Introduction

I find focal length to be a very mysterious thing. I think this is because it’s intangible, unrelated to the normal human experience.

I can physically see things from a high or low angle, I can move my body to get a ‘dolly’ view of something happening, I can dim the lights in the room I’m in. But I can’t change the focal length of my eyes. They see things one way.

So choosing a focal length might be the most arbitrary decision of all. When it comes to choosing focal length, we’re being resigned to the fact that we’re not truly capturing reality. We’re just using a tool to make something that resembles reality.

But of course, while this could be seen as a limitation, it’s also an opportunity. I can use focal length to change the whole world as it appears in my film. I can show things or hide things, bend things and warp things, clarify things or confuse things. Whatever I want my audience to see, I can make them see it, whether or not it’s what they would have seen if they were physically there to witness it.

But I can only do that if I really know what effect the different focal length lenses have, and how I can use them.

Part 1: Changes in Relative Sizes on the frame

I had the idea that shorter focal lengths make for more distorted images, where the relationship in size between two objects at different distances away is less accurate. I did some maths to try to prove this, but it proved my hypothesis wrong. It told me that the ratio in size between objects would be the same no matter which focal length I used.

So I did a few tests with my camera, and then I came across the wikipedia page for “perspective distortion”, which had this very interesting line:

“Note that linear perspective changes are caused by distance, not by the lens per se – two shots of the same scene from the same distance will exhibit identical perspective geometry, regardless of lens used.”

That means that if you’re shooting from the exact same position, a shot with a long focal length lens will look identical to a shot with a short focal length lens, but cropped (if we ignore the change in depth of field). Here is the example wikipedia gives

Here’s my test of the same thing: different shots from the same place:

Short Focal Length Lens:

Cropped Short Focal Length Lens:

Long Focal Length Lens:

That is really incredible. As I zoomed in and out on the camera, it really looked like the space between the pens and pencils was getting significantly further and shorter, like they were being stretched out and squished together. But it turns out it was really just the same image getting bigger and smaller (with regards to the relationship between objects in space – of course, the depth of field has changed)!

I’m really happy I found this because it answers a question I’ve had since the start of the course – does the “flatness” of an image have to be tied to it’s depth of field? The answer is no! I just shot the exact same shot right there, with the same aperture and distance from camera, but they have different depths of field! You just have to be ok with cropping…

I’d still like to know though, why do long focal length lenses crush distance so much?

Part 2: Distance Crushing

Once I got started thinking about how it’s all about distance and not focal length, I made this diagram and it all made sense to me…

It’s the distance that causes the relative size difference, and the lenses are just effectively cropping things out or adding things in to what we would normally see at that distance.

That’s why things shot on a longer focal length lens look closer than they should – they’re being shot from a lot further away than we would normally see things at, but the “cropping”of the shot “tricks” us into thinking we’re not really standing all that far away.

Part 3: Difference between digital zoom and changing focal lengths

I could have done this project and the last one entirely on my phone, but I thought that because I can’t change focal lengths on my phone, I’d be seriously missing out. Now I’ll finally try out both of them and see what the difference looks like.

X70 Short Focal Length:iPhone Full Photo:x70 Long Focal Length:iPhone Cropped:

So, as I’ve now come to expect, the two tighter shots look the same, even though one was achieve through the use of a change in focal length lens and the other was achieved through the use of digital zoom on a shorter lens.

On a completely unrelated note, I think it’s interesting to point out that the images produced by the iPhone look better than the ones shot by the x70, because the colours and curves have been automatically adjusted. This suggests that you really have to treat video footage a bit before using it in a final product.

Part 4: What is a normal focal length?

So I can use a long focal length when I want to exaggerate distances, and a short focal length when I want to crush distances and have a shallow depth of field. But where is the line that crosses from a “long” focal length to a “short” focal length? It must be roughly equal to the “focal length” of our own eyes, because then the way we perceive distance in the film matches the way we perceive distance in real life.

So what is my focal length? I’ll do some tests to find out.

I’ll start by comparing my vision to my phone’s vision.

In this photo I took with my phone up to my eyes, the outer boundaries of the shot are fairly tight – if I look straight forward, I see a fair bit more to the left and right of the photo’s frame. This would suggest the phone’s focal length is “longer” than my eyes. However, and I believe this is more significant, the photo makes the distance between me, the trampoline and the roof of the next house along look much greater. When I look up from my phone’s photo, I feel like I’m wearing a long focal length lens, it looks comparatively crushed in distance!

I’ve always felt like iPhone photos have a certain strange look to them. This exaggerated distance effect may be it.

However, there’s something that doesn’t make sense about this. I found out earlier that it isn’t the lens that exaggerates distance, it’s the distance of the subjects from the camera. Maybe, like earlier, I can just crop it and it looks right again.

To my surprise, the distance look natural now! This means that the thing that was making me think the distances were exaggerated was just the outside part of the image, the parts closest to me. I think what this tells me is that although I can ‘see’ quite a lot to my right and left, all I’m really looking at is what’s straight ahead of me. The things in my peripheral vision, such as the pavement ground in this example, are not properly being registered by my brain. For this reason, it looks very strange to me to have all this information, including the things in front of me and the things around me (as is the case with a shorter focal length), presented to me in one complete frame.

Because I now know and understand why it’s the distance that matters and not the lens, just doing this test with my phone is sufficient.

Part 5: Interesting result of the last finding

In the photo I took of my backyard, it was the outside that was strange and warped. Crop that out and it looks normal. However, the opposite should also be true. If I cropped in really far, I’d eventually “find” what I would get if I shot with a very long lens – an unnaturally compressed-looking image!

So to test this out I got a wide shot in my hallway…

And right in the middle there, the trampoline looks very close to the house behind it, even though they’re actually very far away! Because both the trampoline and the house were far away from me, they end up looking close to each other, regardless of the fact that I’m using a short focal length lens! What an exciting discovery!

Because this part of the frame is so small the flattening “distortion” effect in the middle of the frame is not really a “problem” for creating a natural looking image.

Closing Remarks

I want a shot that looks faithful to what I’m seeing, from the place I’m seeing it, the edges end up looking distorted in the frame. So for any given shot, I can decide upon a position where it looks good to my eyes, and then move back a bit and zoom in. This way, the ‘peripheral’ vision that I saw and still wanted in the shot will be able to be both in the shot and not distorted.

If I’m using a short focal length lens, I should be careful not to have people end up towards the edge of the frame. I think that’s what causes ugly looking conversations in amateur films like the ones below. I should also just avoid using the shortest focal length lens available, just because it’s the easiest to use, unless I have an honest reason to use it.

If I’m using a long focal length lens, I should be mindful of the fact that things’ position in space may seem to not match up with other shots, as we found and discussed in an earlier exercise.

In conclusion, this exercise was not directly very useful to me as a filmmaker, but it was very interesting and I’m very happy to be able to set myself apart again with another piece of in-depth knowledge in one aspect of the shot.