Just a few thoughts on scopes. A 50mm objective will have no clearer or sharper image than a 40mm scope. Clarity is strictly governed by quality of glass and coatings. A 50mm objective will transmit more light in the upper powers of the scope if the quality of the glass and coatings are equal, meaning if you have both scopes set at at 10x the 50mm lens will be useable for a little longer in the evening or brighter in dim light. Move them to 5x and they will be equal. A lesser quality scope with a 50mm lens can be dimmer than a better 40mm scope due to the quality of the coatings and glass that transmit the light/image. I shoot guns with 20mm scopes for coyote hunting and they have been more than bright enough and I have no restrictions on shooting hours, if I can see a coyote coming with my eye I can see him in the scope in theory a 20mm scope is just as bright at 3x as a 50mm scope at the same power, again glass and coating being equal.
The draw back of 50mm scopes is that they have to be mounted higher than a 40mm scope, a 1/4" or more. I tried one and had to build up the comb of the stock so high to get a good cheek weld that I couldn't remove the bolt any longer.
I have had cataract surgery many years ago and they didn't get quite the right lens in my right eye but I'm able to focus my scope enough that I get a clear image. Have you tried to focus your existing scope?
