Focal length question - Why 420mm lens becomes 3717mm on 20D?


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Feb 25, 2006
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The conversion was based on an incorrect CCD width. The CCD width of a Nikon DX sensor for example is 23.7mm. And with an sensor aspect ratio of 3:2, the lens conversion factor for Nikon DSLR is approximately 1.5 times.

Canon's APS-C size sensor on 20D is approximately the same size as Nikon's DX sized sensor, and is much larger than a sensor with a 4.07mm width.

A sensor with a width of 4.07mm with a 4:3 aspect ratio would results in a lens conversion factor of around 8.5. Hence 420mm x 8.5 is approximately 3700mm. This is obviously not correct for the Canon 20D.
 

lsisaxon

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Pynch said:
The conversion was based on an incorrect CCD width. The CCD width of a Nikon DX sensor for example is 23.7mm. And with an sensor aspect ratio of 3:2, the lens conversion factor for Nikon DSLR is approximately 1.5 times.

Canon's APS-C size sensor on 20D is approximately the same size as Nikon's DX sized sensor, and is much larger than a sensor with a 4.07mm width.

A sensor with a width of 4.07mm with a 4:3 aspect ratio would results in a lens conversion factor of around 8.5. Hence 420mm x 8.5 is approximately 3700mm. This is obviously not correct for the Canon 20D.
I think is because the image was cropped to 634x434. Since the sensor is 3504x2336 over an area of 22.5x15.0mm, the effective equivalent focal length of that shot would be 36mm/22.5 mm x 3504px/634px = 8.843. Multiply that by the actual focal length of 420 -> approx 3714mm. So he's using only a small portion of the 8mp sensor.
 

michhy

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lsisaxon said:
I think is because the image was cropped to 634x434. Since the sensor is 3504x2336 over an area of 22.5x15.0mm, the effective equivalent focal length of that shot would be 36mm/22.5 mm x 3504px/634px = 8.843. Multiply that by the actual focal length of 420 -> approx 3714mm. So he's using only a small portion of the 8mp sensor.
Simply put, it is same as taking a 8mp photo with the lens at full telephoto, then crop only a small part of it.... Pretty dumb to say he got such a long focal length actually :sweat: So lets crop all our photos to 1mp, then we also can say our focal lengths are 10'000mm :bsmilie:
 

Feb 25, 2006
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#5
lsisaxon said:
I think is because the image was cropped to 634x434. Since the sensor is 3504x2336 over an area of 22.5x15.0mm, the effective equivalent focal length of that shot would be 36mm/22.5 mm x 3504px/634px = 8.843. Multiply that by the actual focal length of 420 -> approx 3714mm. So he's using only a small portion of the 8mp sensor.
Yes, that is correct.

However, if you look at photo 1/140, or for that matter, the numerous photo from the first one, you will see that the picture size is 434x634, which is the same as picture 140/140, but the sensor width is now 30.84mm instead. One of the reason for this could be due to the picture orientation, but this alone cannot account for the large difference. Hence, the sensor width as stated is probably incorrectly extracted.

If the 20D can produce such resolution from a small crop, effectively less than 0.3 mp, it is really impressive.

Nevertheless all the bird pictures are very nice.
 

lsisaxon

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Nov 29, 2004
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#6
Pynch said:
Yes, that is correct.

However, if you look at photo 1/140, or for that matter, the numerous photo from the first one, you will see that the picture size is 434x634, which is the same as picture 140/140, but the sensor width is now 30.84mm instead. One of the reason for this could be due to the picture orientation, but this alone cannot account for the large difference. Hence, the sensor width as stated is probably incorrectly extracted.

If the 20D can produce such resolution from a small crop, effectively less than 0.3 mp, it is really impressive.

Nevertheless all the bird pictures are very nice.
I think it was heavily processed..
 

lsisaxon

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Nov 29, 2004
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#7
michhy said:
Simply put, it is same as taking a 8mp photo with the lens at full telephoto, then crop only a small part of it.... Pretty dumb to say he got such a long focal length actually :sweat: So lets crop all our photos to 1mp, then we also can say our focal lengths are 10'000mm :bsmilie:
6mp crop to 1mp is only 2.45x. If the original crop factor is 1.5, you will get effective crop factor of only 3.67x. To get effective 10000mm lens, you will still need a 2725mm telescope.
 

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