Understanding Zoom

brdy · Mar 4, 2011

Hi new to camera stuff. I have a Canon G11 had it for about 12months now. Its has 5x times optical zoom. I have asked myself what does this really mean, why because Im thinking of progressing to a dslr body, perhaps the Canon 60D and have been thinking about what lenses i would get to go with it. And I'd like to know what a telephoto lens of 70-200mm lens will give me. My point and shoot G11 has a 28-140mm lens which I have been told is how marketing people come up with term "5x zoom" since 28 x 5= 140 .
Ok if this is so then a lens that is 50-250mm is also a 5X zoom. Now its here Im getting confused,probably because I have never had a dslr with a telephoto lens so have no experience about what sort of image I'm going to get. But am I correct in thinking that whilst both lenes are 5 times zoom the 50-250mm will give me nearly double the zoomed in image that the 28-140mm lens will because the starting point of magnification is different. Or will the camera sensor size make a big difference here.
My girl friend has a TZ10 which has 15X zoom ..Is that same as a 50-750mm on a camera with an APS-C sensor. I assume sensor in the TZ10 would be a lot smaller. As you can see Im confused

PASM · Mar 4, 2011

70-200 gives you X3 almost at the long end. Your G11 doesn't have a 28-140 it has the equivalent (in 35mm terms) of 28-140. I'm guessing but it probably has something more like a 4-20mm lens.

50-250 will give you almost twice the focal range of a 28-140.

Sensor size isn't a consideration when comparing as it's already factored into the 'marketing' of saying the G11 has a "28-140". However the depth of field at given apertures will be different with the G11 compared to an APS-C sensor camera.

tirediron · Mar 4, 2011

You're pretty much spot on. The "x" in zoom is simply the ratio that the narrow end goes into the long end (EG: 50-250 = 5x zoom). The size of teh sensor has a major impact on field of view. Focal lengths are all based on full-frame/35mm sensor size. For instance on an APS-C body with a 1.5 crop factor, the 50-250mm lens is still a 5x zoom, BUT it gives the field of view of a 75-375mm lens.

The TZ10 could be anything, it depends on the sensor size and where it's field of view/focal length "starts" (that is: What the wide end is).

brdy · Mar 4, 2011

Hi tirediron .... Understand now that all lenses are based on 35mm full frame sensor ...so when a 50-250mm lens is put on a camera with APS-C sized sensor that it acts like a 75-375mm. Now my question is this a magnification of field of view or is it merely cropping the full frame view. If its not a magnification then really the lens is the same on both sensors just the APS-C just crops the center bit out of the image. Or am I wrong.. Also if I had two lenses a 20-300mm and a 100-300mm will the resultant field of view of both the lenses at full zoom be exactly the same..

kundalini · Mar 4, 2011

Get all that stuff outta your head. Your lens focal length is just that.... your lens focal length.

If you really want to take it a step further, then your field of view is 1.6 times (for Canon) greater on an APS-C body than a full frame. All said and done, the focal length remains the same.

OrionsByte · Mar 4, 2011

I agree with Kundalini - don't worry about what the zoom factor is, what matters is the focal length. Think of 50mm as a "standard" zoom - that's roughly equal to what you see without any kind of magnification.

So the 28mm-140mm lens on your G11 is going to go a bit wider than standard, and can zoom in a bit closer than standard.
The 50mm-250mm lens is going to start at standard, giving you no wide-angle options, but it will zoom in a lot closer than your G11 can.

That "5x zoom" is a statistic you're only going to see on point and shoot cameras and doesn't really mean anything because two cameras or lenses can have "5x zoom" and have a completely different field of view, but the focal length (when converted to 35mm film equivalent) is an industry standard of measurement.

DerekSalem · Mar 5, 2011

PASM said:
70-200 gives you X3 almost at the long end. Your G11 doesn't have a 28-140 it has the equivalent (in 35mm terms) of 28-140. I'm guessing but it probably has something more like a 4-20mm lens.

50-250 will give you almost twice the focal range of a 28-140.

Sensor size isn't a consideration when comparing as it's already factored into the 'marketing' of saying the G11 has a "28-140". However the depth of field at given apertures will be different with the G11 compared to an APS-C sensor camera.

Actually a 50-250mm will give you *more* than double the 28-140mm. The 28-140mm has already been converted to the 35mm equivalency, like you said...but the 50-250mm hasn't been. Once you convert it to the 35mm equivalent it comes out to roughly 80-400mm.