Calculate captured image area size on macro

I

inspector-71

Suspended / Banned
Messages
17
Name
George
Edit My Images
Yes
Using a cannon f2.8 100mm at 0.31m from the object on a 450d (crop), how can I calculate the area of the part of the object that is captured?

Background: I am trying to photograph defect in a fabric using this set up and would like to use the image to count the number of physical defects and then estimate the defect/m2

Thanks for any help.
 
Is 31cm the 1:1 distance for the lens?
If so the area captured is the sensor size - 22.2 × 14.8 mm
 
Last edited by a moderator:
yes. 31cm is the 1:1.

Thanks for the info.

How would I factor in increasing the distance up to 0.49 (1:3) and then get the size?
 
Last edited:
The simplest 'general' method would be to set up the shot using a tripod, then take 2 shots - the one you want, and one with a ruler lying on top of the fabric - looking at the second shot you can see the exact size the image represents, no need to worry about calculations of magnification, etc.
 
Is it as simple as...

1:1 is equal to the sensor size so 22.2 × 14.8 mm (thanks rapscallion)
so at 1:2, double the area is captured by the sensor so 44.4 x 29.6 mm
 
inspector-71 said:
Is it as simple as...

1:1 is equal to the sensor size so 22.2 × 14.8 mm (thanks rapscallion)
so at 1:2, double the area is captured by the sensor so 44.4 x 29.6 mm
Click to expand...

Yes, though you may find just reading the scale on the lens is not 100% accurate. Won't be far out, depends how critical you need to be.

Just do the ruler thing, remembering to read off an actual image and not just in the viewfinder, which shows slightly less than is recorded.
 
Thanks. i have fired a couple of test shots using a photography scale and compared to the ratio marks on the focus ring. The calculation is close enough to the image with the ruler for my needs

31cm to object 1:1 = 22.2 x 14.8mm
35cm to object 1:1.5 = 33.3 x 22.2 mm
39cm to object 1:2 = 44.4 x 29.6 mm
49cm to object 1:3 = 66.6 x 44.4 mm

(posted for reference)

Thanks all for your help.
 
Lunch time so have some time on my hands. Dug out pen and pad and thoroughly confused myself when trying to calculate the area captured via trig.

Can anybody show the calculation from basic trig? I used the angle of view calculator which gave fov angles of 12.669 by 8.465 degrees.
 
Rapscallion said:
Lunch time so have some time on my hands. Dug out pen and pad and thoroughly confused myself when trying to calculate the area captured via trig.

Can anybody show the calculation from basic trig? I used the angle of view calculator which gave fov angles of 12.669 by 8.465 degrees.
Click to expand...

You'll fry your brain with that.

Internal focusing lenses change focal length at close range, and sometimes distances are quoted from the focal plane (lens focus scales are) or from the front of the lens (depth of field tables).

But the ruler never lies.
 
You must log in or register to reply here.
Back
Top