Suppose an objective lens is having numerical aperture of 0.65 and magnification of 45X. What will be its focal length?
You can`t determine focal length from these data. You need to know working distance in addition to magnification.
For simple single lens you can estimate focal length from lens diameter and numerical aperture. But for lens assembly it is not correct because effective aperture is less than input lens diameter.
If this is a microscope objective working at finite conjugates, then the focal length can be computed from the standard tube length. The conventional microscope will create a real image in the tube that the eyepiece will image to (or near) infinity with the exit pupil at the observer's iris. The standard tube length is generally 160mm but some vendors use values as large as 210mm.
If the microscope use infinity corrected objectives, then the objective creates an image at infinity and a fixed lens in the tube creates the real image for the eyepiece. In this case, you will probably need to contact the vendor to get specific first order properties for the objective.
I confirm James Carter's answer. If you have a DIN microscope objective (160mm tube length, not infinity corrected), then the focal length is given by the following:
flange - object distance = 45 mm
flange - image distance = 150 mm
total track = 150 + 45 = 195 mm
effective object distance (s) = 195 / (m + 1), where m = magnification
effective focal length = m * s / (m + 1)
For a 45X objective, this works out to
However, if the objective is infinity corrected, as James says, the focal length depends on the focal length of the tube lens for which it was designed. If the objective has an M25 thread, it was designed for Leica or Nikon, and the tube lens focal length(TLFL) is 200mm. If the thread is RMS (0.8" x 36 TPI), it could be Olympus (TLFL = 180mm) or Zeiss (TLFL = 165mm). Other tube lens focal lengths are possible. Given the TLFL, the objective focal length is:
FL = TLFL / m
HTH
Hello, I would like to follow on this topic with another question:
Can the quantity 'focal lenght' (at least in case of inf-corr. obj.) be regarded equivalent to the radius of spherical surface in the Debye-Richards-Wolf integral? (The DRW theory is used for calculating the focal field of a collimated beam after being focused by an aplanatic system).
Thank you for your comments.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Optics