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I am quite new to photography, but spent some days understanding aperture, timing, zoom and ISO.

Now I see the values printed on the Canon Powershot G15´s lenses, and I can´t interpret them. I assume that the values are mainstream without anything odd. It reads:

6.1-30.5mm // 1:1.8-2.8 // 5x Zoom.

First value: I always considered 50mm to be "normal", and values like 18mm to be wide-angle. That would make that cam a fisheye?

Second value: I expected something like f/2.8-f/8, with higher numbers adding depth of sharpness but losing illumination. Would this lens give me bright, blurred picture?

I guess I am confusing things, but can´t see how.

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6.1-30.5mm is the actual focal length (zoom) range for the lens, but the sensor in the camera is much smaller than that in a dSLR or film camera, so the focal lengths are also smaller. The G15 uses a 1/1.7" format sensor, so its crop factor is roughly 4.5x. The spec to look for here, if you know how focal lengths translate to field of view (FOV) on film is the "35mm equivalency". For the G15, this is 28-140mm. That is, the field of view on the G15's lens is similar to a 28-140 lens on a 6D, or an 18-88 on a 700D.

1:1.8-2.8 is not the aperture range of the camera's lens, it's the maximum aperture of the lens. Because the lens is a zoom lens, two numbers are given: the first one is for the wide end of the lens, the second for the telephoto end. So at 6.1mm, you can open up the lens to f/1.8, but at 30.5mm, you can only open it up to f/2.8. You can, of course, always use a smaller aperture setting than the maximum.

The 5x Zoom factor is simply calculated by dividing the longest focal length by the shortest to get a relative measurement of how large the zoom range is. In this case, 30.5/6.1 is a 5x zoom. However, note that both a 1-50mm and a 100-500mm lens would also be 5x zooms but vastly different in nature, which is why the absolute description of focal lengths rather than the relative measurement of zoom factor is typically used for purposes other than marketing.

  • Thank you, all 3 make perfectly sense. I mostly found focal length values as 35mm eq. (for better comparability I guess), so I did not realize that the actual value was written on the lens. – zsz85 Apr 15 '14 at 20:58
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Focal length

A 50 mm focal length is considered as 'normal' for a full frame camera, i.e. with a sensor that is 24 x 36mm. With a smaller sensor the focal length for a 'normal' lens is shorter.

The G15 has a 7.6 x 5.7mm sensor, so a 'normal' lens would be 11mm.

The 6.1-30.5mm lens has the same field of view as a 28-140mm lens would have on a camera with a full frame sensor.

A fisheye lens is not only one that has a short focal length, it also has a distorted view. Just a shoft focal length doesn't make a lens a fisheye lens.

Aperture

The 1.8-2.8 apterture is not the largest and smallest apertures, it's the largest apertures for the shortest and the longest focal lengths. At 6.1mm the widest aperture is 1.8, but at 30.5mm the widest aperture is 2.8.

Zoom

The 5x zoom range is just the factor between the shortest and longest focal lengths, i.e. 30.5 / 6.1 = 5.

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The focal length is the actual focal length of the lens, but since you have a very small sensor, it is highly cropped compared to what you would get with a 35mm lens. Since the crop factor is 4.8, it is equivalent to a 29.28mm-146.4mm lens.

The aperture is the range of maximum apertures, 1.8 on the wide end and 2.8 on the tight end. Again, crop factor makes a big difference here. While these would be very fast on a 35mm lens, the sensor in the G15 (and thus the lens as well) are very, very small and the actual focal lengths used are very, very short. Thus, even though 7mm focal length on the G15 may look like 35mm on a full frame camera, it is only actually 7mm and thus has the DoF characteristics of a 7mm lens. This is why you don't get strong (or possibly any) background blur with a point and shoot, despite having an f/1.8 lens.

The 5x is just the longest focal length divided by the shortest. It is pretty much a meaningless number, but consumers like big X numbers that are easy to compare (even if it isn't a valid comparison) so consumer targeted cameras frequently have the x number plastered on to them.

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