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Old Apr 23, 2003, 2:13 AM   #1
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Default Convert Zoom Factor to 35mm?

How do you convert zoom factors (i.e., 5x optical zoom) to 35mm equivilants? I don't have a digital SLR yet (not enough money!), but I'm both curious and think it'd be good to know down the road.

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Old Apr 23, 2003, 3:11 PM   #2
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The equivalency is based on full wide angle equivelent focal length. For example, a camera with a 38mm wide angle focal length at 5x would equal 5x38 or 190mm. At 4x this same camera would be at a 35mm equivalency of 38mm x 4 or 152mm.

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Old Apr 23, 2003, 10:32 PM   #3
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And, while you are learning, most of us here feel that digital zooms are not worth the trouble of enabling because they do not provide nearly as high quality photos as with optical zoom only (AKA - "worthless"). While my camera is advertised as 27x (38mm-1026mm), I leave the 2.7x digital zoom turned off and use it only with the 10x optical zoom (38mm-380mm).
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Old Apr 24, 2003, 1:35 AM   #4
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Quote:
Originally Posted by lg
And, while you are learning, most of us here feel that digital zooms are not worth the trouble of enabling because they do not provide nearly as high quality photos as with optical zoom only (AKA - "worthless"). While my camera is advertised as 27x (38mm-1026mm), I leave the 2.7x digital zoom turned off and use it only with the 10x optical zoom (38mm-380mm).
Oh, I'm well aware of the uselessness of digital zoom. I turned it off in my camera (Sony Cybershot F707) as soon as I bought it.
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Old Apr 24, 2003, 1:39 AM   #5
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Quote:
Originally Posted by Lin Evans
The equivalency is based on full wide angle equivelent focal length. For example, a camera with a 38mm wide angle focal length at 5x would equal 5x38 or 190mm. At 4x this same camera would be at a 35mm equivalency of 38mm x 4 or 152mm.
Uh... how do you find the "wide angle focal length?" Is there a generic converter of "5x optical zoom = XXXmm"? If not, why?

If we must, assume the camera is 11.1MP and has a full 35mm size CMOS (i.e., the Canon EOS-1Ds). I know that digital SLRs with smaller photoreceptors have "focal length multipliers" and I assume this means you multiply the zoom factors as well. If that's true, then let's just ignore that, because it's camera-specific.

Thanks!
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Old Apr 24, 2003, 1:44 AM   #6
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BTW, it might help if I pointed out that I know absolutely nothing about 35mm cameras. To be honest, I don't know why 35mm cameras don't just use zoom factors (i.e., 3x or 5x) instead of all those millimeter sizes. It's confusing--especially if the numbers are only relevant on one camera. 3x zoom is 3x zoom, no matter what camera you're using.
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Old Apr 24, 2003, 7:01 AM   #7
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Let's see if we can put this into perspective. The use of the term "X" for describing "zoom" is only meaningful when you are given a starting point. It's "NOT" like binoculars or a telescope where the term "power" is frequently used to describe what one can expect in relation to what one sees with bare eyes. With a telescope at 6 power, the expectation is that what is seen through the eyepiece appears six times as large as it would looking through a window with the bare eye.

With a camera like a 35mm (based on the size of the film) platform, a 50mm lens is normally considered to be "lifelike" in terms of what you see through it. So in photography, we base many things relative to this. If a lens is smaller than 50mm it's considered "wide angle," if its' larger than 50mm it's considered "telephoto."

The consumer/prosumer fixed lens digicams generally have a range of focal lengths which extend from wide angle to telephoto. These true focal lengths are very small. For example, it could be 7mm at the wide angle end and a 4x would then be four times seven or 28mm. But because the sensors which replace film are very tiny, the field of view is greatly reduced, so what would be an extremely wide angle to a 35mm size piece of film looking through the lens becomes only a medium wide angle like 38mm in terms of 35mm "equivalency."

To get a starting point for the "X" factor, we must depend on the manufacturer's calculations which give us the extremes of wide angle to telephoto "equivalencies". The ratio of the telephoto to the wide angle then gives us the "X" factor in zoom. This ratio may be expressed as 2:1 (2x zoom), 3:1 (3x zoom), etc. The zoom factor then is internally consistent, but has no across-the-board meaning. The majority of these digicams starts with about 38mm as the wide angle. Some may go as wide as 28mm, but 38mm is most common. You must check the manufacturer's specifications to know then what range of focal lengths are contained within the zoom rating.

With digital SLR (removable lens types) which use 35mm platform lenses like you use on a 35mm film camera, the sensor size varies. The Canon EOS-1Ds has a full 35mm film frame size sensor. So the lens chosen gives exactly the same focal length field of view as it would on a 35mm film camera. With many of the Nikon digicams like the D1, D1H, D1X, D100, etc., the sensor has a 1.5x crop factor. That is the field of view when using a 35mm lens is reduced by the crop factor. This makes the image frame field of view look like it would if you were to take an exposed 35mm film negative and take scissors and trim it by cutting away the top, bottom, and sides until the remaining area was 50 percent smaller. Just imagine laying the D1's sensor on the center of the 35mm exposed negative and trimming away everything which wasn't covered by the sensor.

With Canon, the "crop factor" for the D30/D60/10D is 1.6x. For the EOS-1D it's 1.3x. For Kodak Professional Series like the DCS-760, etc., the field of view crop is also 1.3x. This simply means that these sensors are larger than those in the D30/D60/10D, etc. The Sigma SD9 has a 1.7x crop factor, so it's sensor is smaller than the Canon D60, etc.

The reduced field of view of these "crop factor" sensors means that the image impressed on the sensor is like a cropped 35mm film negative. But because the entire resolution (all the photo-diodes which detect light) are contained in this smaller than 35mm negative sized plate, the full electronic "resolution" is vested in this smaller frame. So when cropping and enlarging 35mm film, there is a loss of resolution (you throw away the silver halide which was used to capture the areas discarded) but no loss in the digital crop factor sensor since all the photodiodes were used to gather detail in the reduced field of view. The reduced field of view accounts for the magnifying "effect" once the image is viewed or printed at a fixed size relational to the pixel count in the horizontal and vertical axis.

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Old Apr 24, 2003, 8:49 AM   #8
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Very well explained Lyn!
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Old Apr 26, 2003, 3:10 AM   #9
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Quote:
Originally Posted by Lin Evans
Let's see if we can put this into perspective. The use of the term "X" for describing "zoom" is only meaningful when you are given a starting point. It's "NOT" like binoculars or a telescope where the term "power" is frequently used to describe what one can expect in relation to what one sees with bare eyes. With a telescope at 6 power, the expectation is that what is seen through the eyepiece appears six times as large as it would looking through a window with the bare eye.
Ah... now here's where I was getting confused. This is how I thought they were doing it. That makes more sense than just telling you what factor "all the way zoomed in" is from "all the way zoomed out."

Now, I have a Sony Cybershot F707 with 5x optical zoom. I believe the F717 is identical, so I'll assume it is. On the web page they list this:

"Focal Length: 9.7 ~ 48.5mm
35MM Equivalency: 38 ~ 190mm"

So if I had a digital SLR and I wanted what 10x zoom would be on my F707, I'd want a 380mm lens (ultimately--after focal length multipliers)?

Also, I can assume from these numbers that my CCD is 25.5% of the size of 35mm film (or 35mm is 3.92 times bigger, if you prefer)?

Lastly, are lens (or cameras with fixed lens even) made that are 38mm-380mm (i.e., 10x optical zoom)? Or is this uncommon?

Thanks. As you can tell, I know next to nothing about this stuff.
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Old Apr 26, 2003, 3:12 AM   #10
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Quote:
Originally Posted by Lin Evans
The equivalency is based on full wide angle equivelent focal length. For example, a camera with a 38mm wide angle focal length at 5x would equal 5x38 or 190mm. At 4x this same camera would be at a 35mm equivalency of 38mm x 4 or 152mm.
Now that I know what you're talking about... Did you intentionally use the numbers from my camera? Or is this a weird coincidence?
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