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Old Oct 10, 2011, 11:43 PM   #1
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Default digicam "optical zoom" vs focal length mm.

Hi... I have a pocket digicam which claims 12X Optical zoom. (Camera actually works fairly well.)
I also have a Canon Rebel with a 200mm lens.
Can anyone tell me how the 12x optical zoom on one camera relates to the 200mm lens on the canon?
I gather the 200mm on a digital camera is equivalent of a 300mm lens in 35mm format. Do I understand this correctly?

Many thanks,
"newby" Rusty
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Old Oct 11, 2011, 5:51 AM   #2
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Originally Posted by rustyhill View Post
Can anyone tell me how the 12x optical zoom on one camera relates to the 200mm lens on the canon?
It doesn't.

A focal length of 200mm is a physical characteristic of a lens. It's a measure of how much it bends light and thus its angle of view.

A 12X optical zoom is simply a ratio of the zoom lens' shortest focal length to its longest. It's a ratio between two real measurements, not a real measurement on its own.

A 12X zoom can mean that the lens has a focal length ranging from 25mm to 300mm, but it can also mean that it has a focal length ranging from 10mm to 120mm.

Because different cameras have different size image sensors, and therefore, different angles of view for the same focal length lens, in order to make a meaningful comparison, you need to use the 35mm equivalent focal length. For instance, on your Canon Rebel, a 200mm lens has a 35mm equivalent focal length of 320mm. Meanwhile, the Canon PowerShot SX150 IS, which has a 12X optical zoom, has a physical focal length of from 5mm to 60mm. But because it has a much smaller image sensor than your Rebel, it has a 35mm equivalent focal length of from 28mm to 336mm. So the SX150, with its 5-60mm lens, could have the same angle of view as your Rebel with a 200mm lens, but the only way to know that is by comparing their 35mm equivalent focal lengths.

And, again, the 12X optical zoom specification doesn't tell us anything about the focal length (actual or 35mm equivalent), only its range of focal lengths.
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Old Oct 11, 2011, 10:19 AM   #3
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Many thanks, good informative info.

Maybe my 12x camera will tell me the equivalent 35mm focal length. I hope.

Again, thanks,
Rusty
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Old Oct 11, 2011, 3:17 PM   #4
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Originally Posted by rustyhill View Post
... Maybe my 12x camera will tell me the equivalent 35mm focal length. I hope. ...
The first place to look is in the EXIF data for your photos. Almost for sure it will tell you the physical focal length, and may well tell you the "35mm equivalent". Then shoot some photos at each end of your zoom range.
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Old Oct 11, 2011, 4:17 PM   #5
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G'day Rusty

Very good Q mate ... maybe this sketch will help [made up for my students]



I would presume that your camera manual [maybe on the camera body itself] will say something like "12x zoom, 30mm ~ 360mm equiv" ... meaning that in 35mm film camera equivalents, your lens will show similar results to a 30mm to 360mm lens on a film camera or full-frame digital camera

Hope this helps a bit
Regards, Phil
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Old Oct 12, 2011, 7:38 AM   #6
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Just curious -- what APS sensor camera has a crop factor that makes 28mm the FOV equivalent of 50mm on a FF camera? I would have thought tat you'd have used either Nikon's 1.5 crop factor or Canon's 1.6 crop factor.
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Old Oct 12, 2011, 10:34 AM   #7
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Yes, Ozzie_Traveller, there are a number of flaws in that chart.
  • The alignment of the 14mm (for 35mm film) and 9mm (for APS-C), 24 and 16, and 3002 and 200 are appropriate, and
  • the offset of 17 and 11, 20 and 14, 35 and 24 are correct
... but ...
  • 28 and 18 should not be aligned, and
  • 50 and 28 shouldn't be anywhere near one another
Also, while a 35mm equivalent focal length of 28mm is common as the shortest focal length/widest angle of view for superzoom cameras, it is by no means universal. The same is true of 24mm and compact cameras. In addition, the extension of the lines for superzoom and compact cameras to the left of the 1X mark implies that there are also 0.75X and 0.5X values to be found on the graph, and that's certainly not true.

Lastly, and this certainly ventures into the relm of nitpicking, technically, a "Telephoto Lens" is a lens whose focal length is longer than the lens itself. That definition actually includes some popular pancake "Normal" and and "Wide Angle" lenses.
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Old Oct 12, 2011, 4:45 PM   #8
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G'day fellas

Thanks for your responses - & you're not nit-picking
This 'generic' chart is based upon manufacturer's printed materials, but does -of necessity- have some compromises ... which of the 4 "APS" sensor sizes do I go with? or do I compromise??

Equally -for example-, if I look at any of the Fuji or Panny range of superzooms, I read their "35mm equiv" lens ranges & their manuals describe angle of coverage for the zoom. Equating this 'compromise' with the 35mm table gives me certain results that do not always stack up ... thus another series of compromises

The chart is designed to give my students, often 'newbies' some visual grasp of the angles-of-view of lenses, and the differences between them

I will however go back to the source info & double check the 18 / 28 / 50 alignments - so if I have made an error, thanx for helping to make the chart even better

Otherwise - does it help you to work out lens focal length & angles of view??

Regards, Phil
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Old Oct 14, 2011, 7:12 AM   #9
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Is it completely accurate? No.

But is it a very handy guide and broadly useful? Heck yes.

Nice job.
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Old Oct 14, 2011, 8:02 AM   #10
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Asside from the minor mathematical errors, my only objection is the attempt to equate "1X" to some actual value. Those numbers are ratios. (4X means that the longest focal length is 4 times longer than the shortest focal length.) The implication that "1X" actually means something, and that it is the basis for all the other Xs, is wrong, and while it may succeed in seeming to remove some confusion now, ultimately, it creates a misconception that is hard to remove. I don't like to see something taught that must be untaught later. It perpetuates ignorance and laziness.

Wouldn't it be better to take the time to teach someone the actual truth than to tell them a little white lie because it's easier for you to do and for them to understand?
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Last edited by TCav; Oct 14, 2011 at 8:07 AM.
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