Lenses, Field of View, Crop Factor, and how they effect your image

I frequent an online discussion board where the question of lenses, field of view and “crop factor” pop up on almost a daily basis.

I spend quite a lot of time explaining these concepts and so I figured it was about time I write a post about it. This is difficult because they are all quite intertwined and it is hard to split them up… so it might become a bit confusing, I apologise in advance.

The confusion

I believe a lot of confusion comes about from people incorrectly saying “on x camera your 35mm lens is actually a 50mm” when what they should be writing is “on X camera your field of view is similar to Y lens on Z camera”.

Field of View

There is a fantastic video from the guys over at AbelCine which explains this concept.

In short, a lens has certain characteristics (primarily Depth of Field) and this won’t change regardless of what camera, sensor or format, you are recording to. The only thing which will change is your field of view.

Field of view is measured by degrees and the easiest way to explain this is by starting with what is refereed to as a “Standard” or “normal” lens.

I’ll refer to our friend wikipedia here,

In photography and cinematography, a normal lens is a lens that reproduces a field of view that generally looks “natural” to a human observer under normal viewing conditions, as compared with lenses with longer or shorter focal lengths which produce an expanded or contracted field of view that distorts the perspective when viewed from a normal viewing distance

So a standard lens is one with a field of view of 53 degrees. Now depending on what sensor size or film format you are shooting on you will need a different lens to obtain this field of view. Here is a run down of the most common formats and their “standard lens”… now these aren’t 100% accurate but they are a good starting point.

Full Frame (stills) – 50mm

– 5Dm2/m3

Super 35/APS-C – 32mm

– 35mm film, Alexa, RED, F3/F5, 7D etc…

M4/3 – 25mm

GH2/GH3, af100

Super 16 film – 16mm

16mm film cameras, black magic pocket cinema camera

2/3″ – 11mm

Sony f800 + a bunch of other ENG cameras.

Back to The Confusion

Now the confusion about “a 35mm lens is actually a 50mm on x camera” started about because people began referring all lenses to the full frame sensor of the 5Dm2. To me this doesn’t make much sense because there are really only a few cameras with a sensor this size, they’ve only been around since 2008, and they were never really designed for video use. Compare that so Super35 which has been around for decades, is the standard for film/video production and is what most high end digital cameras are designed to replicate.

Back to Field of View

Now using the information above, we can hopefully see why lenses don’t actually change. A 50mm will always be a 50mm lens, but if you want to replicate the same field of view of a 50mm lens on a different sensor you will need a different lens.

Here is a photo I’ve stolen from Neal Currie (image linked to his site).

A lens works by projecting light onto a sensor. A lens creates a circular image and the size of the sensor has to fit in to the size of that projected circle. Here is a diagram to demonstrate

If we compare this image then to the one above showing the field of view depending on different lenses, we can see how the sensor size and the lens choice pair up.

Let’s say that purple circle is the projection from a 20mm lens. That lens, regardless of what camera it is on, will always project that same image circle. If you reduce the size of the sensor you will be reducing your field of view, but the characteristics of the image are still the same.

Depth of Field

Depth of field, when it comes down to it is really quite simple. There is a LOT written about it so I’ll try and keep this summary short.

There are only 3 things which change the depth of field of an image.

F-stop

The more open your lens is (in other words the larger the iris/the smaller the f-stop number) the shallower your Depth of Field will be.

Focal Distance

The closer your subject is to the lens/the smaller your focal distance is set, the shallower your Depth of Field will be,

Lens Focal Length

The longer your lens is then (a 100mm lens will give a shallower depth of field than a 50mm at the same f stop and focal distance) the shallower your depth of field will be.

Please note that sensor size is not on this list. The size of the sensor does not directly effect your Depth of Field.

“But my 5D has a shallower depth of field than my point and shoot”

Yes and no. We know that sensor size doesn’t directly change your depth of field. Lenses change your depth of field. We know that the longer a lens is the shallower our depth of field will be. And this is where our knowledge of Field of View comes in handy.

To replicate the FoV of a 35mm lens mounted on a S35 camera on a Full Frame camera such as the 5Dm2, we need a 50mm lens. And a 50mm lens at the same f-stop and focal distance will give us a shallower depth of field because it is a longer lens.

It really is that simple.

Why do I care, I just want to get my shot?

Recently someone asked on a discussion forum how much a director should know about cinematography and one of the first responses said they should understand how lenses work. I added that it was equally important that they understand how Field of View changes their image.

Below is a GIF which I love. It demonstrate so perfectly how your lens choice changes the subject.

It demonstrates how you can keep the subject the same size within your frame but the background changes.

The wider you are the more of the background you’ll see and the longer your lens is the more compressed it will become. Here are the two end frames side by side.

You can see the house at the very end of the zoom looks very close to the chunk of concrete… but on the wide it looks VERY far away.

This is important to understand, as you can use it to single your subject out from everything around them or perhaps using a really wide lens to show that they are alone within the space, all while keeping them the same size within the frame.