# Calculate Distance to a Object in an Image

This is rather specialized, but people do ask how to determine the distance to an object or subject in an image. The DSLR Exif data may tell you Focus Distance, except you should realize that the focus distance reported is often seriously incorrect, especially for zoom lenses. You surely want to verify what actual focus distance is required for this lens to report that distance at that zoomed value.

Otherwise, the calculator here will do this, but you must know a few things about the situation:

• The real size of some Object there in real life, the actual or approximate estimated Width or Height size dimension, in feet or meters. Not necessarily the "subject", but some known object there at same distance. The distance result computed here will be in the same units.

You need the original digital image. Size of Object can't work from a cropped image or a paper print. OK, it can work from a resampled image, if it still the UNCROPPED full frame view (can work using original sensor mm, and the resampled image and object pixel dimensions, if Not cropped).

• The lens focal length that the camera image used.
• The sensor size in the camera, in both mm and pixels.
• The size (pixels) of the actual Object in the image.

If using a simple compact camera, smart phone camera, or camcorder, you may not know all of the numbers. Especially not the precise size of the sensor in mm, especially not if in movie video mode. And this part may be rather confusing for novices without a basic understanding of digital images.

• Focal length: the marked focal length applies to infinity distance, and will be somewhat different than marked if up close. We really don't know focal length for macro work, but this calculation will be as accurate as your data when the actual distance is at least a meter or two (3 to 6 feet or more).

Then simple cameras normally specify their minimum and maximum zoom focal length, but we don't know any other value. These do reset to one default focal length when turned on, and then we can zoom them wider or more telephoto. But often we use the default focal length, which we don't know, so manually zoom them to the widest or narrowest view first. But if any Digital Zoom effects are in play, then that is cropping, and all bets are off.

If it is a still photo image (not a movie file), the focal length should be in the Exif data, which you can see there. If a camcorder, and if you think you can duplicate the first focal length (maybe the default focal length), you can take any still picture with it now, and see that focal length in its Exif data (of the still image).

• Sensor size: Specs for simple cameras rarely tell us sensor size, not in any meaningful way, and crop factor is likely more accurate then. DSLR normally do show detailed specifications.

But seeing a specification that your sensor is 2/3 inch or 1/1.8 inch means nothing in terms of actual sensor size (it is just a way to NOT tell us how tiny the sensor size is). If you cannot come up with accurate sensor size as width x height in mm, then your best bet is to use crop factor, which probably is available.

• Even compacts and phones usually tell us Equivalent focal length (compared to 35 mm film cameras). And then crop factor can compute it for us. For example, crop factor can be determined if focal length is described this way:

Focal Length: 4.5 (W) - 81.0 (T) mm   (35 mm film equivalent: 25-450 mm).

From these numbers, we know this crop factor is 25 mm/4.5 mm or 450 mm/81 mm, either one is 5.55 crop factor (ratio of Equivalent/Real focal length). This is due to sensor size, and since we know 35 mm film size, then this ratio specifies sensor size. One choice below can compute sensor size from this Crop Factor (independent of focal length). Note that 720 or 1080 HD video shots are a different size than still pictures in a 3:2 or 4:3 still camera, but the Crop Factor choice includes this.

• Or (not literally being serious), if you do know focal length accurately, and if you can measure the cameras field of view (the horizontal and vertical dimensions that it sees, at least at six feet distance), then there's a calculator that can compute Sensor Size (Option 8 there).

## Calculate Distance of an Object in an Image

 First select the dimension to be used here: Height   Width Enter One of these two options: Sensor sizeHeight dimension mm Or Crop factor and Aspect Ratio Width:Height 3:2 (DSLR) 4:3 compact, phone 16:9 in 3:2 camera 16:9 HD in 4:3 cam 16:9 HD camcorder Lens Focal Length mm Height size of sensor pixels Height of Object in image pixels Height of real Object feet or meters Distance = Same units

Numbers only.  A NaN result will mean an input is Not A Number. Decimal points are OK.

You can use object size in feet or meters, cubits or whatever... The Distance result will be in the same units.

Example: In this image, the tape on the floor measures 30 feet.

Nikon D800 camera: Sensor 35.9 x 24 mm. 7360x4912 pixels. 60 mm f/2.8 D lens.

The Exif says this Focus distance was 3.76 meters, which is 12.33 feet (when it was actually 30 feet). And this was a 60 mm lens, not even a zoom lens, so the cameras distance report is a real crap shoot (worse than useless).

The door measures 80 inches tall, 6.667 feet (or the height or the width might be reasonably estimated, for a distance estimation). The cropped door is 2724 pixels tall (crop it, then look at image size).
So the calculator input specified 24 mm sensor height, 2724 pixel object height, 4912 pixel sensor height, 60 mm lens, and 6.667 feet estimated real object height.

The tape on the floor measures 30 feet, and the calculator says 30.069 feet (0.23%). That 0.069 foot is 0.83 inches. This distance is computed to the Thin Lens node somewhere in the lens, not really known (but this calculated value here is Not to the focal plane at rear of camera). I guessed the node was at the middle of lens, so that could be an inch error. Still, the accuracy seems very adequate.

The small resampled image copy which is shown here is 450x300 pixels, and it can work too (only because the image is still full frame view, NOT cropped at all).
Then (in this resampled smaller image) the cropped door is 168 pixels tall, sensor height is 300 pixels tall (but still 24 mm in camera), and calculator says 29.777 feet (0.75%). Less precision in a smaller image or object due to less possible cropping accuracy. Still, even this is very near 30 feet.

Note this 168/300 pixels or 2724/4912 pixels is simply computing the size is 56% of the 24 mm height of the camera sensor. Then knowing this height in the camera, and also the real life height, and the focal length distance in camera, it calculates distance to the subject.

Then it is similar triangles, just equal opposite angles, which have equal tangents, which are similar height/distance ratios.
Object height on image sensor (mm)/ focal length = Real Object height / Distance to Object.

The size of the Object image in mm is:
sensor height in mm x Object height in pixels / sensor height in pixels.

The calculator will do all of this, but it needs your accurate numbers.

The math the calculator uses is from this diagram:

If we measure that the object size (height or width) is say 10% of the image pixels, then we know it is also 10% of the sensor mm dimension. Then if we know the size of the sensor in mm, and if we know focal length in mm, then trigonometry can compute the distance to the object (Field of view is computed the same way). If we know the actual real life size dimension of the subject, then we can scale the distance accurately too.

Again, three points.

• The marked focal length applies at infinity, and it will be a little different if close, so the focus should not be too close, at least a meter or two (at least two is better).
• Simple cameras typically do not give specifications about the sensor size in mm (the 1/xx inch number is near meaningless). And the 16:9 video height in 4:3 or 2:3 still cameras will be a shorter partial height too. However crop factor can compute sensor size.
• Camcorder 16:9 simply fits the full view circle from the lens. But other movie modes (compacts, phones, DSLR) have to fit into the 4:3 or 3:2 format that is otherwise imposed. The 16:9 width cannot be wider than the still picture frame size.

To make that point, example from the megapixel calculator for 24 megapixels:

Maximum Image Sizes for 24.0 mp

Aspect ratio 1:1 / 4899 x 4899 pixels = 24.000 mp

Aspect ratio 4:3 / 5657 x 4243 pixels = 24.003 mp

Aspect ratio 3:2 / 6000 x 4000 pixels = 24.000 mp

Aspect ratio 16:9 / 6532 x 3674 pixels = 23.999 mp

16:9 in 4:3 chip: / 5657 x 3182 pixels = 18.001 mp

16:9 in 3:2 chip: / 6000 x 3375 pixels = 20.250 mp

(16:9 video will be resampled to 1920x1080 or 1280x720)