Focal Length and Angle of View
/I wrote an article some months ago for the folks at Henry's on the subject of why angle of view might be more relevant to photographers than focal length. It was well received, although one fellow got his shorts in a knot because he didn't like the approach, although he agreed that everything I wrote was true. It's fair to disagree on approach, but when it gets coupled with insults, that's indicative of immaturity and a desperate need to get a life. The internet enables people to trash others by hiding in anonymity. So here's the deal. You don't have to agree and if you really hate what I write, that's cool and move along.
Anyhow back to the premise. I meet lots of newer creatives, whether shooting stills or video and they are rarely concerned about the physical focal length of a lens, which is particularly irrelevant as an indicator when crop sensor cameras get involved as they always reference lenses as if they are full frame even when not. What real creatives are concerned about is the image and what goes in it and what does not. This is partially managed by angle of view.
The Problem
Ever since makers starting building cameras using the focal lengths from 35mm film attached to sensors that were not the same size as a 35mm negative we have had to deal with the silliness of crop factor. Let's consider this real world situation that I heard in a store.
Customer : “I have a Nikon. It has a crop factor of 1.5. If I buy a lens that says it is 35mm, it doesn’t look the same as pictures that I took with my old Nikon film camera. This is so confusing.”
Seller : “You are correct! You have a crop sensor camera, so when you mount a 35mm lens, it’s not really a 35mm look, it looks like a 52.5mm lens”
Customer : “But I want a lens that looks like a 35mm lens looked on my old camera!”
Seller : “No problem, you want to buy a lens that is 23.3mm so it looks like your old 35mm. It’s all about the crop factor.”
Customer : “Forget this nonsense, I will just use my phone.”
The seller is accurate, but unfortunately not helping. This kind of discussion goes on every day in the world of digital photography and breeds all kinds of back and forth about what’s better. It applies less for folks shooting micro four-thirds because the focal length numbers are actually correct for the sensor size.
Let’s Think About Angle of View Instead
What most of us are really looking for is control of what shows up on the image. For the vast majority, we could not care less about focal length numbers, what we really care about is what the sensor captures and how that communicates the image that we want to make and the story that we want to tell.
The combination of focal length and sensor size go together to define the diagonal angle of view, basically a measure of the angle between the two farthest separated points the in the frame, the diagonally opposite corners.
We want to control what goes onto the sensor, and we use position, technique and different lenses to achieve our goals. If we are in a small room and want to get the whole thing in, we would typically go to a wide-angle lens. Does the focal length number really matter? It does not, what matters is if we have enough angle of view to obtain our goal.
This is why I encourage people not to get all hung up on focal lengths, far better to try different lenses and find one that fits your goals. Zoom lenses tend to be great places to start to solve this problem because by moving the elements in a zoom, we can obtain a range of angles of view across the zoom range. In fairness we usually do not think in terms of angle of view, so the idea of focal length is a simple way to define a coverage angle. We all know that an 11mm-24mm zoom has a range of wider angles of view, than a 70mm-200mm lens.
The Meaning of Crop Factor
Contrary to some tall tales, crop factor is simply a mathematical factor that describes the percentage change of a given sensor based against an established standard of full frame 35mm negative size. Some cameras such as micro four thirds actually label their lenses properly for the sensor size in use, but all crop sensor cameras be they DSLR or Mirrorless do not. Thus, we must apply crop factor math to achieve the angle of view that we desire.
I’ve built a chart for you to help you make these comparisons.
Crop Factor and Angle of View Comparison
This chart will not cover every possible angle of view, but it does provide some very common examples, and you will start to see why you may choose a full frame sensor if you want to go really wide, or maybe a micro four thirds for a super telephoto that doesn’t require the assistance of the Hulk to carry around. There are other factors that come to bear based on sensor size, but that’s a different conversation.
| Diagonal Angle of View (in degrees) | 35mm Full Frame Focal Length | Nikon / Sony / Fujifilm | Canon Crop Focal Length | Micro Four Thirds Focal Length |
|---|---|---|---|---|
| 114.4 | 14mm | 9.3mm | 8.75mm | 7mm |
| 94.49 | 20mm | 13.3mm | 12.5mm | 10mm |
| 84.06 | 24mm | 16mm | 15mm | 12mm |
| 75.38 | 28mm | 18.7mm | 17.5mm | 14mm |
| 63.44 | 35mm | 23.3mm | 21.9mm | 17.5mm |
| 46.79 | 50mm | 33.3mm | 31.3mm | 25mm |
| 28.56 | 85mm | 56.7mm | 53.1mm | 42.5mm |
| 24.41 | 100mm | 66.7mm | 62.5mm | 50mm |
| 18.21 | 135mm | 90mm | 84.3mm | 67.5mm |
| 12.35 | 200mm | 133.3mm | 125mm | 100mm |
| 8.25 | 300mm | 200mm | 187.5mm | 150mm |
What we learn from this chart is that with lenses on crop sensor bodies, the focal length numbers that are stenciled on the lens do not provide the same angle of view as on full frame. So, if you have an 18-55mm lens on your crop sensor and set it at 18mm, the angle of view is actually much more like the angle of view of a 28mm on full frame. There are many reasons why makers did not modify the markings on their lenses for crop sensor cameras, very few of which are customer oriented.
If we use the Nikon/Sony crop sensor example we see that to get the same angle of view as is found on a 300mm lens on a full frame camera, we need only mount a lens labeled 200mm. This lens will be likely physically smaller and perhaps lighter and less expensive. However, this lens which is stenciled for full frame but is built for crop sensor will not work completely on a full frame camera.
Some makers, such as Fujifilm, use the full frame numbering system even though they only make one sensor size. So, when Fujifilm released their very fast and optically excellent 56mm, it was built to deliver the same angle of view as an 85mm on full frame. This was necessary to get film camera shooters to embrace the idea of crop sensors, because they thought more about angle of view than they did focal length numbers or sensor size.
What we can conclude from the chart is that to achieve the same angle of view, our actual focal length choices will vary by sensor size. So are we really more concerned about a specific focal length number or the angle of view? The answer, the majority of the time is the angle of view.
Considerations for Purchasing
In an article that I wrote on the fallacy of lens compression, I included an image with coverage frames overlaid on it to show the angle of view for a fixed shooting position and a fixed subject distance. Here I will do the same thing, however this time, I will enhance the frames to include the angle of view and labeled focal length using 35mm full frame as a base line, and then you can use the chart above to find the stenciled focal length for your sensor.
It's really not about the numbers at all, they are just measurements that only matter when you use them as guides for defining the angle of view that you desire. A Bugatti Chiron delivers a maximum of 1500 brake horsepower, yet it doesn’t matter for a drive to the milk store. Choose your lens and angle of view range based on what you want to get out of your images and let the focal length numbers fall where they do.
Conclusions
Angle of view allows the creative to manage what is in and what is not in frame in conjunction with other conscious actions such as framing and composition. Understanding how focal length changes with sensor size can be helpful, but it may be simpler to not worry about the focal length and simply drive to the angle of view that you want.
Have an idea for an article or tutorial? Do you have a question photo or video unrelated to this article? Send me an email directly at ross@thephotovideoguy.ca or post in the comments.
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