What can the focal lenght of objective lens of 5 inches diameter be?
It might be just about any focal length. THe diameter of the lens has nothing directly to do with the focal length. It’s the curve of the faces and the refractive index of the glass that determines the focal length of the lens. You can measure it very closely by projecting the focused image of a flashlight bulb onto a white card in a darkened room and measuring the distance between the lens and the image on the card. If you use a regular flashlight bulb and a variable power supply you can get very accurate by actually focusing on the filament in the bulb. Make sure you’re focusing on the filament and not the reflection of the filament in the back of the bulb. Here is a very good demonstration on youtube that shows what I’m trying to explain.
