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Explaining on simple terms, the Aperture is the “hole” on the lens, through which the light hits the sensor. The bigger the aperture, the larger this hole is and obviously the smaller the Aperture, the smaller the hole. Below you can see what a big and small Aperture look like.
To use the Aperture on your camera, you have to use the f-number. The f-number, is the ratio between the focal distance and effective diameter of the lens (part of the lens on which the light enters), as described below:
f -number = Focal Distance / Effective Diameter of the Lens
The bigger the Aperture, the more light that enters through the lens, but the smaller the f-number.
The smaller the Aperture, the less light that enters though the lens, but the bigger the f-number.
So we can say that they are “opposite”, because when you say that you have a large aperture you mean that you have a small f-number.
For each stop on the f-number that you decrease, the amount of light that hits the sensor is double (for example from f/2 to f/1.4). Of course, for each f-number that you increase (for example from f/2.8 to f/4) the amount of light that hits the sensor is half.
Aperture changes from lens to lens and the smaller the minimum f-number on a lens, the more expensive it will be, due to technical questions such as lens size.
Amount of Light
Using the same ISO and Shutter Speed on all photos, check how the amount of light is decreased by choosing a smaller Aperture (big f-number). To have the same amount of light with a smaller Aperture, you will need to raise the ISO and Shutter Speed.
Depth of field
The Depth of field is no more than the distance between the nearest and farthest objects in a scene that is in focus. The Aperture has a big influence on this, as the smaller the f-number the less that is in focus. The higher the f-number the more that is in focus.
On the next photos I used the following f-numbers: f/1.8, f/7 e f/22. Check how on the photo with f/1.8 the small Eiffel Tower is on focus and the background is totally blurry. This blurry effect is called Bokeh.
Check now how the apertures with f/7 and f/22 look like.
You can easily see how the Aperture affects what’s in focus and what isn’t.
Does this mean that you should only use the maximum and minimum Apertures and forget the rest? The answer is no!
You can see on the first photo how the top part of the Eiffel Tower is sharp and the base isn’t. Whatever points are on the same distance (same geometric plane) to the lens, will be on focus. You could also chose to have the background in focus and the Eiffel Tower blurred out.
You should also not use the maximum aperture of the lens, if you want the maximum sharpness on the photo. The smaller the Aperture (high f-number, like f/22) the more the light will disperse (diffract), because is passing only on a very small hole. This will affect on the end the quality of the photo and create such effects as the “Light Star Effect”. Check on the photo below how I used this effect.
If you want almost all of the photo in focus with maximum quality, you should use an f-number between f/7 and f/11.
Finding the Lens Aperture Lens
If you own a lens, you can always see written what is the minimum Aperture of the lens. On a zoom lens you can have a fixed or non fixed Aperture.
Non fixed aperture: It will be for example, 28-70 f/3.5-5.6 (see photo below). This means that when you have the focal distance of 28mm the maximum Aperture is f/3.5 and with 70mm, f/5.6
Fixed Aperture: It will say for example, 17-40 f/4. This means that for the focal distance of both 17mm and 40mm, the maximum Aperture will be f/4.
Try now to see on this one, what the maximum Aperture is.
If you said f/2.8, you are right 🙂
Having Lens with a big Aperture (very small f-number, such as f/1.8) makes the same lens, bigger, heavier and more expensive. Not everyone needs this type of lens, so it’s always important to know this before you pay more than you need to.
For example, portrait shooters are the tipical photographers that need lens with a very low f-number, so that the subject is in focus and the background totally blurred out.
On the opposite side, Landscape Photographers are photographers that need all of the scene in focus, so they can have lens that have smaller Apertures (higher f-numbers as maximum Aperture).
If you have any question, just leave a comment below 🙂