Looking at a Leupold VX3HD 3.5-10 scope. I like the lower profile of the 40mm objective, but am I giving up too much light?

**Full disclosure, not sure what rifle I will put this on yet.

You're giving up a smaller field of view. At 3.5X, I bet that scope will get you past legal shooting time.

Read the section in this article on exit pupil or EP.

Thanks guys. And forgive me if I have confused myself here...

Based on EP and FOV, it seems the 50mm obj is the better unit to go with??

Yeah 50mm is better if you can fit it on your gun comfortably.

You do get a better exit pupil at a given power with a bigger objective, but it does NOT effect the FOV when comparing otherwise identical scopes with different objectives. You also gain 2-3 oz going from 40-50mm so if weight is a consideration then that’s relevant.

I have a VX-3HD 4.5-14 x 40 on the way. Hope to have it bore sighted then ready to hunt pretty quick. I'm going to do a side by side look at dusk against these:
I've had the VX6 7 years, the VX5 maybe 4. I wanted a smaller obj, and lighter scope for the Mohawk I posted last week.

The short version is that, all things being equal, a larger objective gathers more light, but weighs more, is bulkier and can ruin the balance of many rifles. The larger objective also allows one to use a higher level of magnification in lower light (see EP in the article linked above).That out of the way, all things are not equal, and the previous statement only applies to otherwise identical versions of the same scope. It's also worth pointing out that most individuals over 30 and almost all individuals over 50 have eyes that can accept only so much light, so there's a point of diminishing returns. My personal takeaway is that I prefer high quality scopes with smaller objectives, except when I specifically plan on shooting in very low light, like hunting hogs at twilight. Finally, if low light performance is important to you, your choice of reticle is very important, meaning you'll want illumination or coarse crosshairs at the very least.