Hi there,
A recent thread had brought up the 'Red-Headed" stepchild of photography:
the Z - axis
X is horz.; Y is vert.; Z is depth. Read everything you can but noone writes of it, so I just had to do a spreadsheet for it and it shows the problem of "zooming with your feet". The size of the scene you choose is the field and the distance from lens-to-scene is the depth. The optical axis cuts this into 2 right triangles and it's easy to calculate the third side from this. Watch what happens with the PHYSICAL distortion, the change in magnification of the scene as you move closer:
[pre]
6 Field
Depth Distortion%
1 216.228
2 80.278
3 41.421
4 25.000
5 16.619
6 11.803
7 8.797
8 6.800
9 5.409
10 4.403
11 3.652
12 3.078
13 2.628
14 2.270
15 1.980
16 1.743
17 1.545
18 1.379
19 1.239
20 1.119
25 0.717
30 0.499
35 0.367
40 0.281
50 0.180
100 0.045
[/pre]
For this graph, a 6 ft scene at 6 ft = 11.803%. The measures are which ever you choose; mm., cm., in., ft., meters, the proportions are always the same. This is the distortion from the center of the film to the top, not the corners. Most wedding and portrait shots keep this distortion between 3 - 4%, more will make people look fat thru fish-eye, less than 2% will start to flatten features with tele-photo compression.
Calculating the focal length is easy: depth/field = lens/film size. It works for all formats and sizes. This would never be a problem for landscapes but does show up whenever you shorten the distance like; weddings, portraits, interiors, table-top product shots, macro work.
Have fun with it.
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