50mm – The Basics: What does 50mm even mean?!?

Where the confusion starts:

I think there’s a lot of confusion surrounding the use of millimeters (mm).Especially since we also measure filter diameters in terms of mm, and lots of other things using mm (unless you’re from the USA…).

But how can we contextualize mm when we’re talking about lenses? 200mm is exactly 20cm. Does it mean we can only take photos of objects 20cm away? Does it specialize in objects that are 20cm in size?

Nope, none of the above. When we talk about a lens being a certain number of mm (I will call it XXmm), we’re describing the lens’ Focal Length. Focal Length is quite technical, but we can also explain these differences in terms of angle of view.

Angle of View:

Angle of view is how much the lens “sees” from edge to edge of the frame. 

Small numbers mean wide field of view; this wide angle allows you to capture many things in a frame but most of them will be quite small (ex. 10 – 20mm). Large numbers mean a very small field of view –  called telephoto; this allows you to zero in on the subject while excluding the things on the side of what you’re looking at (ex. 200-600mm).

Here’s a comparison of a wide angle (18mm) shot and a telephoto (200mm) shot:

18mm vs 200mm - same spot

I did not move at all to take these two shots. Zooming from 18 to 200mm does not change the relationship between the different elements, it just crops the photo.

Could I get the same results by staying at 18mm and just walking closer?

Nope. Moving your location changes the relationship between the different elements.

Here’s a comparison where the main element – the No Entry sign – is about the same size in the frame but I’m using 18 and 200mm angles of view:

18 vs 200mm lens - same framing

In the 18mm, you’re seeing a much wider view of the background. In the 200mm shot, the angle that you see behind the main element is mush more restricted.

Sensor Crop

All lenses actually create a circular image, however the shape (generally rectangular) and size of the sensor determines how much of that circle is cropped by only recording data in a specified area (that rectangle that ends up being your photo).

In fact, if you crop your images you’re effectively zooming in post processing – your XXmm is going up. Which is partially why when you talk about sensor size, they use the word “Crop Factor” which will be dealt with in my next post.

If you’re looking for more info – Wikipedia has some here!

Questions?

Something to add?

Fire away below or email me at photography@koreweb.com.

Matt

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