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Why do different (mirrorless) cameras + provided lenses all seem to end up with a 24 mm 35-mm-equivalent focal length?

  • The fixed-lens Canon Powershot G5X has a 1" sensor (actually 8.80 mm high) with a crop factor of 2.7. Its built-in lens has a range of 8.8–36.8 mm, which means the widest angle is 8.8×2.7=24 mm 35-mm-equivalent.

  • The Panasonic Lumix DMC-GX8 has a µ 4/3 sensor and is frequently sold with a 12–60 mm lens. With a crop factor of 2, that means it corresponds to 2×12=24 mm 35-mm-equivalent.

  • The Sony α6300 has a APS-C lens and is often sold with a 16mm–50mm lens. At a crop factor of 1.5, that becomes a 1.5×16mm=24 mm 35-mm-equivalent focal length.

  • The Sony A7R II has a full-frame sensor. It appears to be often sold with a lens down to 24 mm.

That means the widest angle on the four systems described above are all the same! Is there a physical reason for this? The 12 mm lens for the µ 4/3 system is MUCH cheaper than a 12 mm lens for the APS-C or full-frame systems. Why is this? Why does there appear to be this magical limit of a 24 mm 35-mm-equivalent focal length? Does that mean that short of buying expensive wide-angle lenses, moving to a larger sensor with a smaller crop factor does not, in practice, give me a wider angle view?

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The camera lens projects a circular image of the outside world onto the surface of film or digital sensor. Most modern cameras are designed around a rectangular format. This rectangle, known as the “classic format”, has a length that is 1 ½ times height. As an example, the full frame (FX) measures 24mm height by 36mm length. The now popular compact digital (DX) has a format that is 66% of this size. The DX measures 16mm by 24mm. This classic format, when enlarged, exactly matches to make a 4X6 inch print or an 8x12 inch print.

Now the camera lens projects a circular image aimed so that it focuses on the flat surface of film or digital sensor. Only the center portion of this projected circle is photographically useful. The central portion is called the “circle of good definition”. Beyond this image circle, the image is too dim and too blurred to be useful. To retain only the useful central potion, the camera is equipped with baffles and a format mask. It is this mask that sets the format size.

The typical camera lens is designed for a specific format dimension. This is especially true when it comes to short focus lenses. This is because short, wide-angle lenses must be positioned close to film or digital sensor. It is this closeness that is the peril. If the back-focus is short, there is but minuscule space for the lens mounting mechanism. This is especially bad if the camera has a mirror imposed between lens and image plane. To get around this, a trick used is: Make the lens retro-focus. In this design, the focal length measuring point, called the rear nodal, is shifted rearward. This scheme allows for a longer back-focus distance. The longer back-focus, plus clever use of multiple lens elements of different shapes and powers, enlarges the useful diameter of the circle of good definition. What I am trying to say is, short focus, (wide-angle) lens are challenging.

For the above reasons and some others not covered, the focal length considered “normal” for any camera is a lens with a focal length about equal to the corner to corner measure (diagonal measure) for the format. For the FX (full frame 35mm) that’s about 45mm. This value is usually rounded up to 50mm by tradition. For the DX (compact digital) that’s 30mm. By definition, a “normal” is a lens that delivers a view that is not wide-angle and not telephoto.

Key to your question is – If a “normal” lens is fitted to a camera, the angle of view will always be 53°. This is the angle of view that is most often published. This is the diagonal angle of view. With a camera sporting a classic format held in the landscape orientation, fitted with a “normal” lens, the angles of views realized, are 53° diagonal, 45° horizontal, and 31° vertical. Again, this is true for all cameras sporting a “normal” lens coupled with a classic format (length 1.5 times height). Now the angle of view expands when a shorter lens is fitted and shrinks when a longer lens is mounted.

If the camera sports a classic format rectangle and we know the crop factor, we can easily find the format dimensions. For the Power Shot, 2.7 crop factor: Sensor dimensions are 24 ÷ 2.7 = 8.8mm height --- 36 ÷ 2.7 = 13.2mm length, diagonal 15.9mm. Angle of view when fitted with “normal 15.9mm” = 31° vertical 45° horizontal 53° diagonal.

For the Panasonic Lumix DMC-GX8 crop factor 2: Format 12mm height 18mm length 21.6 diagonal Angle of view when fitted with “normal 21.6” = 31° vertical 45° horizontal 53° diagonal.

When it comes to “normal” we can’t get away from these angles of view unless we use a lens other than “normal”, or a format dimension that departs from the classic rectangle. Most cameras mount a “normal” or kit lens zoom centered on “normal”.

  • Even if some parts of this answer are at least correct, most of it is completely irrelevant to the question asked and some parts are simply wrong. The definition of a normal lens is obviously not 'neither wide-angle nor tele', when a wide-angle lens is usually defined as a lens with a shorter focal length than a normal lens and a tele lens is defined as a lens with a longer focal length than a normal lens. That is just recursive nonsense. More importantly: you do not answer the question (first sentence, 24mm equivalent thing) at all! – jarnbjo Nov 14 '17 at 15:13
  • @ jambjo -- The G5X crop (magnification) factor is 2.7. Thus a 24mm is equivalent to 24 X 2.7 = 35mm. The realm of wide-angle is considered to be 70% of “normal” or shorter. “Normal” for the FX is considered 50mm. Thus 50 X 0.70 = 35mm. Most conclude a 35mm is the beginning value for the realm of wide-angle. The 24mm, equivalent to 35mm for DX format also fits the "beginning of wide-angle" designation. – Alan Marcus Nov 14 '17 at 16:27
  • You are just confirming what I am criticising. If you say that a normal lens is neither wide-angle nor tele, a wide-angle lens is shorter than a normal lens and a tele lens is longer than a normal lens, you have actually said absolutely nothing and not described any characteristics of the different lens categories. Not that anyone asked for it. The rest of your comment is also just incorrect or irrelevant. 24x2.7 is for example not anything near 35, but it would also not have had any relevance to the question if you had made the correct answer 65. – jarnbjo Nov 14 '17 at 21:50
  • @ jambjo - Your are correct on one point -- I did error - I was trying to say -- 8.8 X 2.7 = 24mm (35mm (FX) equivalent. Thanks for pointing out this error. – Alan Marcus Nov 14 '17 at 23:11
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This is simply where the market is converging to at the moment. The typical kit lens is still 18-55mm on an APS-C sensor but most people find wider angle more useful, so some manufacturers made a few lens that start at 16mm. Olympus still sells many of their entry-level cameras with a 14-42mm which is equivalent to 28mm, while the 18-55mm mentioned earlier is equivalent to 27mm towards the wide end.

Fuji also initially and still includes an 18-55mm with many of their mirrorless cameras when sold as a kit. They later made a 16-55mm bright zoom and a 16-50mm kit zoom. Pentax was actually the first one out with a 16-50mm when they launched their DA* line yet they sold before and after many 18-55 lenses. The Panasonic common kit were also 14-42mn, then 14-45mm until they started offering the 12-35mm ones (24-70 equivalent). The 12-60mm you mention is still a more advanced offering.

The same happened with full-frame cameras. My main Minolta film lens is a 28-75mm. Manufacturers made several lenses starting at 28mm (28-70, 28-75, 28-105, etc) but eventually wider lenses started getting more popular and now the 24-70mm is probably most popular starting full-frame lens. Although they have 24-105mm ones that are quite popular too.

Fixed lens cameras slowly evolved that way. 35mm used to be a popular wide starting point, then 28 and now 24mm is the most popular, although there are wider and narrow ones too. Most importantly for a fixed lens camera is that cropping is much easier than stitching, so you want to start with a wide angle that can satisfy most needs.

While lenses are evolving, they are shaping photos we see. I used to find a 28mm wide and now the 24mm (equivalent) seems normal and I use shorter focal-lengths to make my photos feel wide.

At this point in time and for a long while now, the widest rectilinear angle of view are available for full-frame only. Canon makes an 11-24mm lens and IRIX makes a 11mm prime which are both widest available angle-of-view among rectilinear lenses. Previously the Sigma 12-24mm was the widest and it had stay unmatched until Sigma also released an 8-16mm for APS-C. You can get very wide angles but not quite as wide on Micro Four-Thirds with a 7-14mm which is similar to what Nikon offered for full-frame cameras with their 14-24mm.

The field-of-view you get in practice depends on the lens you buy. So if you are going for a general purpose lens, then you can find a 24-70mm or so equivalent for any sensor size. Lenses for larger sensors typically cost more, so going to a larger sensor is costly (even the same is almost always more expensive. If you are buying a lens for its wide angle though, then going full-frame is probably going to cost less for the lens alone. The IRIX 11m Firely costs $525 USD, which is cheaper than the Panasonic 7-14mm F/4 for Micro Four-Thirds and the 8-16mm for APS-C. So at the time of writing, your cheapest widest angle is for full-frame.

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    Re: wide rectilinear vs. consumers, I would also point out that beyond a certain point, things can start getting very "stretched" in corners, which might not be what many people are looking for in group photos, etc... at some point, rectilinear becomes distortion for some purposes. – junkyardsparkle Nov 13 '17 at 6:54
  • The question in the title is "Why...?" and the first word of your answer is "No": it may be worth quoting the part of the question your "no" is answering :). – dhag Nov 13 '17 at 14:10
  • No is the answer to the last question... let me see what I can improve. – Itai Nov 13 '17 at 14:20
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That means the widest angle on the four systems described above are all the same! Is there a physical reason for this?

Probably. I'm not a lens designer, but I suspect that if it were possible to design a lens that gave you reasonable image quality and an angle of view that varies from very wide to very narrow, all while keeping the price down, someone would've done it by now. The fact that we see zoom lenses with pretty consistent ranges for angle of view makes me guess that lenses covering about 65° to 20° angles of view probably hit a sweet spot in terms of what's possible to produce at a price and image quality point that's attractive to consumers.

The 12 mm lens for the µ 4/3 system is MUCH cheaper than a 12 mm lens for the APS-C or full-frame systems. Why is this?

The much smaller sensor sees a much smaller part of the image, so the image only has to be good in the center. A full frame sensor sees a much wider view through a 12mm lens, so you need a lens that's sharp and corrects problems like chromatic aberration over much more of the projected image, and that means better glass, more elements, etc.

Why does there appear to be this magical limit of a 24 mm 35-mm-equivalent focal length?

It's not a limit, it's just the lower bound of that engineering/economic/marketing sweet spot for general-purpose kit lenses. You can certainly buy wider lenses, but a lens that's both wider and longer while keeping a reasonable aperture range and still making acceptable images would generally be more expensive, and therefore less attractive.

Does that mean that short of buying expensive wide-angle lenses, moving to a larger sensor with a smaller crop factor does not, in practice, give me a wider angle view?

Well you can't stick your Canon EF-S 18-55mm lens on a full-frame Canon 6D, if that's what you mean. Lenses designed for cameras with smaller sensors, like the EF-S lenses, might project an image that's too small to cover a full-frame sensor. Also, those lenses extend farther into the body and would interfere with the mirror on a full frame camera, so they're physically incompatible with those cameras. But if you've got an EF lens, you'll see a wider view through it when you mount it on a full-frame body.

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There's no magic limit at around an 84° diagonal angle of view, which equates to a 24mm lens on a FF sensor. But there do seem to be a few reasons why it is a popular starting point for relatively inexpensive normal zoom lenses.

  • It is roughly just a bit over half) the focal length of the sensor diagonal in most of the cases you have used as examples. A FF sensor has a diagonal of a little over 43mm. 24mm is 56% of 43mm. A 1.5X APS-C sensor has a diagonal of almost 29mm. 16mm is 55% of that sensor's diagonal. The others are similar.
  • Any focal length shorter than about one-half the sensor's diagonal measurement begins to get very difficult to design for good image quality while also keeping the cost of producing such a lens at a minimum. Price is a very important aspect of "kit" lenses as they contribute an appreciable amount to the total cost of the camera + lens packages most commonly bought together. You can have very high quality rectilinear wide angle lenses with a wider field of view, but they start to get very expensive very quickly.
  • The approximately 85° angle of view yielded by such lenses is what many consumers have come to expect from a "regular" rectilinear zoom lens on the wide end of its focal length range.

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