Question about distance

[ila]

Hi. I'm brand new here, so please be kind. I was out touring Joshua Tree National Park this weekend and took a photo with my Canon 60D, using a 18-200mm lens.

The image below is scaled down by 50 percent so I could upload it here.

In the very center of the image, there is a U shaped dip in the hill. In that dip, there are two people standing up.

How can I tell the approximate distance between where I'm standing on the road and where those two people are?

Thank you, one and all, for any help and education you can provide.
Gratefully,
ila

 

[weepete]

The easiest way would be to find the place in google maps and use the ruler to measure your approximate position to their approximate position. Why do you want to know?

 

[Ysarex]

It's possible but it's going to take a lot of math. Here's your equation: Where D = lens to subject distance, S = linear measure of subject, and V = angle of view of the lens: D = (S/2)/tan(V/2).

In the photo you supplied the taller of the two people is 7 pixels tall. Given your 50% reduction then the taller of the two is 14 pixels tall in your original. We can assume an average human height of 5'11" which will become the linear measure of the subject. To get a value for angle of view we have to begin with the lens focal length you used to take the photo and the pixel dimensions of the photo. Assuming a typical 4000 x 3000 pixel camera and you recorded that 5'11" person in 14 pixels: To get 14 pixels to fill the 4000 pixel width frame we need an increase of 28471%. Assuming you took the photo with an APS class camera and an 18mm lens then to fill the frame with the 5'11" person you would need an 18 x 28571% lens = 5124mm lens. The angle of view then for a 5124mm lens on an APS size sensor is .3 degrees. And that's all the math I'm doing -- got you started.

Joe

 

[Derrel]

Judging by the image, this looks like a wide-angle lens that made the shot. And based on experience, it looks as if there are not "all that many" different depth ranges involved here, so I am going to say the people are only about 250 yards from the camera, and not any farther. This estimate is based on a hunting range estimation distance method, where you break each "zone" into 40-yard chunks and estimate the range of a target.

The lens angle of view and the short focal length is what makes the people in the distance look so,so tiny: their tiny size is NOT based on sheer didstance, but on low lens focal length and low lens magnification.

 

[ila]

Wow! Thank you all for your great responses. I love the different methodologies posed here. I definitely have some thinking to do....

 

[Bill The Lurker]

you need to know the focal length you shot it at.

then you can use similar triangles. the triangle INSIDE the camera from the lens to the sensor, with the projected image of the person on the sensor as the base, is SIMILAR to the triangle on the outside from the lens to the real person with the real person as the bse. means the ratio of base to height is the sme on both triangles

borrow ysarex numbers and look up pixel pitch of a 60d (4.3 microns). your projected image of a person is 14 pixels or 60 microns high in the sensor. that's 0.06mm. assume person is 6 feet high (close enough there's a lot of little errors already here) and assume your focused at infinity,
and you get

focal length in mm . . . distance to person in feet
------------------------ = -------------------------------
. . . . . 0.06 . . . . . . . . . . . . . . . . . . 6

so the distance in feet is roughly 100x the focal length you used in mm. bit less since the people are likely a bit lessthan 6' tall