Wide angle shots and blurness?

I have a Nikon D800 with Nikkor 24-70mm lens. My problem is that my wide angle shots (usually group pictures) lost little bit sharpness after zooming 50%. They appear tact sharp till 50%. Please guide me what mistake I'm making with wide angle shots.

At 50% zoom you are partially zoomed in to the shot. At that zoom level you may start to be able to see faults you can't see when zoomed out, such as blurring due to motion blur, camera shake, or problems due to diffraction, or focussing problems, or too shallow a depth of field.

It's kind of hard to tell you exactly what you did wrong without seeing a shot.

But here's my guess for what it's worth - don't shoot wide angle with the f/stop above f16. Because if you do you may be getting diffraction problems. It's pretty unusual to see camera shake in wide angle shots, unless you did something catastrophically wrong, like setting too slow a shutter speed. You mention you are photographing groups of people. Make sure your f/stop is no lower than f/4 - otherwise the depth of field may be too shallow to capture everyone in the group in full focus.

When you say "zoom" are you talking about when you view the image at 100% in Photoshop? With large sensors, most full images appear on your screen at 16% or less

If at 100% your images are out of focus, then there may be a problem with the auto-focus setup in camera. There's what is called "AF fine tune" Look in your user manual on page 338

NOTE: Before you attempt to use this feature, make sure that the image is not really blurred caused by camera movement during exposure using shutter speeds below about 1/250th second.

There's a program you can buy that is made to fine tune your lenses called FocusTune V2.0

Can oyu link an example with full exif intact so we can see what you did wrong?

What your saying is at wide angle (24 - ~50mm) they're OK, but from 50mm to 70mm they are blurry.

Camera shake is the obvious answer, the increase magnification also increase the effects of camera shake, with the longer shutter times needed in low light it will make it even worse.

You're probably getting blur caused by camera shake in the wide shots too, but it will be less obvious.

Using a tripod will completely eliminate camera shake, but subject movement can still be a problem in low light with longer shutter times.

Increasing the light (flash etc) will make the shutter speeds faster, you should be able to hand hold with a shutter speed the reciprocal of the focal length, nothing complicated, it just means that if your lens is set at 50mm a shutter speed of 1/50th of a second should enable you to hand hold without blur, this drops to 1/70th of a second at the 70mm mark. With practice you can hand hold a lot slower than this.