Beginner How big is the subject on my taken photo.. help please

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Gez Ogion

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Gerry
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I have a bit of an unusual question on size conversion with photography. I know my sensor size (35.6x23.8mm) .. It's a Sony a9ii full-frame

I know my lens focal (600mm) and it has a max magnifcation ratio of 0.2. And at 600mm it has just under a 5 degree angle of view

I know the size of the bird/animal i am taking the photo of.. and i know roughly.. how far away i am. Is there an easy way to work out the size of the animal on the final photo?

eg.. Goldcrest (9cm long) approx 30ft away.. full frame camera with 600mm focal distance.. how big is the Goldcrest from my location on my photo.

This is something I have always been interested in learning!
 
Focal length and sensor size is sufficient, then you can get either size on the sensor from subject size or vice-versa.

If you’re interested in learning and mathematically minded, this is a fairly succinct explanation for a simplified pinhole camera model (which will work well enough in this case):


Normally you use focal length in pixels and work in pixel units then scale from the known sensor size.

There might be better explanations out there, google “camera matrix” and you’ll find the right sort of thing.


Edit: openCV docs a bit more verbose https://docs.opencv.org/4.x/d9/d0c/group__calib3d.html
 
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If you’ve ever done maths at school etc on projective geometry or equations of planes in 3D space it’s just the same same stuff - think of the cone of light collected by the lens intersected by the subject plane and the sensor plane.

If that makes no sense ignore me and carry on… :)
 
andrewc said:
Re-arrange this formula, but you will need your units to be consistent,
Click to expand...
ok.. not sure if i have got this right ... but here goes,,

real object height = distance in M (10) x object height on sensor in mm .. i have a fullframe 35mm which is 1:1 .. so a 90mm bird would be 90mm on sensor?
that gives me 10M x 90mm = 600 then divide by focal length in mm = 600 = 1.5m ...

So if i have done this right.... a 90mm bird at 10 meters away .. when using a 600mm lens on a 1:1 35mm sensor.. would give me a height of 1.5 meters or 59 inches so the bird would appear to be around 17 times bigger than the original ?
 
So we start removing the scale that formula adds to not fill the frame with the subject:

Focal Length = Sensor size x (Subject Distance/ Subject size)

Then we want subject size so arrange to;

Subject size = subject distance * sensor size / focal length

Let’s use the vertical dimension and all mm for consistent units

Subject size = 10000 * 23.8 / 600
= 390mm

So a 390mm tall subject would fill your frame.

You have 90mm bird, 90/390 =0.231 so it fills about 1/4 the height of your frame, or 0.231 * 23.8 =5.498 mm tall on the sensor.
 
Obviously the size on the final photo depends how big you print it. But it’ll be about 1/4 the height of the print.
 
i have a fullframe 35mm which is 1:1 .. so a 90mm bird would be 90mm on sensor?
Click to expand...

1:1 would refer to magnification and is nothing to do with sensor size.

Historical reasons mean we talk about full frame as a crop factor of 1 and describe other formats related to this. But that’s an accident of history and nothing to do with physics or maths.
 
I'm not going into the maths it's too early and I'm too old, but to throw in another factor a 600mm lens might not acrually be exactly 600mm, and many lens will be a bit different again at different distances (focus breathing) Now I admit they wont make a massive difference, but if the size is important you'd need to test the lens at the same distance and compare it to a known size object.
 
So my understanding is you want to measure the size of the bird in the image? Would it not be just as easy to use the imaging software to do this, and probably just as accurate?
 
I think the question is “how much of my image is bird”?

The other simple way of course is with angle of view, you say 5 degrees, not sure where you get that, it’s about 2.3 vertical, 3.4 horizontal, 4.1 diagonal. So we’ll use vertical

image height = 10,000 * tan(2.3) = 401mm

And again your bird is 90mm so about 1/4 of the image height. Simpler calculation same result.

(Note I’ve simplified the trig as it’s approximately correct for a narrow angle of view. Wider angle divide the angle of view by 2 and multiply the result by 2)

a 600mm lens might not acrually be exactly 600mm, and many lens will be a bit different again at different distances (focus breathing)
Click to expand...

See my links on doing the correct maths, you can account for this by shooting calibration charts at the working distance to get the lens parameters. You can also model the lens distortion too, but it’s likely to irrelevant for a long tele as they tend to have little distortion.
 
To be perfectly honest, I'm not sure Gerry has made his question clear.

Is he trying to use the image to measure the bird's actual size? If so, this page is a good starting point...

en.wikipedia.org

Photogrammetry - Wikipedia

en.wikipedia.org en.wikipedia.org
 
Try FOV calculator

I think your FOV at 30ft will be about 50cm wide and 35cm high. But If I wanted to check this and I already had the lens, I'd photograph a ruler from 30'.
 
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