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Old Nov 9, 2006, 12:49 AM   #1
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I was wondering if there's a way to convert mm mesuring in lenses to X zooming?....just so i could have an idea of how much it is...any explanation on both subjects would be really apreciated!
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Old Nov 9, 2006, 7:01 AM   #2
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This topic comes up every other month or so.

The 3x or 4x or 10x 'zoom factor' you see on digicams is simply a ratio of longest focal length to shortest focal length. Because it's a ratio, it's not very useful:

25mm-75mm and 100-300mm zoom lenses are BOTH 3x zoom but they are completely different lenses.

What is useful is to find out what the effective focal length of a given digicam is. the specs for the camera will say this. The camera may advertise 3x zoom but the specs will tell you it's lens is equivelent to 28-84mm

That gets you one side of the equation.

Assuming you are comparing a digicam to a DSLR, every DSLR lens will have a focal length associated with it. Let's say it's 17-85mm. What you have to be aware of is that MOST DSLRs have a 'crop factor' or 'magnification factor' that has the affect of producing images that are more magnified than if the lens was used on a film SLR. You can do searches on the topic to find out more details. But for the purposes of this thread, the important thing is that for Nikon bodies that magnification factor is 1.5. So that 17-85mm lens BEHAVES LIKE a 26-128mm lens on a Nikon DSLR.

On entry level Canon DSLRs the factor is 1.6. Canon also has cameras with a 1.3 factor and cameras with full-size sensors that have NO factor. Every other DSLR maker has cameras with some factor (1.5 or 2.0). The factor will be listed in the specs of the camera body.

So, if you have a digicam with a 28-84mm lens and you want to know what lens to buy for a Nikon camera that covers the same focal range, simply divide 28 by 1.5 and 84 by 1.5 and you get 18.6 - 56mm. So with any of the Nikon DSLRs you would need a lens that covers 18.6mm - 56mm to get the same coverage you had with your digicam.
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Old Nov 9, 2006, 7:15 AM   #3
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'mm' is a focal length, 'x' is a ratio between two focal lengths.
A 10-20mm lens is a '2 x' zoom. 20 is two times ten.
So is a 400-800mm.
One is an extreme wide angle, the other a very long telephoto.
Just ignore completely the 'x' number. It's a load of marketing-driven rubbish.
What you want is a 35mm equivalent.
40-50mm is around a 'normal' field of view - what you see with your eye.
30mm is a fairly wide angle. Less than 20mm a very wide angle, getting near to a fish-eye type field of view.
100mm is enough 'zoom' for head and shoulders portraits accross an average room.
200mm is a good telephoto, good for shots of people accross a tennis court.
400mm is a long telephoto, and about the limit of 'superzoom' point and shoot cameras. Good for a whole body shot at around 30-50 yards.
800mm is a small telescope.
You need to find the 35mm-equivalent focal lengths for this to apply. Actual focal lengths will be much smaller as most digital cameras use sensors smaller than 35mm film and so the field of view with a given lens is cropped. 35mm is the accepted standard with which to compare as most cameras in the past used that size of sensor (film).
Hope this helps.
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Old Nov 9, 2006, 8:01 AM   #4
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ok, how about an example.

i used to own the panasonic fz30. it is a 12x optical zoom lens. the relative 35mm equivalent for the lens is 36mm to 432mm (36x12=432).

In general, most digicams start at around 35mm (some may be as wide as 28mm, or as closed as 40mm) equivalent focal length, and their 'zoom' factors (x3, x4, x6 etc) are calculated from that.

I now have a pentax *istDL and several lenses. it has a sensor size smaller than full from 35mm film cameras - so it has a crop factor of 1.5x

here is a breakdown of my lenses:

18-55mm f3.5-5.6 kit lens = 27-83mm - this is a good wide angle zoom.

28-200mm sigma f3.5-5.6 zoom = 42-300mm range in terms of 35mm equivalent (between 8 and 9x 'zoom' in digicam terms).

28mm vivitar f2.8 macro = 42mm fixed lens.

50mm pentax f1.7 = 75mm fixed lens.

400mm vivitar f5.6 = 600mm fixed lens (keep in mind this lens sounds like a big zoom, and it is! but, it is only doubling the 300mm equivalent on the tele end of my 28-200mm zoom. basically it will provide 1/4 of the viewing area of the 300mm length - if my math is right).

so, in my mind, most digicams give their 'optical zoom factor' from around 35mm on an equivalent scale. then you can use those numbers to compare with dslr lenses to find out in approximate terms what the dslr lenses are providing.
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Old Nov 9, 2006, 5:16 PM   #5
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If you want to convert to the X power listed for binoculars or telescopes divide the 35mm equivalent focal length by 50. It isn't exact but pretty close.
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Old Nov 9, 2006, 6:55 PM   #6
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Another way of looking at the X.

Unzoom the camera all the way (to the widest view).

If the camera is 2x, then imagine a cross dividing the viewfinder in half horizontally and also in half vertically. The maximum zoom will take the view of one of the four portions to fill the frame.

If the camera is 3x then imagine the viewfinder is divided in thirds both horizontally and vertically. The maximum zoom takes one of the nine portions to fill the frame.

For 4x it is one of the 16 small portions in a 4x4 subdivision, for 5x it is one of the 25 small portions in a 5x5 subdivision, and so on.

Part 2. How does the widest view of one camera compare with the widest view of the other camera? This is important too.

First disregard the mm (focal length) printed on the camera or its lens itself. You need the 35mm equivalent for both cameras you are comparing.

The most common point and shoot camera "mm" in 35mm film camera equivalent is around 38mm at widest angle. This corresponds to a side to side angle of view of about 53 degrees. A typical SLR camera has a 50mm lens as standard. Its side to side angle of view is about 40 degrees. A common point and shoot camera with a "wide angle" lens has a 28mm focal length. The angle of view is about 65 degrees.

Then at 2x zoom thefield of view is about (not exactly) half, for 3x zoom the field of view isabout a third, and so on. For example at 3x zoom the 38mm zoom lens e.g. "38 to 111 mm"with widest view of about 53 degrees now hasa view of about 18 degrees.

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