Help on Bahtinov Mask Generator parameters for 18-55mm lens

[sumangeek]

Hi,

I am planning to make my own Bahtinov mask for my canon 18-55mm stock IS lens. I've come across this website (http://astrojargon.n...kGenerator.aspx) which creates templates for the mask. I've experimented with these settings (focal length = 55mm, clear aperture= 55mm/5.6 = 9.82 mm, Edge thickness = 10). Unfortunately the template turns out to be small. I've somewhat "intuitively" chosen the parameters. It would be great if someone explains what focal length and clear aperture actually mean?

Cheers

Suman

[ronin]

I am not sure you could make a mask for a camera lens.

The idea is based on a telescope where the diameter is fixed and there are no internal mechanism to reduce the apparent size. A camera lens is not constructed and doesn't operate the same.

By the time the light has passed through say 10 bits of glass the brightness is reduced and this is "built in" to the f number. What a camera lens states as the f number is the apparent f number to the sensor/film. So I do not think that setting a focal length and an f number would produce anything sensible except perhaps at the minimum focal length and the lowest f number (widest). Even that I suspect would still be an approximation.

Although not 100% sure I do I not think that you can produce anything with a high degree of certainty. If the mask is generated from aperture and focal length then perhaps a simple measure of the diameter of the front lens will suffice (as in ignore whatever the f number is said to be by the camera lens) as maybe the optics of the camera lens will account for this. So generate one using the measured aperture and the selected focal length. I would not be sure that it would work but the diameter should then match the camera lens and I would hope that the focal length would give the correct image.

It will heavily depend on whether the correct inputs are the aperture and focal length or the f number and focal length, on a scope they are much the same but on a camera lens they are not.

[sumangeek]

Hi Ronin,

The mask is also available for DSLRs. But how the aperture and focal parameters are set is something to be seen