The lens equation and the reality

We bought a new camera for our lab, which is going to be used for a bright field imaging microscope and especially bead tracking in two dimensions. We've got our microscope built up and the light beam comes out at the bottom and is already focused by a relay lens.

Until now, I haven't heard about relay lenses at all - our experimental optics lecture was covering a lot of different scenarios, but then the real world says hi and everything is a bit more difficult: Our microscope gives out a parallel light beam, which is focused by the relay lens and you could mount a camera directly after the relay lens. There would be an image at this point and if you moved further away the image would get less sharp.

However, the image we get so far (about 1.5 cm) is too large for our camera chip (it's so small! 6*5 mm^2!), so we are going send the image through another lens and get an overall smaller image, so that the image size will roughly fit the chip size.

Back in the optics lecture, we would use 1/b+ 1/g = 1/f and we would know the focal length we would like to have for a certain object and image distance. Looking up the possible focal lengths of lenses is destroying this illusion of getting any focal length you want! The smallest one we could buy for a certain diameter has a focal length of f = 3.8 cm. Yay. So b and g become larger than we would like to have them, so you need a mirror etc. And the table is finite! It's not a simple paper sheet on which you draw your optical axis... In addition to that, if the lenses and mirrors are not aligned as well, you'd get a tilted image, so we have to be careful.

But all this is awesome, hands-on lab-work and we can play around! It's fun!!