In the first part of this series, we discussed how your choice of lens focal length and angle of view effect what you will be photographing, but WHY is this the case. WHY do wide angle lenses have short focal lengths? Why is a lens angle of view determined by the focal length at all?
It’s time to delve into some really nerdy shit.
How is Focal Length Determined?
When light enter a lens, the glass elements focus that light on what is called the point of convergence.
The light then disperses until it hits the sensor or film or whatever is capturing the image. The distance between the sensor and the point of convergence is the focal length.
If you’ve ever used a projector of any kind, you’re familiar with this concept as you focus the image on whatever screen you are projecting on to.
How Do You Determine the Angle of View of a Lens?
The ‘angle of view’ of a lens at a particular focal length is usually determined diagonally (from corner to corner) not horizontally or vertically across the frame.
This is why ‘fish eye’ lenses come in two varieties:
- Diagonal (which usually covers about a 108 degree field of view across the horizontal axis of the frame)
- Circular (this lens produces a circular image that truly covers a 180 degree field of view across the hoizontal, but to do so, it is highly distorted at the edges and often requires post production to ‘de-fish’ the image)
But Why Is Focal Length Interlinked This Way?
You might be wondering, “So why are only short focal length lenses capable of a wide field of view?”
Well, it doesn’t have to be that way. As we learned earlier, the distance between the point of convergence and the sensor determines our focal length. If we don’t have enough space between the convergence point and the sensor, we can’t focus the image on the sensor. We also need an appropriately LARGER sensor.
This makes having ‘wide angle’ telephoto lenses practically impossible for regular photographers. The lenses (and cameras) would be prohibitively large and unwieldy, and the sensors themselves would be ridiculously expensive.
However, this is EXACTLY why observatories have such enormous mirrors to focus their astronomical images on. That HUGE focal length lens needs a HUGE ‘sensor’ to focus the light on.
It’s also why sensors for satellite telescopes are soooo expensive. Because they’re BIG!
Did You Learn Something?
And if you haven’t already, check out the first part of this series.