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Old Jul 9, 2006, 9:03 PM   #1
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I now have an Olympus C2100UZ camera. It has a 10x zoom lens.

Suppose I buy the new Sony A100 DSLR camera with the kit lens (18-70 mm F3.5-5.6).

Now, suppose I use the Olympus to photograph a scene, using the full 10x zoom & horizontal format (i.e. wider than tall). Then, standing on the same spot, I train the Sony on the same scene, also using horizontal format, using the kit lens at its maximum of 70mm. I assume that the Sony kit lens has a wider angle of view, so will include a good deal more of the scene than did the Olympus.

I then crop the Sony picture down to give me a picture that includes only what the Olympus picture includes.

Question - How much - i.e.what proportion - of the left and right sides of Sony picture will I have cropped?

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Old Jul 10, 2006, 10:49 PM   #2
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After pondering my question some more & comparing the specs of the 2 cameras I haven't been able to work out any exact answer (no surprise there).

But I have concluded that standing on the same spot, my old Olympus C2100UZ at full zoom (its lens, by the way is, I understand, the equivalent of 38 - 380 mm) would give me a much better picture of whatever it had recorded (e.g. a house in the distance) than would the A100 with the 18-70mm kit lens zoomed to its 70mm focal length.

If I had the A100 Sony with an 18-200mm lens fully zoomed however I can see the distant house would be recorded better than with the 18-70mm lens - nearly three time bigger than with the kit lens.

But whether it would be better than the Olympus is unclear to me. Maybe its 10 megapixels would make the A100 the winner?
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Old Jul 11, 2006, 12:50 PM   #3
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Good to see that you went toward important thing... focal length.
Focal length is what decides "how much lens magnifies"/how wide is field of view... zoom number is just empty number which doesn't tell anything concretic.

Now because A100 (like any consumer priced DSLR) has smaller sensor than 35mm film effective focal length isn't same as real focal length. In case of A100 this "cropping factor" is ~1.5 meaning that 18-200 lens would become 27-300mm equivalent.
Difference between 300 and 380mm isn't much and A100 has 5x amount of megapixels...

Now when we consider much higher pixel amount of A100, ~10MP we could crop height and width of picture down to 50% of original and still have 2.5MP of data... when using 300mm equal lens that would give field of view similar to 600mm. That would result "2.5MP camera with 600mm lens".
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Old Jul 11, 2006, 2:16 PM   #4
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Thanks. E.T. You've helped me clear my mind. I see how my earlier conclusion was wrong.

Now that you've put your finger on the fact that actual focal length is what governs the size of the image it produces of the distant object, it seems to me that the 18-70 kit lens at full zoom will record the distant house on the sensor exactly the same size as does the Olympus C2100 at full zoom (because the Olympus lens' focal length range is 7-70mm).

However, the A100 will record more of the house's surroundings because the A100 has a somewhat bigger sensor - (23.6 x 15.8mm) whereas with the Olympus' sensor, is said to be one half inch. (Whether that means it's a half inch measured diagonally or on the long side I can't discover).

Assuming I've now got it right, the Olympus would not resolve the distant house better than would the A100 with the kit lens. In fact, after cropping the A100 kit lens picture down to cover only what the Olympus had covered, the number of megapixels of data comprising the cropped picture would not be much different from the Olympus' picture & could even be a bit more- possibly 2.5 megapixels compared to the Olympus' 2.0.

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Old Jul 11, 2006, 2:55 PM   #5
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Herb wrote:
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Now that you've put your finger on the fact that actual focal length is what governs the size of the image it produces of the distant object, it seems to me that the 18-70 kit lens at full zoom will record the distant house on the sensor exactly the same size as does the Olympus C2100 at full zoom (because the Olympus lens' focal length range is 7-70mm).
That's about right... but it doesn't project same picture to sensor but to a thing called as "image circle".
Then smaller sensors just capture smaller part from total image circle which causes increase in effective focal length.


But for knowing how much details show all we need is amount of megapixels and effective/35mm equivalent focal lengths.
Then we just need to remember that doubling focal length from comparison value halves field of view.

Because of "crop factor" kit lens becomes ~27-105mm. After cropping to 2.5MP we would be at 210mm. Cropping further to 2MP (or 1.9MP for exactly same as in C2100UZ) doesn't go much further... cropping field of view to 420mm equal would give just ~0.6MP picture. That should tell enough.
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