Superzoom Super Focal Length: Technology or Trickery?

"While the megapixel race has slowly come to a halt, there’s a different kind of battle going on, and that’s over zoom power. Ten years ago superzooms commonly had 10X and 12X lenses, and in 2015 the average is around 50X. Late last year Canon’s PowerShot SX60 HS hit 65X and in March 2015 Nikon blew away the competition, releasing its Coolpix P900 with an incredible 83X, 24-2000mm equivalent zoom."

-- DPReview's 2015 Superzoom Roundup

2000mm equivalent focal length... that's an astronomical amount of zoom. Literally. You usually need an astronomical telescope to get that kind of extreme focal length. The question is whether this is an exciting breakthrough in camera technology, or is it some marketing slight of hand?

Crop Factor

Before the invention of digital imaging, the majority of cameras took thirty five millimeter film. Each frame of the film was thirty five millimeters long by twenty four millimeters tall. Because the film was a standard size, a 50mm (focal length) lens on a Minolta would yield the same field of view as a 50mm lens on a Pentax. A longer lens, like an 85mm lens would provide more magnification or "zoom" to the image. Because all the cameras used the same film size, the focal length was a handy way to describe how much magnification a lens would produce. Longer lenses were higher magnification, and shorter lenses were lower magnification.

Unfortunately the situation got more complicated when digital cameras arrived on the scene. It was expensive and difficult to create a full thirty five millimeter sensor, so smaller sensors were introduced such as the aps-c "crop" sensor. When the same 50mm lens is used with one, the image is reduced to a smaller area of the image circle. Lenses are round, and they produce a circular image like the one in the diagram (the actual image circle of a lens is somewhat larger than what is shown). The red frame shows what a thirty five millimeter (full frame) camera records, and the blue rectangle is what an aps-c sensor records of the same image. With a full frame camera, you'd see the mountain plus some trees in the foreground, but notice that with an aps-c camera, the mountain fills the frame. Effectively, the image has been magnified even though the same lens has been used on both cameras. On an aps-c, a 50mm lens feels like, or gives the same magnification as a longer 75mm lens.

Photo Credit (Wikimedia commons) By Self - Self, CC BY 2.5

(Orange Rectangle Added)

To help cope with this situation, photographers came up with the a number called a "crop factor" or "focal length multiplier". The crop factor for an aps-c camera is 1.5 (or 1.6 for Canons). This means if you have a 50mm lens and an aps-c camera, the effective focal length is 50mm x 1.5 which equals 75mm. The crop factor is a very useful number to know. For example, you read an article that says that 90mm is a good focal length for portraits. You know if you buy a 90mm lens, the effective aps-c focal length will be 90mm x 1.5 = 135mm. In that case you need a shorter lens to get the same field of view.

Here is a list of common sensors from largest to smallest, and their crop factors:

Full Frame = 1.0

APS-C = 1.5

Canon APS-C = 1.6

Micro Four Thirds = 2.0

1" sensor (Nikon 1, Sony RX100, Canon GX-7) = 2.7

1/2.3" sensor (Canon SX60, Nikon P900, Pentax XG-1) = 5.6

iPhone 5S, iPhone 6 = 7.2

Is this magnification real? The answer is (as it always is in photography) "it depends". A Nikon D810 with a full frame lens takes 36 megapixel images. It also has a "crop mode" in which it only uses an aps-c sized area of the sensor. It's useful if someone is upgrades from an aps-c camera, and wants to use their DX (smaller image circle) lenses. In crop mode, the camera creates 16 megapixel images. Has the D810 actually magnified the image? No. It's just thrown away half the pixels. 16 megapixels is still a good size image, but the point is that the focal length was not magically increased by just cropping.

Now consider the case where you have two 24 megapixel cameras, one with a full frame sensor and one with an aps-c sensor. In this case, the magnification is real. Both cameras have the same number of megapixels, but the aps-c camera's sensor is smaller with proportionally smaller pixels. Those pixels take a smaller image coming from the lens (the blue rectangle in the diagram), and blow it up to the full 24 megapixel image size. That's useful if you want to get more reach out of the same lens! In that case, why not make even smaller pixels for more focal length?

Small Pixel Problems

There are two problems with having smaller pixels. The pixels (actually photodetectors) are like little buckets that collect light while the camera's shutter is open. With a smaller pixel there is less surface area to collect the photons streaming in from the lens. Less photons means that the camera circuitry has to amplify the number of photons in order to get the same brightness level as a larger pixel. This amplifies the noise as well. It's why smaller sensors don't handle low light situations as well--so few photons have been collected by the little buckets that the amplified noise dominates the image. But wait, what if the shutter was left open longer so that more photons could land on the smaller pixels? Wouldn't that solve the problem? Yes it would, but here is the second problem. Smaller buckets can't hold as many photons as larger buckets. If the shutter is left open longer, some of the buckets will fill up completely and become over-exposed. What this means is that smaller pixels have both less sensitivity and less dynamic range. This isn't such a big deal stepping down from full frame to aps-c or micro four thirds, but as sensors get even smaller, the inherent problems with small pixels begins to degrade image quality.

Effective Focal Length

Look closely at the table of crop factors above, and you'll notice that as the sensor gets smaller and smaller, the crop factor gets larger and larger. For a micro four thirds camera, the crop factor is 2.0. A 90mm lens suddenly becomes 180mm. What about a 70-200mm lens? That becomes a 140-400mm lens. These are pretty standard lenses, but the equivalent focal length numbers are going through the roof! Finally we come to the super-zoom cameras: SX60, Fujifilm S1, Nikon P900, Pentax XG-1 and the Samsung WB2200. The crop factor is 5.6. Now that 70-200mm lens becomes 392mm - 1120mm of "equivalent" focal length. This is how they come up with astronomical zoom numbers. But it's not so much the lens getting longer as it's the chip getting smaller.

And it is a *tiny* imaging chip. It measures 6.17mm x 4.55mm. To get a sense of how small this is, imagine a standard U.S. postage stamp (which is one inch tall and zero point seven inches wide). You could fit fifteen of these chips onto the stamp and have room left over. To see how this chip crops the image circle, look at the orange rectangle in the diagram. Instead of seeing the trees and the mountain, all it sees is the very tippy top of the mountain. The chip has sixteen megapixels, which means the pixels are also tiny in comparison to other camera's pixels.

Conclusion

The pixel size in the superzooms is nearly identical to those in the iPhone6, and they produces about the same quality images. Photos like that are fine for casual use and on the web. But if you look at them at actual 100% size, the detail is poor even in bright, sunlit shooting conditions. The photos look much better scaled down to 50%, but at that point is it really a 16 megapixel image or is it a 4 megapixel image? A midrange DSLR or mirrorless camera can easily produce sharp detail at 100%. And this is where a buyer might feel shortchanged by the marketing. The P900 superzoom looks like a midrange DSLR. You could imagine a camera buyer browsing a shelf full of cameras in this price range: Canon T6i, Nikon D300, Sony A6000, Pentax KS-2 and the Nikon P900. They all look about the same with the same with similar specs, except the superzoom has these amazing numbers! 83x magnification! 2000mm of (effective) focal length! It seems like the camera has capabilities the others can't match. The truth is that it's really a 360mm lens with a cell phone sized sensor.

This is fine, as long as the buyer knows what they're getting. These cameras can be a lot of fun, and they can take amazing shots that would be impossible without a telescope. If the only intention is to post photos to the web (where 99% of the photos end up anyway) then the images are good enough for that. There could also be scientific uses. If naturalists are identifying birds in an area, they don't need fine art type images.

Nikon P900 sample images from DPReview

Footnotes

Pixels vs. Photodetectors - Cameras list their resolutions in megapixels, but that's misleading. Pixels are display elements on a screen--they *emit* light. The photodetectors or photosites on a camera sensor *absorb* light. The detected signal is then converted into pixels to be displayed on a display device. Pixels and photodetectors are fundamentally different things, but trying to make that distinction while explaining other complicated subjects seems like a bad idea. Photodetectors store charge. When a photon hits light sensitive silicon, it creates an electrical charge. It's this charge that is stored in photodetectors, not actual photons.

Regarding the pixel size difference between the iPhone-6 and the chip in the superzooms: the iPhone-6 pixel size is 1.16 microns, and the superzoom pixel size is 1.33. To put this in perspective, the Nikon D810's pixel size is 4.89 microns. In surface area, which is what counts for sensitivity, the D810's pixels are 13.5 times larger.