Camera Aperture: Technical Details You Need to Know
So let’s talk about camera aperture! What is it? How does it work? Why do we care? All these questions and more shall be answered!!!
In its simplest terms aperture can be considered as the amount of light let in by the lens to the camera sensor. The wider the opening, more light gets exposed to and captured by the sensor.
This is why aperture is so important. By controlling how much light enters the camera, we can control the exposure of our image.
Sounds straightforward, right? So let’s take a closer look at the terminology associated with aperture.
Aperture (the is the actual opening in your lens) size can be expressed in terms of an f-stop – which is simply a term that indicates the aperture measurement.
When talking about aperture’s and f-stops a higher f-stop numbers indicate a smaller aperture (less light), smaller numbers indicate a larger aperture (more light). Here’s a nifty graphic to help you visualize this.
But what does f/1.4, f/2.8, and f/8 actually mean? Are they just random numbers? Do they mean something? If so then how are they calculated?
It’s actually a pretty simple formula:
f-stop = focal length / diameter
In this formula focal length is just the distance from your lens to your image sensor. Diameter is the diameter of the entrance pupil – the effective aperture size as seen from the front of the lens. So if you were looking at your lens from the front, how large would that hole look? This is the concept of entrance pupil diameter. It’s basically just the effective aperture size as seen from the front of your lens.
So, for example, if you have a focal length of 50mm and your entrance pupil diameter is 12.5mm then you get an f-stop of 4 (which would be expressed as f/4). It should be obvious that as the diameter of your aperture (your denominator) increases the resulting f-stop number will decrease.
Now that we know how to calculate it, what does it actually mean?
Well, at its heart f-stop is a measurement of how much light your lens is letting in. But how do we use that information?
Large apertures (small f-stop numbers) result in shallow depth of field. This means that objects close to your camera will be in focus, but objects further away will start to blur.
On the other hand, if you stop your aperture down (higher f-stop numbers), you will increase your depth of field. So now both close and far away objects will appear more in focus.
But what is the reason for this?
Think about it this way, when you’re taking a picture, light is entering your lens and reflecting off of your subject. That light then passes through your aperture and hits your image sensor.
Now, the size of your aperture obviously affects how much light hits the sensor (that’s why it’s so important for controlling exposure), but it also affects something else – the angle at which that light hits the sensor.
Remember that when we talked about entrance pupil diameter we said it was basically just the effective aperture size as seen from the front of your lens. So if you have a large entrance pupil diameter (large effective aperture size), then that means light is hitting your image sensor at a shallower angle.
Conversely, if you have a small entrance pupil diameter (small effective aperture size) then that means light is hitting your image sensor at a steeper angle.
Now what does this have to do with depth of field? Well think about it – when objects are further away from our camera they appear smaller in our frame. So if we’re using a large aperture then the angle of incidence (angle at which light hits our sensor) will be shallower for those distant objects and thus they will be out of focus.
On the other hand, if we’re using a small aperture then the angle of incidence will be steeper for those distant objects and they will appear more in focus because more light from them will actually hit our image sensor!
A small (or narrow) aperture allows distant objects to remain in focus
Now you might be thinking, “If large apertures mean shallow depth of field, and I always want my images to be in focus, then why would I ever use a large aperture?!
First of all, as we just discussed, large apertures also mean more light enters the camera. So if you’re in a low light situation (say indoors without flash), then using a large aperture can help make sure your image is properly exposed.
Secondly, sometimes you WANT your background to be blurred out! This is often the case with portraits where you want the focus to be on your subject and not on whatever is going on behind them.
An example of bokeh (the aesthetic quality of the blue produced in out-of-focus parts of an image)
Aperture Priority Mode
Many cameras have a shooting mode called Aperture Priority. This allows you to select the aperture you want and the camera will automatically adjust shutter speed to maintain a proper exposure.
Why would you want to use Aperture Priority Mode? There are two main reasons:
You can control depth of field. So if you want a shallow depth of field (for those nice blurring backgrounds), just dial in a low f-stop number and let the camera do the rest!
Less worry about shutter speed. In general, as long as your shutter speed is above 1/focal length of your lens, you should be fine hand holding your camera (ie no tripod necessary). So if you’re using a 50mm lens, then as long as your shutter speed is 1/50th sec or faster, you can probably get away with not using a tripod. Of course there are other factors (movement of your subject, how steady your hands are, etc), but this is generally good rule of thumb (this is known as the reciprocal rule btw).
But wait!!! I hear some of you shouting “What if I’m in low light?!”. Well don’t worry, that’s where larger sensors come in handy!!! If we have a larger sensor capturing more light then we can afford to have a smaller aperture and still get proper exposure while maintaining fast enough shutter speeds for hand holding!
Do you see now why aperture is so important and how having a larger sensor gives you an advantage in low light situations? I hope so!