The image becomes larger due to increase in focal length. The f/5 (again I’m talking about using a sensor at prime focus) will “stretch” the light out by making it bigger (increased magnification). If the focal ratio is the same, the brightness is the same (photons per unit area), the increase in in focal length will make it bigger. An example, my f/2.8 camera lens will require a heck of a lot less exposure than my f/10 C8, even though the C8 is bigger. In visual use, you are using an eyepiece to make an image of the image at the focal point. If you have the same magnification, the bigger scope will be brighter due to more light being collected and distributed over that image (that is the same size). Here is an extreme case, get that 100mm refractor and 150mm reflector, point it at the Orion Nebula, on the 100mm refractor, put the lowest power eyepiece you can (say 15x) and on the reflector, put a very high power eyepiece (say 400x), you will notice that the smaller image will be brighter. Now place them at the same magnification (say 50x), in this case the 150mm reflector will be brighter as the image scale is the same and the 150mm collects more light. Point of the matter, the image scale (without an eyepiece) and your camera is a function of focal length. If you keep the focal length the same (hence image scale), the larger aperture will be brighter, the focal ratio is a handy way of quantifying this. In your case the 150mm would be brighter since it’s image scale is similar. Think about how much light you are collecting and how you are spatially distributing that light.
Edited by AlienRatDog, 22 October 2019 - 08:02 AM.