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Estimating distance of subject (1 Viewer)

andyotter

Well-known member
I've been digiscoping for about four months now and am often interested in approximately how far away the birds I'm snapping are.

I read bf member gillean55's method (relate num of focus turns to a series of measured distances), but that didn't fit with my style of working. I considered getting a cheapish s/h laser rangefinder, but deciced lugging around another bit of equipment just to satisfy my nerdiness is going a bit far.

So I settled on using the bird's length along with EXIF data to calculate an estimate and wandered off to a nearby car park to collect some data.
My approach was nothing new; it was sketched out here back in 2005:
Take a six inch ruler and put it at a measured distance, say 40 yards. Look through the scope and see how much of the field of view it fills. Make a note of this. Repeat with other distances. Say the ruler spans half the field of view at 40 yards. You then know that an object which is roughly six inches long and spans half the field of view is 40 yards away. As you say, this method does depend on knowing the rough size of the object you are looking at - not too difficult with birds.
Cheers, Pete.
(I used a grid-lined cutting mat and did all my calcs in metric units)

I tested my 2 digiscoping EPs (24x & 40x) at two distances (50m & 100m), taking shots at 4 camera focal lengths. Sticking all the data into a spreadsheet to find relationships, I was pleased to find (as suspected) that results were proportionally related, allowing me to calculate a constant.
namely:

(mag . fl . realW)/d = constant

where
mag = ep magnification,
fl = focal length as read from EXIF data
frameW = real width of field of view (as est. using 'ruler' in data collection or bird when scoping)
d = distance to subject​

With my Nikon FSIII ED60 + P5100 camera setup this constant is approximately* 7.5 so, rearranging the formula:

(mag . fl . realW)/7.5 = d
*I was pleased to find my data fell into a 5% band either side of 7.4 which is fine for the range of accuracy I'd want in this context

so with my 40x EP on, a 30cm bird which takes up about a 1/4 of the width at full zoom (fl=26.3) gives the calculation:

(40 x 26.3 x 1.2m)/ 7.5 = 1200/7.5(approx) = 160m (approx)

I'm very pleased with this approach as I can concentrate on the watching and photographing in the field and leave the calculation for particular shots at home.

Of course, it's easier for me with fixed EP's but I think I'd still use it if I had a zoom; I'd just take an extra (quick & dirty) 'reference' shot at one end of the zoom and use that to calculate the distance.

I hope this is useful to somebody.
(My apologies if I just taught your grandmother how to suck eggs!)
 
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I tend to use google maps. I've made various lengths of cardboard and marked off the scale depending on the zoom level used on the map. When you zoom the map in and out, the scale automatically changes, so it's easy to make different measuring scales for different zoom amounts. Out in the field I make a note of where I was stood and where the target was. Using google maps in the satellite mode you can generally pick out key landmark features right down to particular trees etc. Then it's just a case of placing my scale on the screen and measuring off the distance. If it's a place I go to regularly such as a lake or somewhere like that then I already have a list of known distances such as the length/width of the lake and I can make a good guess while out in the field.

Paul.
 
I tend to use google maps. I've made various lengths of cardboard and marked off the scale depending on the zoom level used on the map. When you zoom the map in and out, the scale automatically changes, so it's easy to make different measuring scales for different zoom amounts. Out in the field I make a note of where I was stood and where the target was. Using google maps in the satellite mode you can generally pick out key landmark features right down to particular trees etc. Then it's just a case of placing my scale on the screen and measuring off the distance. If it's a place I go to regularly such as a lake or somewhere like that then I already have a list of known distances such as the length/width of the lake and I can make a good guess while out in the field.

Paul.

So do I, Paul; I find the 50m scale quite useful. I have out sat images of our regular haunts pasted into a file on the Mac with an isometric(triangular) grid overlaid which makes it easy to make rough estimates of distances from any point to point.
When I first did this, I was gobsmacked at how far away the birds were, now like you I know that island is 80m away and that willow 150m.

I still occasionally use sat images as a cross-check, when I can't quite believe what the figures are telling me - so far it's always been my sense of estimation that's been out ...
 
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There is an on-screen distance measuring tool available in Google Maps:

In the standard interface, bottom left of the screen above the copyright notice, click on "Maps Labs," then Enable the "Distance Measurement Tool". Click "Save changes".

Once enabled you will get a little icon next to the map scale that looks like a ruler. Click it to start the measuring tool. Then click Point A, then click again for Point B, and voilà!

You can also zoom in and out, as well as grab-and-pan before clicking the second point (if you want to stay closely zoomed-in), and you can manually move the markers once placed to fine tune them.

This is how I started estimating distances for my photos, since as someone else said, as long as the satellite photos are reasonably current, you can pick-out individual trees if need be. Given that Google Maps driving and cycling estimates are very close, I figure this is accurate enough for my purposes.
 

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