Total focal length and magnification

Question

I would like to realize an image of the cantilever. The dimensions are (450x50) microns. I would like to use a self-constructed telephoto lens. This consists of two lenses. The image magnification can be 4 to 8. The magnification of the image is nothing more than the ratio between the two focal lengths of the two lenses. E.g. for f1= 320mm f2=40mm the total f= 35.5mm and the imaging magnification is 8 . Since typical telephoto lens focal lengths are over 50mm, I would like to ask if the total focal length matters? I.e. for f1= 800mm and f2=100mm the total focal length is f=88.89 mm but the image magnification remains the same.

Answer 1

The longer the focal length of your lens, the farther away the object and image planes will be. For a given aperture this will reduce the amount of light that your lens can gather. Since large lens systems are considerably more expensive than small ones, one would ideally image a small object with a smaller lens closer to that object. The technical way to specify the "brightness" of a lens system is the Fstop ratio, i.e. the ratio of the focal length divided by the aperture of the lens. Lenses with large Fstop are easier to manufacture but produce darker images.

The second consideration is depth of field. Only objects that are located exactly on the object plane will appear sharp on the image plane. Out of plane objects will appear blurred and that blur will increase very strongly in bright systems with small Fstop ratios. In order to chose the "correct" focal length and Fstop, you would also have to know the amount of blur you are willing to accept for parts of your cantilever that are above or below the image plane and specify the required image brightness. In practice the lens design will be a compromise between these quantities.