Yup, that would be true, and more so on low cost scopes, but even expensive scopes will dim some on high power, it's just the nature of light transmission and magnification. I've got a Japanese built 20-60x60 Redfield spotting scope that I paid $500 for 15yrs ago, it's a fantastic scope, but on 45-60 power, it'd better be mid day with bright sun, or it will just get dimmer and dimmer as the power goes past 40x and on up. With the zoom lense, it's real easy to see the effects of magnification on light transmisson.