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View Full Version : Optical zoom to focal length ratio?



greogory
07-27-2004, 05:11 PM
I'd like to know if there is a guideline for determining the relationship of the optical zoom mulitplier on a digital camera to the focal length of a lens for 35mm SLR cameras.

I guess what I'm really trying to ask is, what optical zoom mulitipliers would be the approximate equivalency of 28mm, 50mm, 100mm, 125mm, 200mm, etc., lenses?

I have a Sony Mavica CD350 with a 3x optical zoom to which I can attach Sony accessory lenses of .7x wide angle and 2x telephoto, respectively. How do I know what the equivalency of the native 3x zoom is to the focal length of lenses standard lenes and, if I were to a the .7x and 2x lenses, by what range have I increased my focal lengths?

Thanks for any help or pointers.
Greg Robinson

D70FAN
07-27-2004, 06:26 PM
I'd like to know if there is a guideline for determining the relationship of the optical zoom mulitplier on a digital camera to the focal length of a lens for 35mm SLR cameras.

I guess what I'm really trying to ask is, what optical zoom mulitipliers would be the approximate equivalency of 28mm, 50mm, 100mm, 125mm, 200mm, etc., lenses?

I have a Sony Mavica CD350 with a 3x optical zoom to which I can attach Sony accessory lenses of .7x wide angle and 2x telephoto, respectively. How do I know what the equivalency of the native 3x zoom is to the focal length of lenses standard lenes and, if I were to a the .7x and 2x lenses, by what range have I increased my focal lengths?

Thanks for any help or pointers.
Greg Robinson

Your Mavica MVC-CD350 has an equivelent focal length of 41mm to 123mm. So if you were to take a 35mm camera and put on a 41mm to 123mm lens you would get approximately the same result.

The 0.7 wide angle would give you a 28.7mm wide angle, and the 123mm with 2X adapter would be 246mm. So...

With the 0.7x WA adapter your CD350 would have a zoom range of 28.7mm to 86.1mm.

With the 2X adapter the zoom range would be 82mm to 246mm.

All of the above are 35mm camera equivelents.

Hope that helps.

greogory
07-28-2004, 02:54 PM
So what one of the things I infered from your reply, which I truly appreciated by the way, is that the focal length:optical zoom multiplier ratio is going to be camera dependent and that there's not a general across the board equivalency that may be applied to all digital cameras, is that an accurate inference? If so, I would assume that's because there's not an industry standard "default" focal length for the built-in lenses used by different manufacturers in different camera models.

greogory
07-28-2004, 02:57 PM
The goofy "purple smiley" that found its way into my last note was simply supposed to be a colon denoting the relationship between "focal length (to) optical zoom multiplier", I simply forgot to select "disable smilies".

Jake Conner
07-28-2004, 04:33 PM
Yes, what you inferred is true.

Puck
07-28-2004, 05:13 PM
So what one of the things I infered from your reply, which I truly appreciated by the way, is that the focal length:optical zoom multiplier ratio is going to be camera dependent and that there's not a general across the board equivalency that may be applied to all digital cameras, is that an accurate inference? If so, I would assume that's because there's not an industry standard "default" focal length for the built-in lenses used by different manufacturers in different camera models.

The equivalent 35 MM focal length varies in digital cameras because digital cameras come with various sensor sizes. A 35 MM camera has a frame size of 36 x 24 mm and a diagonal of 43 mm. A "normal lens" for a 35 mm camera would have a focal length equal to the diagonal or 43 mm although 50 mm is usually said to be a normal lens. All consumer digital cameras have much smaller sensors. For instance, I have one camera with a 1/1.8" sensor and another with a 1/2.7".

Measurements for the 1/1.8" sensor are 7.18 x 5.32 mm with a diagonal of 8.93 so a normal lens for this camera would have a focal length around 9 mm.

Measurements for the 1/2.7" sensor are 5.27 x 3.96 mm with a diagonal of 6.59 yielding a normal focal length of about 6.6 mm.

The smaller the sensor the smaller the focal length needed to approximate the focal length of a 35 mm camera lens.

The zoom multiplier is nothing more than the largest focal length divided by the smallest focal length of your camera's zoom lens. For example, I have a camera with a 10X zoom (5.8 to 58 mm or 38 to 380mm stated in 35mm equivalent). It is indeed a zoom of 10 X but it does not magnify the "normal" view by 10 times. If we assume a 50 mm lens yields a "normal" view" then this lens at max zoom has a 7.6x magnification. That's between the view given by 7x and 8x binoculars.

D70FAN
07-28-2004, 05:30 PM
Yup. What Jake said in brevity, and Puck elaborated on. Nuf' said.