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Old May 13, 2009, 8:41 PM   #1
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This will probably be elementary school math for most of you photo buffs but here it is. I have a Kodak P850 camera that goes from 35 mm equivalency of 36 to 432. It is a 5 megapixal camera. I know that I can crop some but lets just say the crop only brought the equivalent megapixal down to 4.5. I am tempted to get a camera with 28 to 280 (panasonic tz5) mm equivalency which is a 9 megapixal camera. If I cropped enough that the result was the equivalent to a 4.5 megapixal camera (half of 9) how much virtual zoom would the pictures look like in the TZ5. The reason for the question is that I really like the wide angle ability of the tz5 (tz7 too costly for me) but I know I will miss the zoom ability of the P850 but want to know basically how much zoom I will be losing. Thanks.

Bob
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Old May 13, 2009, 9:16 PM   #2
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It makes my brain hurt thinking about it. ;-)

If you crop a photo to make it look like you used twice as much optical zoom, you end up with 1/4 the original pixels.

So, if you cropped it enough so that you ended up with 1/2 the original pixels, you'd probably end up with it looking about like you were using somewhere around a 420mm setting instead of a 280mm setting.

Here's a page with some formulas that might help you compare models:

http://www.digicamhistory.com/Figure%20of%20Merit.html
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Old May 13, 2009, 10:09 PM   #3
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Thanks Jim, that answered my question. Much appreciated. Also, thanks for the link.

Bob
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Old May 13, 2009, 10:24 PM   #4
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1/2 the area means the linear measure of each side times the square root of 2, to word it poorly...

So, your proposed "digital zoom" of cropping to a roughly 4.5 mp area of a 9mp image would be a "magnification" of 1.4x. This makes the 280mm equivalent become a roughly 390mm equivalent, not significantly different than the 430mm equivalent you are used to.
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Old May 14, 2009, 9:38 AM   #5
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Quote:
Originally Posted by dwig View Post
1/2 the area means the linear measure of each side times the square root of 2,...
dwig has it right. In general the "digital zoom" ability with a higher pixel count camera goes as the square root of the pixel ratio. In Bob's example the ratio is two so it is the square root of two or 1.4142136... If the pixel ratio was four (e.g., 3Mp to 12Mp) it would be the square root of four or two.
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Old May 14, 2009, 9:47 AM   #6
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Thanks guys. Please remind me not to answer math related questions without checking formulas. :-) I simply multiplied 280mm by 1.5x and assumed that would get me close to what you'd have after cropping for half the original pixels.
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Old May 14, 2009, 4:53 PM   #7
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Quote:
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Thanks guys. Please remind me not to answer math related questions without checking formulas. :-) I simply multiplied 280mm by 1.5x and assumed that would get me close to what you'd have after cropping for half the original pixels.
That is exactly what I would do if I wasn't near a calculator, and if I felt a bit fussy I'd knock down the result by five percent.

For this kind of thing there is no reason to be very exact about the result. However knowing the exact answer can help figuring things out. In this case, the difference between measuring one side of a rectangle vs. measuring the area. Knowing that will mean being able to do the same calculation for any pixel ratio.
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Old May 14, 2009, 6:19 PM   #8
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....I did not know that !!!!!

I wrote mainframe computer code in 12 different languages and AIX/UNIX in 3 different shells........ I understood/stand all of them, same with SQL and a couple other dbases but that math question and the answers.......well I am in AWE.

None of that makes sense to me but it is interesting to read your varied answers.
__________
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