Focal length and Crop Factor – Photography for Beginners Series
Today we are talking about what focal length actually means and I will be explaining as simply as possible what people mean when they talk about the ‘crop factor’ in relation to your lens focal length.
As always I am keeping it simple for you and we are not going to get heavy on the science at all.
So what does focal length mean?
Focal length is the distance between the lens and the camera sensor. Strictly speaking, it is the distance between the ‘optical centre’ of the lens and the camera sensor. The thing is though, does it matter what the ‘optical centre’ of a lens actually is? I really don’t think so and these things bore me rigid. I am also not going to discuss how focal length actually works. I am not scientific and I would do a very bad job of that. Feel free to look all of that up for some bedtime reading. I am going to move on from that real quick because what is MUCH more important for you to understand right now is how focal length affects what you see when you look through your lens. That is what we are going to focus on here.
A focal length of around 50mm is considered to be ‘standard’. Now what do I mean by standard? Well let’s imagine you are taking a portrait of your friend with a 50mm lens. When you look through a 50mm lens they will pretty much look the same distance away from you as they actually are and the angle of view will be pretty normal to your eyes.
Lenses with a shorter focal length than 50mm (and more specifically, lenses of 35mm or shorter) are considered ‘wide angle’. If you stand in the exact same spot and look through a wide angle lens at your friend again then they will appear to be further away from you than they actually are and you will see much more of the scene to either side of them.
Telephoto lenses are those with focal lengths longer than 50mm. So let’s say you are looking through a 200mm lens at your friend this time. Again you are still standing in the same spot. This time your friend will appear to be much closer to you than they actually are and there will be far less background visible than before.
So let’s think about that 18-55mm kit lens that most beginners end up getting along with their first camera. We call that a zoom lens. A zoom lens is a lens which has various focal lengths and you can zoom in and out easily and quickly. You will see numbers etched on to your lens somewhere. On the kit lens that I am talking about the numbers might say 18-55mm f/3.5-5.6.
But what do these numbers even mean? When you first get into photography they just look confusing!
Well the first numbers – the millimetre numbers – refer to the focal length of the lens. With the kit lens it means that it starts with a focal length of 18mm but you can zoom in to a focal length of 55mm.
The second numbers are f/stops (which if you listened to episodes 2 and 3 you know ALL ABOUT by now). These numbers refer to the widest aperture your lens can achieve. Now with the 18-55mm kit lens the numbers say f/3.5-5.6. This means that when you are using your lens zoomed out to 18mm (wide angle) you will be able to open no wider than f/3.5. When you are zoomed in to 55mm you will not be able to open your aperture wider than f/5.6.
You might have less numbers on your lens. Yours might be a 50mm lens for example, and if so it might just say 50mm f/1.8. This simply means that the focal length of the lens is fixed at 50mm (it doesn’t zoom in and out) and the widest the aperture will open to is f/1.8. A lens which has a fixed focal length and doesn’t zoom in and out is called a ‘prime lens’ and we are going to talk more about zooms and primes in Episode 16 if that interests you.
So let’s go back to 50mm. I have just told you that when you look through a lens with a focal length of 50mm that everything should appear pretty standard – not closer or further away and the angle of view should be similar to what you actually see.
You might not like me when I tell you that is not always the case!
Because everything I have just said only applies to full frame cameras.
But what is a full frame? Well, full frame DSLRs have a much larger sensor than entry-level cameras or compact system cameras (CSC/mirrorless system). We call the sensor on these cameras a ‘cropped frame sensor’. A cropped sensor allows the camera to be much smaller and lighter and cheaper to manufacture. In the early days of digital all cameras had a cropped sensor because the cost of creating a full frame sensor was just enormous. Over time and with advances in technology full frame did become more feasible and eventually more affordable.
But even today full frame DSLRs are MUCH heavier and larger than cropped frame cameras and they cost MUCH more so it tends to be a purchase that is only made when someone starts to get serious about their photography.
Most of you listening to this podcast will be shooting with a cropped sensor. These are still great cameras. You just need to be aware of some focal length implications…
Because the sensor has been cropped it captures the light from a smaller area of your scene. So when you look through a 50mm lens using a camera with a cropped frame sensor you will not see as much of the scene as I would see if I looked through a 50mm lens on my camera which has a full frame sensor.
We call this the ‘crop factor’.
I do not want you to get hung up on this. There is loads of science behind it all but I believe all you need to know is this;
Because the sensor has been cropped, this means the scene it captures is cropped too. So when you look through a cropped sensor camera you will see less of the scene than someone using the same lens on a camera with a full frame sensor. The edges of the scene are cropped off for you so your scene will be more magnified. And that is basically what a cropped sensor does. It adds magnification to your lens. In fact it adds about one and a half times the focal length.
So, for example, if you put a 50mm lens on your cropped sensor camera it will behave like a 75mm lens. So it will actually behave like a telephoto lens. 50mm x 1.5 = 75mm
If you have a crop sensor camera and you really want a standard angle of view like you get with a 50mm focal length – as in you want to look through your lens and see pretty much what you see with your eyes – then you would need a focal length of 35mm. Because 35mm x 1.5 will give you that 50mm feel that you are looking for.
Why is this important?
Well it’s important that you know when you buy a lens for your cropped sensor camera that you are aware of the magnification that will be added to it. I wouldn’t want you to go and purchase a 50mm lens and wonder why on earth it is bringing your subject much closer when you were told that 50mm was a standard angle of view. It is important that you know about it but it certainly isn’t necessary to dwell on it in any way. Just accept it and move on (like I do).
I am going to let you mull that over before next time. Next time you will be learning which focal length is best to use in different scenarios. So for example which is the best focal length for portraits and which is best for landscape and why. I hope you tune in for that.
TEA BREAK TASK
For now I want to set you a very short task and that is simply to check the focal length of the lens (or lenses) you have. I want you to take note of whether they are zoom lenses or prime lenses and I want you to also take note of the widest aperture you are going to be able to achieve with them. This should all be etched on to the lens itself or, if not, on the manual or box it came in.
Until next time…