I was hoping to keep things simple, but it sounds like I need to explain things further. ;-)
I'm sure most have heard of the Twilight factor, but I guess fewer understand what it really means. It was derived from a study by Zeiss scientists to determine the limit of acuity in low light for different magnifications. It doesn't tell you anything about how bright the scene appears, only the level of resolvable detail. Twilight in their terms is not the limit of colour vision (photopic) that would provide the full stop for most birding, but more akin to moonlight levels, when hedgehogs are most active.
Just to keep it simple, I'll just illustrate if for a 5mm EP which might be the maximum for the average 65 year old.
7x = 15.7
8x = 17.9
10x = 22.4
12x = 26.8
It tells you that by moonlight, an average 65 year old would be about 70% better off in seen detail with a 12x60 than a 7x35, but there would be no difference at all in apparent brightness. Obviously individuals will have different maximum pupil dilation, and it doesn't give you any detail on the tradeoff at sub optimal exit pupils. My own pupils dilate to around 5.5mm a while an 8x42 looked brighter than a 12x50, I could see more detail with the latter.
Leica did a comparable study, but instead of acuity limit, they looked for the threshold for target detection. Like the Ziss study, it was conducted on young adult males. Holger Merlitz has published a paper describing how the results would be affected by the pupil reduction as we age. Although this is quite a different metric to the Zeiss study, it shows an even clearer benefit due to magnification. I won't try to extract some illustrative numbers from that paper but try to describe a comparison I did which convinced me of it's validity.
The view from my back fence is across a ploughed field to a small wood and hedgerow about 150m away. There was some moonlight, but at least part of the illumination was was from the street light glow of the town hald a mile away. I had to hand a 7x36, 8x42, 10x42, 10x56 and a 12x50. I started with the 7x trying to both determin the approximate level of detail I could see in the open, and also in the shadow of the hedgerow. I switched to the 8x and there was a slight improvement in detail, but to my surprise I could now see an animal moving. It might have been a dog or perhaps a muntjac deer I couldn't tell, but I rechecked with the 7x and I couldn't see it. Next the 10x42. The view was dimmer, but now I could see it was a fox. With the 10x56 I could not only see the fox, but also the rat it was hunting a few yards away. Again the view was dimmer with the 12x50, and the detail not as clear as the 10x56, but I could still detect the rat. I checked again and it was still invisible with the 8x42, and the fox invisible with the 7x.
Unless you are a 20 year old whose pupils still dilate to 7mm or more, based on the work of Zeiss and Leica and my own comparisons, I think you would be better off with a 10x56 than a 7x50..... when looking for hedgehogs by moonlight.
David