Focal Length
When looking at a lens, there are two numbers that are always displayed. Aperture and Focal Length. The aperture is usually shown as 1:1.8, 1:4, 1:3.5-5.6. If you want to read more about the aperture number and what it means go HERE. Focal length is usually shown as a number followed by "mm", e.g, 50mm, 85mm, 18-200mm. It does not represent the length of the lens but instead, it represents the the distance between the point all the light rays converge and the film or digital sensor. Below is diagram showing you what that looks like if we were to use a simple pinhole camera.
In the image above, the triangle is the subject of the picture. The four lines converging at the hole in the box represent the light entering the camera, where they cross is where the focal length is measured from. The white square in the box is the medium for capturing the image such our film and digital sensors today.
Field of view (FOV)
The purpose of focal length is to determine the lens Field of View (FOV). FOV is the amount of degrees visible while looking through the lens. The larger the focal length the narrower the FOV. This effect is easy to reproduce by creating a circle with youth thumb and index finger. Now move it in close to your eye then away from your eye while looking through it. When your arm is fully extended you see very little, but as you bring it closer, you begin to see a whole lot more. If you measured the distance from your eye to the circle you created, that is focal length.
Field of view (FOV)
The purpose of focal length is to determine the lens Field of View (FOV). FOV is the amount of degrees visible while looking through the lens. The larger the focal length the narrower the FOV. This effect is easy to reproduce by creating a circle with youth thumb and index finger. Now move it in close to your eye then away from your eye while looking through it. When your arm is fully extended you see very little, but as you bring it closer, you begin to see a whole lot more. If you measured the distance from your eye to the circle you created, that is focal length.
Your camera lens actually does not capture a rectangular image like I have shown above, instead it creates a circular image. However, your cameras sensor is rectangular and only picks up the rectangular part in the middle of that circle.
If you did the experiment above you could do some math to get the FOV from the circle you created. If you compared the FOV you calculated to what others have calculated, it would be different. This is because people are not born with standard sizes but lucky for us, cameras are not human.
When looking at the focal length of a lens, it represents a standard based off a 35mm (full frame) sensor. Because this is the standard, the FOV for each lens can be calculates to a standard. A Nikon 50mm will give the same FOV as Sigma and Tameron. This also holds true between camera makers; Nikon 50m has the same FOV as a Canon 50mm.
Digital Camera Sensors
In the days of film, the size of full frame film was a uniform 35mm across, measured diagonally. With the advent of digital cameras, with digital sensors, this was no longer the standard. In fact, every camera maker has their own size sensor and many have more than one. This poses a problem because using focal lengths to describe the FOV of an image is no longer a standard. Let me show you why.
When looking at the focal length of a lens, it represents a standard based off a 35mm (full frame) sensor. Because this is the standard, the FOV for each lens can be calculates to a standard. A Nikon 50mm will give the same FOV as Sigma and Tameron. This also holds true between camera makers; Nikon 50m has the same FOV as a Canon 50mm.
Digital Camera Sensors
In the days of film, the size of full frame film was a uniform 35mm across, measured diagonally. With the advent of digital cameras, with digital sensors, this was no longer the standard. In fact, every camera maker has their own size sensor and many have more than one. This poses a problem because using focal lengths to describe the FOV of an image is no longer a standard. Let me show you why.
I tried to illustrate with the diagram above what the FOV of two different sensor sizes would be. The focal length for both of them are the same but notice how the darker sensor has a much smaller FOV. This is because the sensor is not able to capture the same size image as the lighter colored sensor once the image has entered the camera. In the digital photography, this is called the crop factor. Essentially the smaller digital sensor is just capturing a cropped image that its big brother, the full frame sensor, can capture. Because of this the FOV is different as well. A digital camera, with a smaller sensor, at 50mm focal length, is not the same FOV as 50mm on a full frame camera.
This issue poses a problem for digital photographers. how does someone know what the FOV for a smaller sensor is if the standard is created for full frame? No worry, it is easy to figure out what the 35mm equivalent is for a smaller digital sensor. To the right of the diagram above are some popular sensors and their crop factor. All you need to do is multiply the focal length on the lens with the crop factor for your sensor and you have your 35mm equivalent. Therefore, for a 50mm lens on a Nikon DX sensor will be the same FOV as a 75mm lens on a Full frame camera.
This issue poses a problem for digital photographers. how does someone know what the FOV for a smaller sensor is if the standard is created for full frame? No worry, it is easy to figure out what the 35mm equivalent is for a smaller digital sensor. To the right of the diagram above are some popular sensors and their crop factor. All you need to do is multiply the focal length on the lens with the crop factor for your sensor and you have your 35mm equivalent. Therefore, for a 50mm lens on a Nikon DX sensor will be the same FOV as a 75mm lens on a Full frame camera.
Perspective
One common misconception is that the focal length of a lens determines the perspective of the picture. This simply is not the case. The only thing that can change perspective of an image is how close you get to the object you are trying to photograph. The focal length of your lens really determines how much scene you are looking at is cropped to give you the picture your camera captures. Below are some comparisons to illustrate this point.
Above are two pictures; the one on the left is at 35mm and the one on the right is at 135mm. It is difficult to see the perspective has not changed because the one on the right is zoomed in. Below are the two same pictures but the left one is cropped to give the same perceived focal length as the one one the right. You will notice that it is really difficult to see the difference. I can only see that the depth of field being different.
Focal Length Tips
Now you know the basics about focal length there are three tips to help you with your photos.
1. Many zoom lenses do not have a fixed aperture, therefore as you zoom in your aperture will get smaller throughout the zoom range. To understand why this is important to know please read about aperture HERE.
2. As a general rule for photography, if you can get closer then do so. Taking a few steps closer to your subject will make your images more compelling then just zooming in with your lens.
3. The more you zoom in the more noticeable camera shake will be if you are shooting hand held. There is a tip to help you get sharp images using long focal lengths. Take 1 over the focal length (35mm equivalent) and that should be your minimum shutter speed. Therefore if you are using a focal length of 100mm on a full frame camera you should be using a shutter speed of 1/100 or faster. With the advent of image stabilization such as Nikon's VR or Canon's IS the game has changed. However, there are still a lot of great lenses that do no have built in stabilization and this tip should really help you.
1. Many zoom lenses do not have a fixed aperture, therefore as you zoom in your aperture will get smaller throughout the zoom range. To understand why this is important to know please read about aperture HERE.
2. As a general rule for photography, if you can get closer then do so. Taking a few steps closer to your subject will make your images more compelling then just zooming in with your lens.
3. The more you zoom in the more noticeable camera shake will be if you are shooting hand held. There is a tip to help you get sharp images using long focal lengths. Take 1 over the focal length (35mm equivalent) and that should be your minimum shutter speed. Therefore if you are using a focal length of 100mm on a full frame camera you should be using a shutter speed of 1/100 or faster. With the advent of image stabilization such as Nikon's VR or Canon's IS the game has changed. However, there are still a lot of great lenses that do no have built in stabilization and this tip should really help you.
All photos and content are copyright © Ian Dalton 2014