focal length

srmdnadem wrote:

Hi Mester...

"Mister" or "Master"? I like "Master" better.

...how can I get a specific focal length from this lens? For example, how can I get focal length=40 mm or 50 mm? please

the answer will be

focul length will be 27-88mm (35 mm equivalent). So , if I do not move the lens to enlarge the view. Is that mean the focal length will be 27 mm or not, please?

that is my enquiry

I'm guessing you want to know how do you get a 40mm or 50mm *FF equivalent* focal length with a 17-55mm 1.6x lens (27-88mm FF equivalent).

The way I'd do it is move and/or zoom to get the perspective/framing combination I wanted. However, if you are curious what the nominal focal lengths would be to get the FF equivalents, multiply the actual focal length by 1.6 to get the FF equivalent or divide the FF equivalent by 1.6 to get the actual focal length.

For example 25mm on 1.6x is equivalent to 25mm x 1.6 = 40mm on FF. 40mm on FF is equivalent to 40mm / 1.6 = 25mm on 1.6x.

Below is some additional related information which you may find helpful.  If not, don't read it. 

The same applies to the relative aperture with regards to DOF, light gathering, and diffraction.  For example, f/2.8 on 1.6x is equivalent to f/2.8 x 1.6 = f/4.5 on FF.

Putting it all together, we see that 25mm f/2.8 1/100 ISO 400 on 1.6x is equivalent to 40mm f/4.5 1/100 ISO 1000 on FF where by "equivalent to", I mean all will have the same perspective (if taken from the same position), all will have the same framing, all will have the same DOF, all will have the same diffraction (not that it matters at these wide apertures), the same amount of light will be projected on the sensor (resulting in the same noise), and all will have the same motion blur.

Hope that helps!