A guide to understanding How Lenses Work and How Focal Length Affects Perspective.


It usually goes something like this: someone posts a photo that they’re proud of on their blog or Facebook. They get a bunch of likes and inevitably the question comes, “What lens did you use?”

We’ve all done it. I’ve done it. Even the photography rockstars out there were once newbies, and I promise, they did it too. It’s a question that if stripped down to it’s most basic reality is actually more of a statement. On the surface, it’s an inquiry about equipment, but underneath that question is the statement, “I don’t understand what makes that photo look like that.” It’s ok to admit. We’ve all been there and if you’re reading this, you just might be there right now.

My goal in writing this is to help you gain a better understanding of how lenses work and how focal length works to affect the visual perspective of an image. But before we get into the practicality of choosing a particular lens for a given subject, let’s get an understanding of how lenses work.


How Lenses Work and How Focal Length Affects Perspective. 



We all know that lenses are labeled with different millimeters, but what do those millimeters actually mean?

First, let’s look at what it DOES NOT mean:

  • The millimeters DO NOT mean the physical length of the lens.
  • It DOES NOT mean the width of the lens.
  • It DOES NOT mean the distance from your subject.

What it does mean:

“…a calculation of an optical distance from the point where light rays

converge to form a sharp image of an object to the digital sensor or

35mm film at the focal plane in the camera.”

– quoted from www.NikonUSA.com

 

Now in English. Remember that light rays, when uninterrupted, always travel in a straight line. Imagine for a moment that there is a string being stretched from every given point on a subject through your lens and reaching the sensor. Those lines or strings are not parallel. Those strings will all reach a point where they converge and then continue past that point of convergence until they hit the sensor. That distance, from the point of convergence to the sensor, is the focal length and it’s measured in millimeters.

Below is a graphic that I created to help you visualize what’s happening. This is only representational and not intended to be true to any particular scale.

focal length
Hopefully, what you notice is that the longer the millimeters, or focal length, the narrower the field of view is, and the focal distance is further from the sensor.

Try this simple exercise: Take the tube from a paper towel roll, or just roll up a sheet of paper. Choose an object across the room and look at it through the tube. Take note of everything you can visibly see around that object and your distance from it. Now cut the tube in half and look at the object again. Move closer until everything in view looks the same as before you cut the tube. Now repeat the process again. Notice how the perspective of the object relative to its surroundings changes as you move closer. It’s not a perfect representation because your eye isn’t changing the magnification like a lens does, but it will you give an idea of how the straight lines of light move and change angle as you move closer or further.

So, there it is in a nutshell. That’s the basic mechanics of how lenses work. Now let’s look at how focal length affects perspective.


The Rules of Choosing a Focal Length



Now that we know how this stuff works, how do we apply these principles and make them work for us to create awesome images? Before we go too far with this next section, let’s just keep in mind that photography is both an art and a science, so although there are some rules, they are largely up to the opinion and interpretation of the photographer. They’re more like guidelines than actual rules.

First, at the risk of stating the obvious, the wider the angle (or shorter focal length) you use, the closer you must be to your subject and the more lens distortion you will create. What’s lens distortion? That’s the effect of using a curved lens to optically change the perspective of an image to make it appear further away. Wide angle lenses will almost always have some degree of a “bubble” effect. Some are more apparent than others, but almost all of them will require some correction in post-processing to decrease that bubble effect and make the image look more natural.

Conversely, the opposite is also true. The further you move back from your subject and use a longer focal length, the more natural the image will appear. Moving back and zooming in on the subject will have the opposite of the bubble effect and instead make the image flatter. This is commonly referred to as “compression.” The guideline at work here is if your subject is a person, or group of people, it’s more flattering if you stand back and zoom in. Have you ever heard the saying that “the camera always adds 10lbs”? That’s what happens to a person when you use a wide angle lens. It’s literally stretching the image making them appear bigger. Whereas, if you use a long focal length, it will help “compress” that person.

Now remember though, these are just guidelines. You most definitely can create beautiful people portraits with wide angle lenses. It just requires a skillful knowledge of composition and subject. Stay tuned for a future article specifically on the topic of shooting with wide angles. Many people avoid wide angle shooting when working with people as the subject because they don’t know how to make it look good. I want to help turn that weakness into a strength. But, that’s for another time.


How Lenses Work and How Focal Length Affects Perspective. 


Now that we’ve covered all that boring, although necessary stuff, we can talk about the actual process of deciding on what lens to use for any given subject. This is the fun part. This is where the artistic and creative vision of the photographer really comes into play, because again, photography is both an art and a science. Knowing how lenses work is the science part, but using them to make your vision become reality is the art. Now that you know how lenses affect a subject, you can take that knowledge and apply it.

It’s up to the photographer to decide what the image should look like,  frame width, depth of field, angle, exposure, color or black & white, etc. The decision on whether to shoot up close with a wide angle, or move back and zoom in can have a huge effect on the final image. The images below are a good example.

How Lenses Work and How Focal Length Affects Perspective. 

They look very similar. Composition and subject are nearly identical in both frames, but there’s one big difference. It may be difficult to explain just from looking at it. There’s a noticeably different feel to both images. But why?

The image on the left was shot with a 35mm lens, while the right image was shot with an 85mm lens. Same relative composition. The subject never moved. Only I moved when switching from one lens to the other. You can get a good idea of how the angle of those straight lines of light going through the lens are different from one image to another. Aside from the technical aspects, let’s consider these images from a purely artistic perspective. The right image brings you, the viewer, closer to the subject and you can almost feel that sense of closeness. Like you’re standing there next to him yourself. Whereas the left image takes you further away. It feels more removed and more formal.

Look again and you may also notice that in the left image, the background seems further away. In the right, the background seems closer, but nothing has physically moved in either frame. So why the difference? Remember that a wide angle lens makes things appear further away, while a long lens makes things appear closer. In an example like this, even though the subject never moved, the distance from the subject to the camera changed. In the left image, the relative distance between the subject and the background is further than the distance between the subject and the camera. In the right, image it’s the opposite.

Maybe this animation will help visualize…

Which lens should I use? Understanding focal length.

Notice how even though nothing in the scene is moving, the perspective changes dramatically as the focal length changes.

Why does this effect happen? Look at the graphic below.

Relative Distance


Notice that the distance between the subject and the background remains unchanged. Only the distance between the camera and subject change. In the top example, the subject is closer to the camera than the background making the background appear further away. In the bottom example, the opposite is true. The subject is further from the camera than the background, so the background appears closer. These distances are always relative and changing. This effect adds to that sense of closeness or distance.

If you pay attention to the difference between photojournalism vs portraiture, you’ll probably notice that most photojournalism is shot with wide angles that put the viewer right in the middle of the action. It makes you feel like you’re right there as a participant in the scene. Whereas, most often, portraiture is shot with longer lenses with the intention to flatter the subject and give the viewer a sense of “looking at” instead of “being with.”

As photographers, this concept is where we live. What feeling are we trying to capture in an image? Are you trying to bring the viewer “into” the scene? Or do you want them to look “at” the scene. While these are some tools and principles that help us achieve that, they’re still only guidelines. As the saying goes, “beauty is in the eye of the beholder.” Only you as the artist can decide for yourself if what you see in the camera represents how that scene made you feel.


what's in my gear bag

Honestly, I don’t have all the latest and greatest gear. I’ve been using essentially the same items for the last 5 years. I started out by buying the middle or low-end versions of most lenses because I couldn’t afford the fancy expensive ones. What I’ve learned from that, is that you can do a lot with a little if you work on developing your skills more than your gear list. My newest (And favorite lens) is my Sigma 35mm f/1.4. I got it last year and it hardly ever leaves my camera. I’ve done many sessions with just that one lens. I don’t want to make this post about gear reviews though. So here is my complete gear list which has served me solidly since I started my business.

Cameras:

Lenses:

Accessories:

I hope this was helpful to you. I feel like I learned a few things myself just through the exercise of explaining it and that’s what this is all about. Learning and growing.

Please, by all means, feel free to continue the conversation in the comments below.

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