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Subject: "Distance to subject from subject size in frame." Previous topic | Next topic
dankeny Gold Member Nikonian since 29th May 2006Tue 23-Jul-13 06:43 PM
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"Distance to subject from subject size in frame."


Roland, US
          

Assume you know the height of the subject. How to you get the distance. My logic (?)

tan t = a/b where t = the angle theta between sides a and b, a is height and b is the distance we are solving for.

if the subject is the exact size of the diagonal (Max angle of view) then the distance is given by

b = a / tan t

Lets say the lens is 400mm on the 7100. theta = 4 degrees, tan theta = .07
Assume the subject fills the frame diagonally and is 14 feet (or meters or cubits, come to it)

b = 14 / .07 = 200 feet (or meters or whatever)

if the subject is a fraction of the the frame say 720 px, then the distance increased by the inverse of the fraction. for simplicity the diagonal of the D7100 is about 7200 px. So the distance is:

b = 200 feet * 7200 / 720 = 2000 feet.

If you followed me so far, you may be saying, so what? The what is that it's wrong. And I don't know why.

David

  

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Replies to this topic
Subject Author Message Date ID
Reply message RE: Distance to subject from subject size in frame.
Covey22 Moderator
23rd Jul 2013
1
Reply message RE: Distance to subject from subject size in frame.
dankeny Gold Member
24th Jul 2013
4
Reply message RE: Distance to subject from subject size in frame.
larrycurrlymoe Silver Member
28th Aug 2013
10
Reply message RE: Distance to subject from subject size in frame.
larrycurrlymoe Silver Member
28th Aug 2013
11
Reply message RE: Distance to subject from subject size in frame.
elec164 Silver Member
23rd Jul 2013
2
Reply message RE: Distance to subject from subject size in frame.
MarkM10431 Silver Member
23rd Jul 2013
3
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dankeny Gold Member
24th Jul 2013
7
Reply message RE: Distance to subject from subject size in frame.
gpoole Platinum Member
24th Jul 2013
5
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dankeny Gold Member
24th Jul 2013
6
     Reply message RE: Distance to subject from subject size in frame.
elec164 Silver Member
24th Jul 2013
8
Reply message RE: Distance to subject from subject size in frame.
dankeny Gold Member
24th Jul 2013
9
Reply message RE: Distance to subject from subject size in frame.
agitater Gold Member
28th Aug 2013
12
     Reply message RE: Distance to subject from subject size in frame.
dm1dave Administrator
28th Aug 2013
13
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agitater Gold Member
28th Aug 2013
14
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dm1dave Administrator
28th Aug 2013
15
                    Reply message RE: Distance to subject from subject size in frame.
agitater Gold Member
28th Aug 2013
16

Covey22 Moderator Expert in various fields including aviation photography Awarded for his contributions to the Resources and The Nikonian eZine Charter MemberTue 23-Jul-13 07:49 PM
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#1. "RE: Distance to subject from subject size in frame."
In response to Reply # 0


US
          

My guess - you're calculating for Angle of View, not Field of View, which is why your D is off? Then again, my math is poor. No guarantees here.

Try this instead:

Distance = (Obj H) * (Focal Length) / (Sensor Size Height)

Parameters per your post:

Object 14 feet high
Focal Length 400mm
Since this is a D7100, approximate size of APS-C for Nikon is about 24 x 16 assuming landscape orientation.

D = 14 feet * 400 / 16
D = 5600 / 16
D = 350

Closer?

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Get out of the car.
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dankeny Gold Member Nikonian since 29th May 2006Wed 24-Jul-13 01:04 AM
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#4. "RE: Distance to subject from subject size in frame."
In response to Reply # 1


Roland, US
          

The funny part is, that gives me the right answer, but it cannot be accurate. With that formula, every shot of an object would be the same distance, since its dimensions are fixed. Pixel height rather than actual, but then how to account for actual dimensions?

David

  

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larrycurrlymoe Silver Member Nikonian since 13th Feb 2009Wed 28-Aug-13 02:44 PM
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#10. "RE: Distance to subject from subject size in frame."
In response to Reply # 1


Calgary, CA
          

Although a lover of math at an earlier time in my life, I believe that it would be much nicer if camera/lens manufacturers would just ad the focus distance that the lens reports in the EXIF data:)
$0.02
Turbo

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larrycurrlymoe Silver Member Nikonian since 13th Feb 2009Wed 28-Aug-13 02:50 PM
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#11. "RE: Distance to subject from subject size in frame."
In response to Reply # 1


Calgary, CA
          

I used to love math problems when I was much younger, today I look for a simpler answer to the questions. Why don't the Lens/Camera manufacturers just add the focus distance of the lens in the EXIF data? Certainly the question of DOF comes into play, but the lenses today are pretty smart and accurate. At what distance did the lens focus? Manufacturer's please tell us!
Turbo

larrycurrlymoe: not just a funny moniker, I can't dance either!

turbostrackandtour.blogspot.com

  

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elec164 Silver Member Nikonian since 15th Jan 2009Tue 23-Jul-13 08:55 PM
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#2. "RE: Distance to subject from subject size in frame."
In response to Reply # 0


US
          

David, what makes you think it's wrong?

Using Armando's formula (with correction for your use of the diagonal instead of height) you get about 198 feet.

Then using Dr. Clark's Plate Scale calculations I come up with about 1994 feet with 720 diagonal pixels on the subject.

Pete

Pete

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MarkM10431 Silver Member Nikonian since 15th Apr 2013Tue 23-Jul-13 11:44 PM
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#3. "RE: Distance to subject from subject size in frame."
In response to Reply # 2


jacksonville, US
          

if it has teeth and bites can i hit it with a rock? if I can I need to move away. if not I can take the picture. all the math my poor brain can handle anymore

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dankeny Gold Member Nikonian since 29th May 2006Wed 24-Jul-13 10:58 AM
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#7. "RE: Distance to subject from subject size in frame."
In response to Reply # 2


Roland, US
          

Pete,

I'm still trying to digest Dr Clark's plate scale calculations. I'm hoping to boil this down to a rule of thumb for using a certain lens/camera combination.

David

  

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gpoole Platinum Member Fellow Ribbon awarded for his excellent and frequent contributions and sharing his in-depth knowledge and experience with the community in the Nikonians spirit. Nikonian since 14th Feb 2004Wed 24-Jul-13 01:51 AM
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#5. "RE: Distance to subject from subject size in frame."
In response to Reply # 0
Wed 24-Jul-13 01:54 AM by gpoole

Farmington Hills, US
          

Your geometry is a little off. The problem is that the triangle you are have used is not a right triangle. The right triangle in the frame is from the center of the image to the corner, not corner to corner. The formula is actually:

tan(t/2) = (a/2)/b

Also the angle of view specified is only at infinity focus. For a lens that focuses by extending the lens, the angle of view decreases as the focus distance decreases. Many modern lenses focus by moving internal elements relative to each other instead of by extending the lens. This internal focusing changes the focal length of the lens as focusing distance changes. All this means that the formula above applies only when the lens is focused at infinity.

Gary in SE Michigan, USA. Co-organizer of the Southern Michigan Chapter
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dankeny Gold Member Nikonian since 29th May 2006Wed 24-Jul-13 10:54 AM
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#6. "RE: Distance to subject from subject size in frame."
In response to Reply # 5


Roland, US
          

Thanks, Gary. Revising my geometry. ...

David

  

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elec164 Silver Member Nikonian since 15th Jan 2009Wed 24-Jul-13 03:21 PM
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#8. "RE: Distance to subject from subject size in frame."
In response to Reply # 6


US
          

>Thanks, Gary. Revising my geometry. ...
>

Perhaps this is getting a bit Geeky, but while Gary's formula is mathematically correct, for longer focal lengths (100mm and longer) your formula is considered a reasonable approximation.

If you crunch the numbers of your example using both formulas, and assuming I didn't err,the difference is only about 3 inches or so in the 14 foot coverage.

But Gary does give a pertinent bit of information that the specified focal length and FOV is only accurate at infinity focus. But I would think that at the distance of your example, the deviation would most likely be minimal and would be more of a concern at closer focus distances.

But it does provide a big wrinkle in your attempt at figuring a rule of thumb.

Pete

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dankeny Gold Member Nikonian since 29th May 2006Wed 24-Jul-13 07:14 PM
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#9. "RE: Distance to subject from subject size in frame."
In response to Reply # 0


Roland, US
          

The back story and my conclusion. Walking in Pelican Valley in YNP, several clicks from the nearest road. I saw this:



I took this pic and enlarged enough to confirm it was a grizzly. My son was with me and I showed him what I was looking at. Of course, we started to stalk the little fellow to get a better look. I grew up in Wyoming and I know full well that there is a fine line between stalking and being stalked. I have no question who is the better at that game. I may be a carnivore but I buy my meat in packages.

To make a long story short. This was a close as I could get, without being caught spying.



The first pic he measures 85 pixels, top of hump to feet (best I can tell). The usual estimate is 1 meter. That yields a distance of 1210 meters. Certainly in the ball park as I estimate nearly a mile, and Joe guessed closer to a half. 1200 meters is about 3/4 mile.

The second pic is the rub. He measures 700 pixels (as near as I can tell). That puts him at 146 meters. That's just way off. He was definitely on the same soccer pitch as me, maybe on the same half.

Clearly, the focus is well short of infinity. I can only assume that is the problem. My conclusion is that distance estimates using this method are rough at best, and best when the lens is focused close to infinity.


David

  

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agitater Gold Member Nikonian since 18th Jan 2007Wed 28-Aug-13 08:38 PM
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#12. "RE: Distance to subject from subject size in frame."
In response to Reply # 9


Toronto, CA
          

>The second pic is the rub. He measures 700 pixels (as near as
>I can tell). That puts him at 146 meters. That's just way off.
>He was definitely on the same soccer pitch as me, maybe on the
>same half.

If pixels had a fixed size, it would all be easy to figure out. I think that's where your problem lies. Pixels vary in size depending on display density (usually measured in pixels-per-inch or PPI) and print density (usually measured in DPI). Zoom in or crop or zoom out and any height or distance calculations go way off the rails. Even without zooming or cropping or magnifying, a pixel is not a size measurement but rather is used to describe the amount of data provided on a display or captured in an image.

For example, using ACDSee Pro, CaptureNX2, Lightroom or a number of other products, I can render your image at the exact same physical size (either on a collection of 21" monitors or printed at 16" x 20"), but at various resolutions, i.e., the exact same physical size with more display pixels or with fewer display pixels, or with more dots or fewer dots. You can make all the different versions of the same image look essentially exactly the same when viewed from a distance of one meter or so.

So it follows that anyone trying to use the number of pixels comprising the height of the bear as a factor in a distance calculation (or vice versa) based on either the angle of view at a particular focal length or the field of view at a particular distance would end up completely frustrated. A pixel is not a measurement factor unless you're in a mathematically compensated and tightly closed environment.

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dm1dave Administrator Awarded for high level knowledge and skills in various areas, most notably in Wildlife and Landscape Writer Ribbon awarded for his excellent article contributions to the Nikonians community Nikonian since 12th Sep 2006Wed 28-Aug-13 08:52 PM
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#13. "RE: Distance to subject from subject size in frame."
In response to Reply # 12
Wed 28-Aug-13 09:03 PM by dm1dave

Lowden, US
          

David is measuring his pixels based on the pixel density of the D800 sensor.

Regardless of the resolution of the output device a full frame image from the D800 will be 7360px wide by 4912px high. The only thing that changes with the resolution of the output device is how many of those pixels are displayed per square inch.

Using the fixed pixel density of the sensor – the image is always recorded (FX mode) as 4912 pixels high – out of those 4912 only 700 of those pixels covered the height of the bear.

Dave Summers
Lowden, Iowa
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agitater Gold Member Nikonian since 18th Jan 2007Wed 28-Aug-13 09:13 PM
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#14. "RE: Distance to subject from subject size in frame."
In response to Reply # 13


Toronto, CA
          

>Using the fixed pixel density of the sensor – the image is
>always recorded (FX mode) as 4912 pixels high – out of those
>4912 only 700 of those pixels covered the height of the bear.

Absolutely Dave. More or less. In one shot the bear's height requires 85 pixels of display (according to the OP), in the other it's 700. So the question I always ask is, how big is a pixel?

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dm1dave Administrator Awarded for high level knowledge and skills in various areas, most notably in Wildlife and Landscape Writer Ribbon awarded for his excellent article contributions to the Nikonians community Nikonian since 12th Sep 2006Wed 28-Aug-13 09:33 PM
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#15. "RE: Distance to subject from subject size in frame."
In response to Reply # 14


Lowden, US
          

>> So the question I always ask is, how big is a pixel?

It doesn’t matter how big the pixel is as long as it is measured against a fixed standard. That standard in this case is the number of pixels that the camera can record and the angle of view.

Dave Summers
Lowden, Iowa
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----------------------------------------------------------------------------------


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agitater Gold Member Nikonian since 18th Jan 2007Wed 28-Aug-13 09:36 PM
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#16. "RE: Distance to subject from subject size in frame."
In response to Reply # 15


Toronto, CA
          

It's an interesting exercise.

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