Originally posted by Canada_Rockies Hyper focal distance is the distance at which an enlargement viewed at normal distance appears acceptably sharp. In the very old film days, the enlargement size that was considered as standard for this was 8 x 10 inches. (Yeah, I know it doesn't fit 3:2, but it's perfect for a 4x5).

Therefore, the multiplication is correct, because the image must be enlarged 1.5X as much from the digital sensor as from the full frame sensor. This will magnify the "circle of confusion" half as much again, in the print, making the print less sharp.

Did some quick math.

Lets use some numbers, since 1.5 crop factor seems to be the number we're trying to compare with.

Conditions:

- Lets say the coc for our APS-C is 0.02. Using our crop factor, our coc for full frame is 0.03 (0.02x1.5).

- We will be using a 50mm lens at f/16.

Question:

- We are curious if using the focal length equivalents, would give us the same result as using the proper coc value.

Example 1: 50mm on Full Frame coc.

[(50*50) / (16 x 0.03)] + 50 = 5258mm

Example 2: 75mm on full frame coc (50mm equiv on dSLR???)

[(75*75) / (16 x 0.03)] + 50 = 11768.75mm

Example 3: 50mm on cropped frame coc.

[(50*50) / (16 x 0.02)] + 50 = 7862.5mm

Example 2 and Example 3 shows very different results.

However, Ex.1 * 1.5 = Ex. 3. So you can probably multiply the final result of your full frame calculations by 1.5 and get consistent results, but you should not be multiplying your focal lengths by 1.5

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Last edited by AVANT; 01-14-2008 at 12:29 PM.
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