Deerhunter:
I feel your pain! But, first and foremost, it is all about light! If there isn't enough, you might as well leave the camera and lens in the case until enough light can be arranged to create a good image. Photography is literally Writing with Light!
It is like those many poor souls at events. You see them flashing away from all over the grandstands, not understanding that if there is an image to be had, it will be provided by the event lights, and not the little pissant on board flash.(Good for up to ahhhhh...6 feet!

) Even the expen$ive add-on flashes won't cut it at long distances, and 150 yards is 'waaay too long! Or, maybe you have one of those big anti aircraft searchlights you see at events!
As was suggested, a little basic knowledge of your camera and exposure will pay great dividends. A camera is only a convenient place to store the imaging stuff, and display those WOW factor lenses we are all so fond of. (Lens envy?

) It takes a photog to create an image...the camera is just the tool.
1. Be sure there is enough light!
2. Get closer. 150 yards? That's a faaaar piece, even with a rifle, and we are talking cameras here. If you can't take a head and shoulders protrait of your subject at 15 feet, get closer, dear. Or is it get closer to the deer? 150 yards is not good! The deer's image will be the size of an ant. Landscape, with small black dots!
Turn the image stabilization off, and use a good tripod! Flimsy is better than nothing, but sturdy is better.
3. READ! Your camera manual is a good place to start. Take a class. Read a book. Very basic, to start. When you begin to understand the relationship of sensitivity (ISO), shutter speed, and aperture, which can be learned in about 1/2 hour, and understood in about twice that, then you will be on your way taking photos, instead of trying to make your camera do something it was never designed to do. We have all been there.
Patti KNOWS which end of the camera to pour the electrons in!
4. You have BEAUTIFUL equipment! (Envy is such an UGLY emotion!) Forget new cameras, lenses, or mi$cellaneou$ attachment$. THIS problem can't be solved by throwing money at it!
Today's cameras are better than most of us will ever be! Take advantage of that. Learn to use what you have, and practice, evaluate, figure out why something did not work, practice some more, and finally, in a surprisingly short time, you will be taking pixes you are proud of. Guaranteed!
TRUST me, I know these things, having spent a great deal of my time in graduate school teaching these principles. THEN get out there and take some pictures!
All the best.