Camera Lens Distance Calculation

Every now and then someone would ask: "How far are you from the animal?" Typically the image was shot with a long lens. I would then mutter an estimate of the distance, but rarely was I confident about its accuracy.

 

Question: Can a lens adequately indicate, or provide an close estimate of, the distance of the subject from the camera? For example, if I am framing the face of a large animal with a 500mm lens on a full frame camera and the face fills the frame, what is the animal's distance from where the camera is?

Distance indications on a lens are only approximate, and approach infinity asymptotically. The important thing is the framing. If range is important, than a. time-of-flight laser rangefinder is linear in its determinations. IMO, that's only important if you intend to return home with a trophy other than a photography.

Mary, I ask my self the same question sometimes more out of curiosity than anything else. I am always stuck with a SWAG.

Good hunting.

Every now and then someone would ask: "How far are you from the animal?" Typically the image was shot with a long lens. I would then mutter an estimate of the distance, but rarely was I confident about its accuracy.

 

Question: Can a lens adequately indicate, or provide an close estimate of, the distance of the subject from the camera? For example, if I am framing the face of a large animal with a 500mm lens on a full frame camera and the face fills the frame, what is the animal's distance from where the camera is?

 

If you know the dimension of the face then yes you can. Your 500mm lens focal length is probably very close to 500mm unless you have serious focus breathing at such a distance. The nodal plane of the lens is also difficult to know but at reasonable distance it's not that important.

Ed, I don't think Mary was thinking of using the distance scale on the lens, but the size of the animal's face in the image.

 

Mary, it would be possible to know the distance to the animal in the way you suggest, but only if you were able to get close enough to measure its face. You could, of course, get a range of values.

Here's a link to an online calculator you may find helpful: https://www.scantips.com/lights/subjectdistance.html

It shows that with a standard 24 x 36 mm sensor and a 500 mm lens, the subject distance will be about 21 times the size of a subject that just fills the short dimension of the frame.

A 500 mm lens has a 4.1 degree horizontal angle, a, of view, so tan(a) - size/distance or distance = size x 14, roughly. So a 1 foot diameter "face" filling the frame would be roughly 14 feet away.

 

The distance markings on the lens barrel are crude at best.

 

The portion of this bison head is about 1.5 feet across, shot at around 400mm (5.2degrees of view) or distance = size x tan(5.2) ~ 16 feet which is roughly true.

 

For example, if I am framing the face of a large animal with a 500mm lens on a full frame camera and the face fills the frame, what is the animal's distance from where the camera is?

I was just about to give the example of a bison portrait - Edwin beat me to it. Here's another calculator: Field of View Calculator - Rectilinear and Fisheye lenses - Bob Atkins Photography

A lot more complicated than I thought. :confused:

A lot more complicated than I thought. :confused:

Back to Square one.

It's much easier with a range-finding reticle in the lens, such as for a rifle scope. In that case you can accurately associate the angle subtended by the subject with the focal length, then calculating a very accurate range value. I'm not aware of any photographic lenses or viewfinders that include this feature. It would have to be embedded in the individual lens, rather than the camera's viewfinder, as the value of the reticle's markings will vary with focal length. I do seem to remember that the Hasselblads that went to the moon had some kind of hash marks for reference, though I'm not sure how they were intended to be used.

Hunting or golf rangefinders can be had for as little as $20 online - after a bit of usage, you can get very good at estimating without the device. The number of heads high approach was used by the military in days past, and can work pretty well. Here is another one Link https://lifehacker.com/estimate-distances-with-your-arm-and-this-rule-of-thumb-5351728

Hunting or golf rangefinders can be had for as little as $20 online - after a bit of usage, you can get very good at estimating without the device. The number of heads high approach was used by the military in days past, and can work pretty well. Here is another one Link https://lifehacker.com/estimate-distances-with-your-arm-and-this-rule-of-thumb-5351728

I found a video explaining this "jumping thumb" method (video link). So this method is much simpler to use than depending on the camera. :oops:

Mary I thought you want to estimate the distance after making the shot and have gone home. If you are still at the scene then there are many easy way to do that.

ary I thought you want to estimate the distance after making the shot and have gone home

That too. It is not simple. :(

The correct formula would be

 

d = 2w*tan(a/2)

where w = width of the subject in feet, and a = the subtended angle (estimated from the lens FOV).

 

The estimate is no better than the accuracy you know the width (w) of the subject. For a long range hunter, anything less than 300 yards away is "point blank," i.e., no correction needed.

 

In layman's terms, 16 feet is too d...n close to an adult bison, unless there is a substantial barrier in between.

Edited April 22, 2019 by Ed_Ingold

d = 2w*tan(a/2)

Sure, simple as 1-2-3! :D

The correct formula would be

 

d = 2w*tan(a/2)

where w = width of the subject in feet, and a = the subtended angle (estimated from the lens FOV).

 

The estimate is no better than the accuracy you know the width (w) of the subject. For a long range hunter, anything less than 300 yards away is "point blank," i.e., no correction needed.

 

In layman's terms, 16 feet is too d...n close to an adult bison, unless there is a substantial barrier in between.

 

Yes. thanks for the correction.

 

And yes there was a "substantial barrier" between me and the bison.

And yes there was a "substantial barrier" between me and the bison.

In Yellowstone, the minimum recommended distance from the bison is 25 yards (75 feet). - Or risk interference from the ranger.

Here's an "easy" way to do it - but it requires some preparation beforehand. You need a foot-long ruler and a tape measure - and something that acts as an "always with you reference" (aka your thumb viewed at arm's length). Measure a distance with the tape measure - say 20 or 30 feet and put the ruler there vertically. Now move back to the starting point and look at the ruler with your arm outstretched and compare to the size of your thumb (pick some wrinkle as reference; you may want to move the ruler to a more appropriate distance and you can also use any other item than your thumb for reference). Now you know what a foot-high target at that given distance looks like in relation to that length on your thumb. If the same target was half the distance, it would appear twice as large in relation to the thumb reference and if it was twice as far away, the size would appear half as large. Now all you have to do in the field is guesstimate the size of the animal you are looking at and compare to the known length of your thumb reference. So in the above buffalo example: the animal appears twice as large as your reference length on your thumb (calibrated at a 30 foot distance) and hence is 15 feet away (or as Ed calls it "too darn close"). This gets more and more inaccurate the greater the distance between you and the animal. Edited April 22, 2019 by Dieter Schaefer

In Yellowstone, the minimum recommended distance from the bison is 25 yards (75 feet). - Or risk interference from the ranger.

 

First, that is for an encounter on foot. I didn't specify whether or not on foot.

Second, it was not in Yellowstone or any other NP.

First, that is for an encounter on foot. I didn't specify whether or not on foot.

Second, it was not in Yellowstone or any other NP.

No implication on your distance from the bison at all Edwin. Btw, your bison closeup is in remarkable detail. :)

 

I stated the restriction in Yellowstone for general reference only. Surely you could be on a vehicle, not to mention there was a barrier between you and the bison. Was it a river?

 

Anyhow, now I have confirmed to myself that I won't bother to wonder about estimating subject distance using camera data. ;)

No implication on your distance from the bison at all Edwin. Btw, your bison closeup is in remarkable detail. :)

 

I stated the restriction in Yellowstone for general reference only. Surely you could be on a vehicle, not to mention there was a barrier between you and the bison. Was it a river?

 

Anyhow, now I have confirmed to myself that I won't bother to wonder about estimating subject distance using camera data. ;)

 

Sorry if I came across as testy. I appreciate your comments. Yes vehicle

In the past I periodically thought about this, especially when somebody asked. I remembered my geometry as well as tricks I learned in the military...all too complicated. My response usually was "in the distance".:)

In the past I periodically thought about this, especially when somebody asked. I remembered my geometry as well as tricks I learned in the military...all too complicated. My response usually was "in the distance".:)

I could write a program for my programmable calculator or using Excel macro to calculate the distance with the 3 parameters as input. The actual subject size, The size of the subject in the frame and the lens focal length. However, it's not precise due to the fact that I don't know the nodal planes of the lens or if the lens has any focus breathing when focus closer than infinity.