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I'd like to understand how to determine how powerful zoom is going to be in particular cameras/lenses based on provided information. What is the best way to do this?

I understand the zoom factor (e.g. 40x or 30x) doesn't really mean anything as it is merely a quotient/ratio of max length to min length of the lens and on its own it can probably be ignored.

I thought the actual max length of lens is going to be what I should be looking at, but I was surprised recently when comparing two cameras:

  • Canon PowerShot SX720 with lens 4.3-172mm (40x zoom)
  • Panasonic Lumix DMC-TZ80EB-S with lens 4.3-129mm (30x zoom)

Based on the max length of both I was expecting Canon (with max length 172mm) is going to allow me to zoom in closer than Panasonic (with max length 129mm), but it's actually the opposite.

Why is that? What other information I should be looking at?

4 Answers 4

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What other information I should be looking at?

Sensor size

The angle of view involves both the lens' focal length and the size of the sensor. A larger sensor will show a wider angle of view with the same focal length. A smaller sensor will show a narrower angle of view with the same focal length.

If images from two differently sized sensors are displayed/viewed at the same size, the image from the smaller sensor has been enlarged more to be viewed at the same display size as the image from the larger sensor.

Greater enlargement has disadvantages, too. Any blur in the image is magnified more. Any noise in the image is magnified more. The larger blur and noise are, the easier it is for our eyes to see them.

In the case of the Panasonic TZ80EB-S, the sensor is 1/2.3". The Canon PowerShot SX720 also has a 1/2.3" sensor, so that's not the difference in your case. The Canon, having the longer focal length, should be providing a more magnified view when both images are displayed at the same size.

Sensor resolution

The Panasonic has an 18.1MP sensor.
The Canon has 20.3MP sensor.

If one views both images at "100%" on a computer or tablet screen, the image with the higher number of megapixels will have a greater enlargement than the image with fewer megapixels. This is because viewing at "100%' isn't the same amount of enlargement for images with different numbers of pixels. "100%" means that each pixel in the image is displayed by one pixel group on the screen. If you're using a monitor with 96ppi pitch, then a 100% view of an 18.1MP 4:3 aspect ratio sensor will be like looking at a piece of a 51x38" enlargement. With the same monitor, a 100% view of a 20.3MP 4:3 sensor will be like looking at a 54x40.5" enlargement.

Again, the Canon has more pixels so should provide more magnification when viewing at "100%".

Based on the max length of both I was expecting Canon (with max length 172mm) is going to allow me to zoom in closer than Panasonic (with max length 129mm), but it's actually the opposite.

Why is that?

Digital trickery

The Canon camera includes a 4X digital zoom. Canon is pretty honest about it. Using 4X digital zoom and then viewing the results at the same display size as before using digital zoom would give you the same enlargement ratio as a 160X optical zoom would. In other words, you'd be seeing the same amount of enlargement as a 17.2-688mm lens on a 1/2.3" sensor. Or the same amount of enlargement as a 4.3-172mm lens on a sensor 1/4 as wide, 1/4 as high, and with 1/16 the area of a 1/2.3" (6.17 x 4.55mm) sensor.

Digital zoom is basically the same thing as cropping after the fact. The camera is only saving the central part of the photo and discarding the edges. Of course this reduces resolution, as only pixels in the central portion of the sensor are recorded. It also magnifies any blur and noise by the same amount. So for most of us, digital zoom doesn't really count because what is really happening is that the camera is just using less of the total area of the sensor to make things look bigger by blowing up the smaller image size to the same display size.

Panasonic includes what it calls "Extra Optical Zoom (EZ)", but in reality this is a form of digital zoom.

Notice that at the 42.5X setting the total resolution is reduced from 18.1MP to 9MP. That's to be expected if the sensor is linearly cropped by a factor of the square root of two (1.414). Guess what? 42.5 divided by 30 equals 1.417 (42/30 = 1.417). That's pretty close to the square root of two (1.414)!

Notice that at the 61.2X setting the total resolution is reduced from 18.1MP to 4.5MP. That's to be expected if the sensor is linearly cropped by a factor of 2. Guess what? 61.2 divided by 30 equals 2.04! Close enough.

Panasonic is Busted

Panasonic is trying to fool you into thinking you're getting optical zoom up to 60X when the lens used is only 30X longer at the telephoto end than at the wide angle end. They even include an additional setting for "digital zoom" to try and fool you. But notice that the 4X digital zoom can only be used at 2X when "Intelligent Zoom" is used?

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    Thanks, it appears I must have been fooled by the Extra Optical Zoom (EZ), which I thought was actually optical. As for digital zoom I understand it's best kept turned off, images can always be cropped afterwards if needed.
    – tsw_mik
    Jun 13 at 0:17
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In complement to the other two excellent answers: don't be too focused on zoom power.

  • A long zoom (above 300mm (35mm eq.) requires some serious skill to aim at a moving target, and IMHO the "compact" format without a viewfinder requires even more skill, and to make things worse on these cameras the zoom is electric so not as fast as a manual zoom.
  • Even with image stabilization, you will need high speeds, that would require a wide lens and good ISO capability.

TL;DR: zoom, yes, usable zoom, less so.

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The max focal length controls how big things appear on the sensor inside the camera, relative to their size and distance.

And the size of the sensor controls how big that is compared to the entire frame. So for instance if a lens projects a 1cm image of a subject onto a 1cm sensor, it will fill the frame. If the same lens projects that same 1cm image onto a 2cm sensor, it will only fill half the length of the frame.

The standard way to compare focal lengths across cameras is to use the 35mm equivalent focal length. This tells you what the image would look like by comparing to a well known standard format used film cameras and typically higher end digital cameras. You can calculate this quite simply, or you can just google "[name of your camera] 35mm equivalent focal length".

From that I find for the Cannon:

Focal Length (35mm equivalent): 24 - 960mm
Focal Length (actual): 4.3 - 172mm

For the Panasonic:
24-720mm in 35mm terms

So the Cannon with 35mm equivalent focal length a bit longer than the Pansonic will be able to make distant things appear a bit bigger in photos, assuming you don't crop photos from either camera and you don't use the digital zoom features on either camera. I'm not sure why you're seeing the Panasonic making things appear larger.

As noted in the comments in the case of these two cameras both sensors are the same size (1/2.3 inch) so the difference is down to the Cannon having a lens with a longer focal length.

Of course you can always crop on your computer to make things look bigger. When you do that you'll lose resolution. So to make a more complete comparison you'd need to also take into account the resolution and image quality of each camera.

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  • (in 35mm equivalent terms, around 35-50mm is considered 'normal'. Anything below that is called wide, anything above that is called long)
    – bdsl
    Jun 12 at 23:49
  • Both cameras have the same size sensors, 4:3 aspect ratio 1/2.3". The Canon camera does not have a smaller sensor than the panasonic.
    – Michael C
    Jun 12 at 23:51
  • @MichaelC yes you're quite right. So the Cannon shows things bigger simply because it has a longer actual focal length. And it seems I misread the question. I don't know why things would appear bigger on the Panasonic. Maybe it's digital zoom on the Panasonic making things bigger.
    – bdsl
    Jun 13 at 0:01
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    Thanks, I will use the 35mm equivalent values for reference in the future.
    – tsw_mik
    Jun 13 at 0:18
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Its industry practice is to label camera lenses as to wide-angle, normal, and telephoto. First let’s clarify “normal”. A camera sports a “normal” lens when its focal length is approximately equal to the corner-to-corner (diagonal) measure of its format. Such a lash up delivers a horizontal angle-of-view of approximately 45⁰. This value is approximately true for most rectangular formats. Now the diagonal measure delivers a greater value, its 53⁰. I deem the diagonal angle of view to be somewhat pointless, nevertheless, the industry pounces on this value when publishing camera specifications. This is because it’s the larger value thus good for advertising puffing.

That being said, a “normal” angle of view occurs when the focal length and the diagonal measure are approximately equal. A wide-angle lens is one that is approximately 70% of “normal’ or shorter. A telephoto lens is one that is approximately twice “normal’ or longer.

Thus, my advice, cut through the puff by finding (computing) the diagonal measure of the camera’s format. Both the canon SX720 and the Panasonic DMC TZ280 sport an image chip that measures 4.62mm height by 6.16mm length. The diagonal measure of this rectangle is 7.7mm. Thus the “normal focal length for both cameras is 7.7mm. Wide-angle starts at 7.7 x 0.70 = 5.4mm ---- Telephoto starts at 7.7 x 2 = 15.5mm. Making sense of the zoom range. The zoom range of the Canon is 4.3mm through 172mm. The 40x is derived from 4.3 times 40 = 172mm. The Panasonic 4.3mm through 129mm thus 30X claim is 4.3 times 30 = 129mm.

On crop factor: The venerated 35mm full frame measures 24mm height by 36mm length. The diagonal measure of this rectangle is 42.3mm. To compute the crop factor, we divide the two diagonal measures. Thus 42.3 ÷ 7.7 = 5.6. This value tells me ---- the two cameras under consideration are smaller formats. They are 1/5.6 = 0.18 x 100 = 18% of the size of the full frame. Another way to view the 5.6 crop factor. If a display image 8 x 12 inch is produced from a full frame, to make an equivalent display image from either of these smaller format cameras, we must apply 5.6 X more magnification.

The disadvantage of smaller format cameras is the imaging recording approach and optics is challenged when more magnification is required to deliver a comparable display image. This is challenging however technology marches on however the target also moves forward.

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