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matoshi
9th of June 2010 (Wed), 18:12
Hello all,
just wondering if there are some general rule of thumb measurement for focal length and footstp. for example. 1mm = 1 footstep away?

thanks guys..

SkipD
9th of June 2010 (Wed), 18:15
Hello all,
just wondering if there are some general rule of thumb measurement for focal length and footstp. for example. 1mm = 1 footstep away?

thanks guys..No.

Please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance? (http://photography-on-the.net/forum/showthread.php?t=672913).

philwillmedia
9th of June 2010 (Wed), 18:18
Hello all,
just wondering if there are some general rule of thumb measurement for focal length and footstp. for example. 1mm = 1 footstep away?

thanks guys..

How big is a footstep

Wilt
9th of June 2010 (Wed), 19:49
((subject distancein footsteps + n footstep toward subject) : subject distancein footsteps) = FL(after): FL (before)

So if subject distance = 10 footsteps, and you take one addition footstep toward subject, and you start with 100mm lens, then...

(10+1):10 = FL after : 100mm, so 110mm = FL after.

IOW, in this example, starting with 100mm lens at 10 footsteps away, then moving forward one footstep will change subject size the same as if you stood at original position and mounted a 110mm lens.

The above 'equation' totally ignores the issue that there is a change of perspective (relationship of subject to the surroundings) if you moved camera position!

matoshi
9th of June 2010 (Wed), 19:53
Wow...thank you.
Now i really need a calculator to figure this out....
:)

((subject distance in footsteps + n footstep toward subject) : subject distance in footsteps) = FL(after): FL (before)

So if subject distance = 10 footsteps, and you take one addition footstep toward subject, and you start with 100mm lens, then...

(10+1):10 = FL after : 100mm, so 110mm = FL after.

IOW, in this example, starting with 100mm lens at 10 footsteps away, then moving forward one footstep will change subject size the same as if you stood at original position and mounted a 110mm lens.

The above 'equation' totally ignores the issue that there is a change of perspective (relationship of subject to the surroundings) if you moved camera position!

philwillmedia
9th of June 2010 (Wed), 19:57
The above 'equation' totally ignores the issue that there is a change of perspective (relationship of subject to the surroundings) if you moved camera position!

Yes, but whilst the focal length remains constant, the size of your footstep is probably different to mine and everyone elses.

Wilt
9th of June 2010 (Wed), 20:05
Wow...thank you.
Now i really need a calculator to figure this out....
:)

No, simple proportionality!
If you started 100' away with a 60mm lens, moving yourself to 50' away is like mounting a 120mm lens

to 50': from 100' = from 60mm: to 120mm

If you started 75' away with a 30mm lens, moving yourself to 112.5' away is like mounting a 20mm lens

to 112.5': from 75' = from 30mm: to 20mm

Wilt
9th of June 2010 (Wed), 20:08
Yes, but whilst the focal length remains constant, the size of your footstep is probably different to mine and everyone elses.

But the proportionality applies, regardless if you are a Lilliputian or a Giant of Brobdingnag

matoshi
9th of June 2010 (Wed), 21:26
No, simple proportionality!
If you started 100' away with a 60mm lens, moving yourself to 50' away is like mounting a 120mm lens

to 50': from 100' = from 60mm: to 120mm

If you started 75' away with a 30mm lens, moving yourself to 112.5' away is like mounting a 20mm lens

to 112.5': from 75' = from 30mm: to 20mm

Thank you. Now it make mroe sense to me....

Thalagyrt
9th of June 2010 (Wed), 22:26
Yes, but whilst the focal length remains constant, the size of your footstep is probably different to mine and everyone elses.

Wilt already said it's still valid but didn't fully clarify why it's still valid. The units (footsteps, meters, feet, miles, astronomical units, distance from the earth to the sun, height of my mom, distance you can throw an iPhone 4, arm span of a lawn gnome) cancel out and leave only focal length when the calculation is performed. You can even do this with different units of course, so long as you convert to one standard base unit.

d = distance
a = FL after
b = FL before

(d+v)/d=a/b

((d+v)/d)b=a

10 meters, step forward 1 meter, original focal length 100mm:

((10m+1m)/10m)100mm = (11m/10m)100mm = (1.1)100mm = 110mm.

SkipD
10th of June 2010 (Thu), 03:58
Wow...thank you.
Now i really need a calculator to figure this out....
:)While the calculations shown in several posts above allow you to choose a focal length that will keep a "primary subject" framed roughly the same at different distances, the image will NEVER be the same if the camera is used at different distances. The perspective (relationship in sizes between elements in the scene that are at different distances from the camera) changes whenever you change the camera's position.

As I mentioned in a previous post, please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance? (http://photography-on-the.net/forum/showthread.php?t=672913). You may just learn a lot that will help you improve your photo composition.

RichSoansPhotos
10th of June 2010 (Thu), 05:53
No.

Please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance? (http://photography-on-the.net/forum/showthread.php?t=672913).

Something I have been looking for, made my day, hopefully I will get a grip on it soon:lol:

Thalagyrt
10th of June 2010 (Thu), 11:42
While the calculations shown in several posts above allow you to choose a focal length that will keep a "primary subject" framed roughly the same at different distances, the image will NEVER be the same if the camera is used at different distances. The perspective (relationship in sizes between elements in the scene that are at different distances from the camera) changes whenever you change the camera's position.

As I mentioned in a previous post, please read our "sticky" (now found in the General Photography Talk forum) tutorial titled Perspective Control in Images - Focal Length or Distance? (http://photography-on-the.net/forum/showthread.php?t=672913). You may just learn a lot that will help you improve your photo composition.

Yep, absolutely correct of course. That calculation only applies for the exact plane of focus - anywhere else and you start having to apply various non-linear algorithms that explain the mathematics behind the lovely thing called perspective... I don't think that math lesson is short enough for this thread though. :lol: