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:
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
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!)
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:
(I used a grid-lined cutting mat and did all my calcs in metric units)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 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
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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