Focal Length vs Effective Focal Length
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Reading photographic tutorials for beginner photographers or ad copy for new gear, we see a term used very often: effective focal length. What is effective focal length? How do we compare focal vs effective focal length?
If you enjoy the technical side of the photographic process, you will like this discussion. To begin learning about focal length vs effective focal length, let’s first look at a few definitions and then some basics of lens design.
Focal Length Defined
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Focal length is usually measured in millimeters (mm) for photographic lenses. It is not a measurement of the actual physical length of the lens but rather a measurement of the distance from the focal point of the lens to the film plane or sensor. With some lens designs, the focal point of the lens may not even be within the confines of the lens.
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For a deeper explanation of this definition and some other terms and concepts we’ll highlight in our article, have a look at this in-depth tutorial from Sophie Morel (PDF). It discusses the focal length formula used to describe the actual focal length of a lens.
Focal length also determines angle of view and apparent perspective, or whether a lens is wide angle, normal, or telephoto.
What Is Normal Anyways?
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The important thing to look at is the image area which is 24x36mm, a standard that has been continued into digital cameras. It’s very important to know this when we consider the term mentioned at the outset, effective focal length.
Measuring the diagonal across the 24x36mm rectangle yields a figure of 43.3mm. With any format size, a lens of the diagonal length would give a field of view of about 53 degrees.
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The Nifty Fifty
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With 35mm cameras, though, the standard for a normal lens is generally viewed as 50mm, not 43.3mm. There are a couple of good reasons for this.
50mm is not far off from 43.3mm, but more importantly the optical formula for a lens that has an image circle covering the Full Frame 35mm format at a focal length of 50mm is extremely simple to design and make, resulting in low cost and incredibly good optical performance in regards to distortion, aberrations, and sharpness.
The 50mm focal length has another characteristic that makes it a great normal lens. When you view through the viewfinder of a 35mm camera with a 50mm lens minted, the magnification is 1X, meaning that when you look at the scene without the camera and through the camera, you see the same magnification.
When considering magnification, a 50mm lens provides 1X magnification on the 35mm format. Telephoto lenses are sometimes referred to by their magnification over the normal lens. So a 200mm telephoto is a 4X telephoto.
The Standards
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All of this brings us to the question of focal length vs effective focal length we started with. The terms effective focal length and apparent focal length are all based on the Normal Lens for the Full Frame 35mm format.
From the early part of the 20th Century on, much of popular photography, both amateur and professional, has been dominated by 35mm cameras with their normal lenses. There are lots of larger formats used by professionals, each with their own focal length normal lenses.
Crop Factor and Effective Focal Length
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The larger formats don’t need to be advertised to the masses, they have their users determined by specific needs. They still get their ads, marketing, and tutorials, but are generally not thought about when discussing crop factors and focal length vs effective focal length. That’s all based on the 35mm Full Frame format.
Two of the more popular digital photography formats besides Full Frame are APS-C and MFT formats. Based on the size of the formats, many APS-C cameras have a crop factor of 1.5X or 1.6X and MFT has a crop factor of 2.0X.
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So, now the question is, what is crop factor? Crop factor is a calculation of how the lens focal length on these smaller formats compares to Full Frame. If we look at the normal lens focal length, an APS-C camera has a diagonal close to 28mm, but that lens would not be wide angle on APS-C, it would be showing about the same view as a 43.3mm lens on Full Frame. If you multiplied 28mm by the APS-C crop factor, that’s pretty much the result you get.
Now, if we look at the Nifty Fifty normal lens on a Full Frame camera, an APS-C camera would use a 35mm lens and an MFT camera gets the 25mm lens. They each have the same “normal” angle of view and apparent perspective compared to each other.
All of this means we could describe the 35mm lens on APS-C or 25mm lens on MFT as having the same effective focal length as the 50mm lens on Full Frame 35mm. The focal lengths have not magically changed, it is more a matter of semantics describing how the lenses behave in their intended formats.
Real Focal Lengths
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That’s where the ads and tutorials can sometimes get a little confusing. A lens ad may say that their new 400mm lens for their APS-C camera has an effective focal length of 600mm. How they get that is multiplying the APS-C crop factor of 1.5X to the 400mm lens to get the “effective” length of 600mm.
Another of saying it is that the 400mm lens on that APS-C format camera sees the same field of view or angle of view and apparent perspective as a 600mm lens on Full Frame would see. The actual focal length of that lens is 400mm. It’s focal point is 400mm away from the plane of the sensor.
But since a 35mm lens on that format sees the same angle of view and apparent perspective as the Nifty Fifty on Full Frame, then the 400mm lens on APS-C is a 12X telephoto lens just like a 600mm is 12X on full Frame. So, viewed in this light, a crop factor calculation is a form of focal length calculator.
What Does It Mean For You?
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If you base your photography endeavors on what you use and what you want to do with it and when and where you want to expand or change things, you’ll be just fine with whatever effective focal lengths your lenses are.
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