A Losers Guide to Photography: Perspective Compression

Perspective Compression, the New Way

Background

I had a conversation at work with a coworker about the benefits of Medium Format cameras. One of the main talking points was what he called "compression" or "lens compression", or what I think is a better fitting term Perspective Compression. I wanted to know what would justify the cost expense of upgrading from an APS-C or Full Frame camera to a Medium Format camera, other than professionalism. I wanted tangible evidence to prove Medium Format was worth an extra 100%+ dollars. We had a long conversation about "compression" and the confusion surrounding that term because of it's interchangeability with other components of photography, like JPEG or PNG.

I spent some time thinking about a better term to define 'the background appearing closer to the foreground'. I came up with Perspective Compression because it can be measured in terms of high and low and the words describe what is actually happening, hopefully alleviating confusion surrounding this lens phenomena. 

Why is this important? A coworker of mine was trying to explain to me how Perspective Compression works. Not that I didn't understand the principles behind Perspective Compression, I wanted to know how Medium Format cameras experience Perspective Compression differently. 


Defining Perspective Compression

So what is Perspective Compression? Perspective Compression is the measurement of the distance the background appears in relation to the foreground. There is no need to complicate it. 

Perspective Compression can be measured by pre-pending it with High or Low. "High" is when the background appears closer to the subject. "Low" is when the background appears to be further away from the subject. So  High Perspective Compression is when the background appears closer to the subject than is appears to the human eye. A low Perspective Compression is when the background appears to further away from the subject when comparing it to the human eye.



In order to better understand what is happening, lets define Focal Length.


Focal Length 

To put it simply, Focal Length is the measurement of the Focal Point/Nodal Point to the sensor plain. So when a camera says it's a 50mm lens, it's telling the photographer the Focal Point/Nodal Point is 50mm in front of the sensor plain. I created a diagram below to better show what I am talking about. 

                            


StudioBinder defines Focal Length as, "the distance from the focal point where light rays converge in the lens to the sensor or film in the camera when focused at infinity". 

It doesn't matter how long the barrel of the lens is and how many glass elements there are; what matters is the distance between the sensor plane and the focal point. This is important because this matters most when comparing lenses on a per-system basis, not when comparing system to system.

Let's Talk About It

The argument behind why people believe Medium Format cameras have a higher Perspective Compression is because the Focal Length is larger than that of any other format. It's important to remember the Perspective part, because it's simply defining where in relation the photographer is to the subject.  The logical reasoning behind Medium Format having a higher Perspective Ratio is because the Focal Plane is shifting further away from the sensor, which should yield a higher Perspective Compression. We can prove this to be false. 

Several Youtuber's have done comparison videos using equivalent focal length cameras. Per the Medium Format focal length provides a higher Perspective Compression ratio argument, we should see the background closer to the foreground, but we don't. Perspective Compression is NOT derived from a higher focal length, but rather the Compression the lens provides is in relation to the perspective of the photographer/camera. 

I did some experimentation and I created a small python script to help with the conversions between different sensor sizes. 

Code can be found here: Python Code

I want to get the equivalent focal lengths per photographic medium. I want to create a nice portrait, so I am going to use a 85mm lens of a Full Frame camera and move my way up. 

Full Frame Equivalent: 85mm, f/8.0
APS-C Equivalent: 57mm, f/5.3 Micro Four Thirds Equivalent: 42mm, f/4.0
Medium Format Equivalent: 137mm, f/12.9

Per my discussion with my coworker, I can hypothesize that there is a correlation between the Focal Length and the Perspective Compression. My expectations are, as the Focal Length increases, so will the Perspective Compression. Meaning, a higher Focal Length will result in a higher Perspective Compression.

First we will test what it will look like on a Full Frame Sensor to give us a baseline. For all of the images, I want the models elbow to be right above camera bottom:

Full Frame Sensor @85mm, f/8.0



So you can see the settings I used. Pretty standard. The distance from the model is what is going to change. Lets watch what happens.

APS-C Sensor @57mm, f/5.3


Micro Four Thirds @42mm, f/4.0

Now, according to Medium Format enthusiasts, a 137mm Focal Length lens should yield a higher Perspective Compression ratio. Lets see if that is true.


Medium Format Sensor @137mm, f/12.9


I want to test something else. I want to test equal focal lengths. The standard I will follow is 85mm f/8.0 for all sensors.

Full Frame Sensor @85mm f/8.0


APS-C Sensor @85mm, f/8.0


Micro Four Thirds @85mm, f/8.0


Medium Format @85mm, f/8.0


This is interesting. There is a lower Perspective Compression ratio at equal focal lengths the larger the sensor. So, the argument Medium Format provides a higher Perspective Compression ratio seems to be invalid at equivalent and equal focal lengths. In order to get the same Perspective Compression look smaller sensors have at smaller focal ranges, Medium Format cameras require longer Focal Length lenses. This is interesting because, as a general rule of thumb, the longer the lens, the more expensive the lens. This isn't always 100% true, but it's pretty accurate. 

The biggest factor to controlling Perspective Compression is the camera's position in relation to the subject. For each of the experiments with equivalent Focal Lengths, the photographer has to move further/closer to the subject to maintain the same framing of the subject. 

Conclusion

I want to admit I don't know everything about photography and I am sure I missed something important. From my testing, I can firmly say there isn't much of a difference between sensor sizes when it comes to Perspective Compression. The biggest factor is the photographers position in relation to the subject to control Perspective Compression. 

Future Goals

I want to learn more about the math behind Perspective Compression and Focal Length. 


Resources: 

https://antongorlin.com/blog/effective-focal-length-calculator/
https://www.studiobinder.com/blog/focal-length-camera-lenses-explained/
https://dofsimulator.net/en/



Comments

  1. DJ ChesleyMarch 26, 2022 at 5:06 PM

    Very interesting. I like that you used python as well!

    ReplyDelete

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