I don't think you are going to see much difference between 40mm and 50mm. Pupils only dilate to about 7mm, diminishing when you get older. If you are 50+ i would think a 5mm dilation would be common. With that said a 50mm at 10x provides you with a 5mm exit pupil compared to a 4.44mm for a 40mm at 9x. As you go up in magnification you will be able to see more detail but the image will be dimmer
