Lens Magnification Question

[dem]

The only reason I can think of for this is a fairly extreme amount of focus breathing, that is, an effective focal length of about 70mm at closest focus.

+1. Entering 72 mm instead of 105 mm into a calculator gives a magnification of 1:4.

105 mm corresponds to infinity focus. A 30% reduction in focal length at minimum focus distance is quite a lot but possible for a telephoto zoom lens.

[george013]

+1. Entering 72 mm instead of 105 mm into a calculator gives a magnification of 1:4.

105 mm corresponds to infinity focus. A 30% reduction in focal length at minimum focus distance is quite a lot but possible for a telephoto zoom lens.

I did find this one too but not with calculations. Just find a focal length that matches the 450, 105 and the M=0,25.

When using the other formula M=f/(do-f) I get do = 525. Together they're 656.25.

When Quentin wants to check his focal length at 105 and a focus distance of 450 he should measure the angle of view. When that is known the image distance can be calculated based on sensor size and that angle of view.
Using the formula M=(di-f)/f will give for a M=0.25 di=1.25f.

Can you tell me if these formulas using the M-factor are valid from there nodal points? I do think so and if so the focus distance should be corrected with the distance between the focal points.

George

[george013]

M=f/(do-f)=(di-f)/f=0.25

f/(do-f)=0.25 --> 1.25f=0.25do 
(di-f)/f=0.25 --> 1.25f=di         


0.25do=di
450-do=di

0.25do=450-do --> 1.25do=450 --> do=360 and di =90

1.25f=0.25do --> 1.25f=90 --> f=72
1.25f=di     --> 1.25f=90 --> f=72

The trick seemed to be to first calculate the focal length. I used the code block to get rid of variable spacing.
Hopely without faults

George