A Review of F-Stops and how they work.
Every DSLR camera manual gives an overview of what F-stops are about, but I have found that looking at it from all the angles lets me understand it better and made me ‘instinctively’ aware of what my camera and lens are capable of and what they are doing. You need this awareness ingrained in you in order to make quicker decisions about F-stops, focal length, light, ISO and shutter speed.
Unfortunately a full understanding of F-Stops and the benefits of faster lenses will make you realize that the weak link in the kits for sale at the entry level is not the camera but the Kit lens that is typically supplied with it. Investment in one or two high quality lenses becomes a priority in most photographers’ minds. The Canon 18-200 mm F3.5 IS Zoom is one of the most versatile lenses one can have in your bag.
How do F-stops work and what is a high F-stop lens? A quick review.
Like much in photography with, F-stops you have to think in opposites. A small number means a big F-stop. A large F-stop calls for a faster shutter speed. Another opposite includes the realisation that investment in lenses will far exceed the investment in the camera for any serious photographer.
Keep in mind that F-stops are fractions of aperture value:
- A seemingly ‘lower value’ F-stop such as F2.8 is a larger aperture than F3.5.
- I repeat: A high F-stop (low number) means a larger Aperture.
- It also means you need a faster shutter speed to avoid over exposure.
- A higher F-stop lets in more light over a shorter period of time, increasing sharpness and limiting depth of field.
Firstly the lens size, expressed in mm is not the diameter of the lens, but the distance of the focal point (middle) of the lens from the sensor. The physical lens size does not come into it. A cropping factor comes into play on DSLR’s. See the DSLR Cropping Factor article.
Lenses are typically rated by their focal length in mm and maximum aperture (opening size of the shutter) expressed in F-numbers. The F-numbers refer to fractions of the focal length, not the size of the lens. For example a maximum F2 for a 50mm Lens would mean that the lens shutter can open to ½ of 50mm, i.e. 25mm. F4 would mean ¼ of 50mm diameter, i.e. 12.5mm and so on. The cropping factor on DSLR cameras does not come into it. The aperture is the same regardless of the camera that the lens is mounted on.
Diameter is not equivalent to Area or light.
50% less diameter provides exactly 25% less area for light to pass through. In the figure on the right, each of the four smaller circles has 50% of the diameter of the larger circle. The dark blue areas correspond exactly to the white areas, so that each smaller circle’s area is exactly ¼ of the area of the larger circle. Halving a diameter is referred to as a full F-stop. If you had to half the diameter of the smaller circles again (by a full F-stop) the area(light) would again be reduced to 1/4 of the previous F-stop.
Shutter Aperture with
50 mm Lens
|% lens open||100%||50%||25%||12.5%|
|Diameter||50 mm||25 mm||12.5 mm||6.25 mm|
In practice lenses do not come in whole F-stops only. So we see lenses with maximum aperture specifications like the following: F/1.2, F/1.4, F/ 1.8, F/2.8, F3.5, F4, F/5.6.
DSLR cameras will allow F-stop adjustments (lens closing stops) at F/1.4, F/ 1.8, F/2 F/2.8, F3.5, F/4, F/5.6, F/8, F/11, F16, F/22, F/32 etc. and more… The bold F-stops are whole stops, each ¼ the area of the former. The numbers in between gives the photographer half or third stops in between for greater control.
The following table provides a clue to why these are popular stops. The bolded lines are full F-stops, and the blue lines are half stops, with third of F-stops in between. Each half step requires a doubling of shutter speed to get the same light exposure. - Again think in opposites: A shutter speed of 1/60th of a second is twice as fast but is half as long as 1/30th of a second.
|Aperture mm||Area mm2||F-Number||Lens opening diameter-mm||Lens Opening area-mm2||% Light||(Roughly)
Fraction of light
I keep a subset of this table on a laminated card in my bag, because when working manually it gives me great info in balancing shutter speed and aperture.
So a smaller F-stop (larger number) allows exponentially less light through, so that the difference between F 1.8 and F 5.6, is ten times the area of light.
The technology to allow a large F-stop like F1.2, F 1.4 or F 1.8 is complex and expensive, especially in zoom lenses and this is reflected in the price. To take advantage of a large F-stop you need a camera that is capable of a very high shutter speed to capture the correct exposure. Fortunately the Nikon and Canon DSLR cameras are up to it.
Both Nikon and Canon entry level DSLR camera ranges have shutter speeds of 1/4000. This is fast enough for just about any light conditions, but if you are photographing extreme light, such as a zoom view of the full moon you might consider using a neutral density filter to give you even more control of shutter speed and bright light.
One affordable option for a large F-stop or fast lens is the 50 mm F1.8 Prime lens available for both Canon and Nikon for less than $100.00. At this price it is an easy decision to buy. It also does not take up much room in your camera bag.