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Thread started 13 Jan 2008 (Sunday) 09:27

# how many feet =XXmm?

Jan 13, 2008 09:27 |  #1

if i have a sigma 12-24mm, and i am taking a photo at the 12mm side of it...how many feet would i have to step back with a 24-105 to get the same frame?

haha does this make sense?

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Jan 13, 2008 09:50 |  #2

Twice as far as you are with the 12mm FL.

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Jan 13, 2008 10:08 |  #3

Mayby this can help http://photo.net/learn​/making-photographs/lens
If not I have the mathematical formula incase you want to calculate from scratch. Good luck.
Greetings

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Jan 13, 2008 10:14 |  #4

As Wilt said, double the focal length is (as a good approximation) the same as halving the distance. So, if you were taking a photo of a subject 10 feet away at 12mm you would need to be 20 feet away at 24mm (i.e. step back onther 10 feet). If you were taking a photo of the moon, you would need to step back about a quarter of a million miles

Graeme
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Jan 13, 2008 11:07 |  #5

i am looking more for a formula. what if i am using a 17-40 f/4. and what to get the same shot w/ my 70-200?

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Jan 13, 2008 11:14 |  #6

bigcountry wrote in post #4693686
i am looking more for a formula. what if i am using a 17-40 f/4. and what to get the same shot w/ my 70-200?

You'd have to have roughly four times the distance between the object and you, than you had with the it 17mm to it at 70mm.

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Jan 13, 2008 11:27 |  #7

bigcountry wrote in post #4693248
if i have a sigma 12-24mm, and i am taking a photo at the 12mm side of it...how many feet would i have to step back with a 24-105 to get the same frame?

For one thing, you will NEVER get the same image unless you were shooting a two-dimensional subject such as a wall or a sign. Here's why.....

This is part of Photography 101, and is all about controlling perspective in your images.

Many inexperienced photographers choose focal lengths merely to be able to frame a subject from whatever camera position they feel is convenient at the moment. They probably don't even realize that there is a huge composition advantage in finding a better vantage point for the shot. The reasons are that distance between the viewer (or camera) and subject is what changes perspective and a different angle, combined with a perspective change can potentially make a huge difference in the quality of composition in a photo.

When I am trying to be completely "in control" of my images, I will - when possible - choose my camera position based on what it does to the perspective. Then, and only then, I will choose a focal length to fill the camera’s frame with the intended image.

Here's a simple example of how perspective control can work for you:

Let's assume that you are taking a photo of some friends in a scene that has mountains in the background. You stand 20 feet from the people and view the scene. A 50mm lens will let you fill the frame with the group of people and some of the background quite nicely, so you take a shot. Then you realize that the mountains are rather small in the background.

Back up to to 40 feet (twice the distance) from the group of people and view the scene, you will see that the mountains are now larger relative to the people - twice the size they were before, in fact. However, the people are smaller in your viewfinder. You now need a 100mm lens to keep the people the same size as in the first image, but the mountains now appear twice the size that they were in the first shot.

Why is this? It's because the additional twenty feet that you put between yourself and the people is insignificant relative to the fifteen miles between your viewing spot and the mountains.

When you are closer to subjects, perspective still comes into play. If you shoot a portrait from a location very close to the subject, the nearest objects (a nose, for example) will be larger relative to more distant objects (such as an ear) than they would appear from a greater distance. That is why experienced portrait photographers like to use a little more distance - and thus a little longer lens – than some beginning photographers would choose when shooting conventional portraits. The subject will usually be happier with the perspective achieved by the greater distance.

***************
Reinforcing what others have said (your "formula"): Double the distance, double the focal length to keep the primary subject the same size in the image.

Skip Douglas
A few cameras and over 50 years behind them .....
..... but still learning all the time.

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Jan 13, 2008 11:48 |  #8

bigcountry wrote in post #4693686
i am looking more for a formula. what if i am using a 17-40 f/4. and what to get the same shot w/ my 70-200?

Let's state this so you do not have to keep asking the same questions for different combinations of focal length comparisons...

Shooting distances (with same subject area in the frame) are always proportional

12/24 = 1/2

17/70 = 1/4.1

as Skip's message states, this is to keep the same amount of subject area at the plane of the subject. But , as he states, the 'perspective' is different for different focal lengths (i.e. the relationship of objects in front or behind the subject) because the camera position is the different to retain same subject area in the frame.

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Jan 13, 2008 12:48 |  #9

Wilt wrote in post #4693882
Let's state this so you do not have to keep asking the same questions for different combinations of focal length comparisons...

Let me ask the question this way...
now if i have a canon 300mm 2.8 IS. and a canon 15mm fisheye, how far back would i have to go get the equal distance?

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Jan 13, 2008 12:56 |  #10

bigcountry wrote in post #4694189
Let me ask the question this way...
now if i have a canon 300mm 2.8 IS. and a canon 15mm fisheye, how far back would i have to go get the equal distance?

What John McEnroe was famous for saying...

Dave
Image editing OK

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Jan 13, 2008 12:59 |  #11

bigcountry wrote in post #4694189
Let me ask the question this way...
now if i have a canon 300mm 2.8 IS. and a canon 15mm fisheye, how far back would i have to go get the equal distance?

You have to step back outside the curvature of the local space/time continuum. Tricky.

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Jan 13, 2008 21:37 |  #12

bigcountry wrote in post #4694189
Let me ask the question this way...
now if i have a canon 300mm 2.8 IS. and a canon 15mm fisheye, how far back would i have to go get the equal distance?

(300mm/15mm)*n = the same framing,
where n is the distance you are standing when using the shorter lens.

It is the proportional distance. 300mm is 20 times the 15mm FL, so you stand 20 times as far!

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Jan 13, 2008 22:03 |  #13

bigcountry wrote in post #4693248
if i have a sigma 12-24mm, and i am taking a photo at the 12mm side of it...how many feet would i have to step back with a 24-105 to get the same frame?

haha does this make sense?

Jurgen
50D~700D~EOS M~G11~S95~GoPro Hero4 Silver
http://www.pbase.com/j​urgentreue
The Title Fairy,, off with her head!!

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Jan 13, 2008 22:15 |  #14

Thanks for the explanation Skip. I cringed after reading the post.

5DMKII, Rebel xti, 24-105 f/4L, Canon 70-200 f/4L, Canon 17-40.

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Jan 13, 2008 22:50 |  #15

ok, let me ask you this.

if i have the canon 35mm 1.4 L, and i am shooting away from my subject at five feet at f/1.8, how many feet back and at what apeture and focal length would i have to go w/ a 70-200 2.8 IS?
Thanks!

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how many feet =XXmm?
AAA
 x 1600 y 1600

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