture opening, and f/8 would be 100/8 or a 12.5 mm opening.
With a 50 mm or a 300 mm lens it works the same way.
50 mm lens at f/2 = 50/2 = a 25 mm opening
50 mm lens at f/8 = 50/8 = a 6.25 mm opening
300 mm at f/8 = 300/8 = 37 mm
300 mm at f/22 = 300/22 = 13.6 mm
To save room on the lens barrel, usually only the bottom number of the fraction is used, but you can see why 16 is actually smaller than 4.
The scale might seem odd too, but it is logarithmic, every number is divided the square root of 2 (approx. 1.414) to get the next value. So you go from 1.4 to 2 to 2.8 to 4 to 5.6 and so on. Equally important is that every step to the next number (sometimes called an f-stop or just stop) represents a reduction of light reaching the sensor or film by half. If you go from 4 to 5.6, only half as much light hits the sensor, from 5.6 to 8 is half again. The reverse works too, so you might say you want to open up one stop, to double the light hitting the sensor.
Enlargement refers to the relationship between how large something appears on the camera sensor, and it’s actual size. Contrary to some reports, the sensor size has no bearing on this. If something is 1 cm on the sensor, and is actually 2 cm, the magnification is said to be 1:2 (sometimes called 1/2 or 0.5). If on the sensor your subject is 5 mm, but in reality it’s only 1 mm, then you have a magification of 5:1, 5/1 or just 5. You are seeing your subject at 5 times actual size.
But remember, you magnification is different from your focusing distance. The focusing distance for a certain magnification is determined by the focal length of your lens.
DEPTH OF FIELD:
Depth of field (or depth of focus) is defined as the distance in front of and behind your actual focus point, that still appears sharp to our eye. Everything that is closer, or farther away, gradually get more out of focus. The smaller your aperture, the greater your depth of field. The greater your magnification, the smaller your depth of field.
So you see that depth of field is controled by a combination of your magnification and your chosen aperture. If you change one of these, you will also change your depth of field. With macro photography, depth of field is very limited. It is important to know beforehand what your depth of field will be with a certain combination of megnification and aperture.
To calculate the depth of field, you can use the formula shown here. Your aperture is “f” and your enlargement is “v”. Lets step though it with the following example. Suppose you want to photograph something at 1:2 with an aperture of 8. The calculation will look like this: above the line you get 0.0666 x 8 x (0.5 + 1) = 0.7992. Below the line you get 0.5 x 0.5 = 0.25.
Your result is 0.7992 / 0.25 = 3.1968. So we get a depth of field of about 3.2 mm.
When you’re shooting, you can check your depth of field with the camera’s preview button. This will close the aperture to your preset, and allow you to see the actual depth of field through the viewfinder, or on your display with live view. You can then adjust the aperture as needed to achieve the results you want.
Remember, you don’t always have to go for maximum depth of field to get a great shot. Sometimes it might be much better to choose a larger aperture to help soften a messy background, this will help isolate your subject.
In the picture of the grasshopper, I decided to use a larger aperture to focus only on the eye.
In the spider with prey image, I chose as small an aperture as possible to get everything in focus. The background is very even so there was no danger of it getting messy.
With the dandelion picture, I also used a small aperture, and in this way the flowers in the background came in to view in an attractive way.