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Old Feb 18, 2007, 2:22 PM   #1
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I really like superzoom cameras. I like DSLR's, too, but have a limited budget, so my love of DSLR's is not often expressed by a purchase.

The Panasonic FZ20 had a maximum aperture of 2.8 from wide angle all the way to 12X optical zoom. My Fuji S5200 has a max aperture of 3.2 at its full telephoto 10X optical zoom setting. However, most superzooms don't have such wide apertures available at full zoom. I have the Fuji S9100. I think it tops out at f4.9 or so at 10.7 optical zoom. The soon to appear Olympus SP-550 is about the same at its full 18X optical zoom.

What I would like to know are the difficulties of getting these wide apertures at full telephoto. How come Panasonic could do it with the FZ20 but hasn't done it with newer cameras. How come the other camera makers haven't done it either?
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Old Feb 18, 2007, 3:09 PM   #2
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When you zoom in, youreduce the field of view.When you reduce the field of view, you reduce the amount of light available for the exposure.

On a full frame camera, a 50mm lens has a field of view of 46 degrees but a 100mm lens has a field of view of 24 degrees, or about half. So, if they both had the same diameter objective lens,about half the light would get through the 100mm lens as would get through the 50mm lens. That'sa fullf-stop.

When they design a lens, manufactureres can do things to increase how much light gets through at different focal lengths, but it usually requires more moving parts and more optical elements, so the lens design will probably suffer in other respects (sharpness, CA, etc.).

But that's why nobody makes a 500mm f/2.0 lens. They could, but nobody would be able to lift it. Or afford it.

I conceed that this is a GROSS oversimplification of some of the nuances of lens design. If anybody has a different take on this, I'd be pleased to read it.
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Old Feb 18, 2007, 4:34 PM   #3
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On the other hand, some companies (Leica, for one) can work miracles with glass. After all, who else makes an f/1.0 lens?
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Old Feb 18, 2007, 6:15 PM   #4
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The f stop measurement is a ratio of the aperture diamater to the focal length. So, if a 50mm lens has a 25mm aperture, that's a 1:2 aperture to focal length ratio, or f/2.

However, put that lens onto a digicam with a sensor 1/4 the size of 35mm film, and since it's cropped down to 1/4 the size it becomes a 200m f/2 lens. Also, since it's only using the center of the lens, you could use a much smaller lens without losing image information and achieve the same measurements.

However, f stops are also considered by many to be a standard measurement of light input for determining exposure level. So, on a digicam with a smaller sensor but high megapixels, it might achieve an exposure at ISO 100 with an f/2 lens that is more in line with what you should get at f/4, so it's conceivable that a camera maker may decide to relabel the f values to be more in-line with their actual exposure performance.
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Old Feb 18, 2007, 6:38 PM   #5
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f/2 is f/2 no matter what the focal length or sensor size and as Corpsy pointed out is a function of the effective diameter and the focal length. What does change is the depth of field. f/2 at 6mm will be almost front surface to infinity compared to f/2 at 500mm which wil be some razor thin zone.

A constant aperture zoom is more expensive and difficult to produce than a variable aperture zoom which is evident if you compare the prices of the DSLR zooms. As well they tend to be much larger because of the diameter to focal length ratio which must be maintained.

To answer the OP's question, there are three reasons: cost, cost, cost!
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Old Feb 18, 2007, 9:48 PM   #6
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Bob is right, that f2.8 on your digicam is about the equivalent of f11 on a 35mm camera as far as the DOF is concerned. This is why it is so difficult to get the nice blurred background in portraits with digacams.
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Old Feb 19, 2007, 11:49 AM   #7
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"To answer the OP's question, there are three reasons: cost, cost, cost!"

I can understand that it is expensive. However, Panasonic did it with the FZ20. Other cameras, including 12X optical zoom superzooms,had a maximum aperturerange of f2.8 to f3.7 or so. I don't think their sensors were bigger than the sensors in the newest models. The Fuji S6000fd and S9100 are big cameras with bigger sensors. Why are they f2.8 to f4.9? What magic did Panasonic do with the FZ20 that seemingly can't be replicated today? I have the S9100 and like it. I am not expecting miracles. I just would like to know what tricks/techniques the FZ20 used.

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Old Feb 19, 2007, 12:51 PM   #8
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The FZ10, 15 and 20 had a 1/2.5 sensor. When they went to the FZ30 they went to a 1/1.8 sensor and it was no longer practical to have an objective lens large enough to maintain f2.8. I think f3.7 is still better than anyone else has managed with a 1/1.8 sensor and 12X.

The smaller series (current FZ7 &FZ8 )evidently had to make some lens compromises to keep the size small even though they have kept the 1/2.5 sensor.

Why the FZ10 & 20has a better aperture than any other superzoom with a 1/2.5 sensor is probably just a matter of lens design and size of the objective lens.

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Old Feb 19, 2007, 1:06 PM   #9
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Thanks, Slipe. You answered my question.
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Old Feb 19, 2007, 1:08 PM   #10
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There is no magic in making a f=6.0-72mm f/2.8 lens. Unless you call what Leica does magic (and it may well qualify.)

Nobody's doing it anymore because as image sensors get more sensitive (more megapixels), they also get larger. In order to have the same magnification on a larger image sensor, they have to increase the focal length of the lens. And making longer lenses brighter is expensive.

The magic wasn't entirely in the lens of the FZ20, some of the magic was in the small sensor.
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