nikon - Focusing issue with kit lens

I have been using my camera (Nikon D5100) for around 5 years. Recently I am facing an issue when I use the kit lens. If I take a group photo (of a group of people) standing not very far from them, the autofocus works well but after clicking if you examine the image by zooming in, it shows that none of the persons in the photo are in focus. It keeps happening.

But If I zoom on anyone and then click, it works fine even though I maintain the same distance from object.

So I decided to zoom first and once I got the proper focus after zooming, then I change the mode to Manual (hoping that focus remains locked) and zoom out and take the pics. Unfortunately, it goes wrong again as I mentioned in the first case.

What could be the issue here? anyone has any input?

Answer

Your lens is not parfocal. If you had a parfocal lens, you would know it. Mainly because the prices of true parfocal zoom lenses ($10K and up) mean only those who really need them and know why they need them buy them.

A parfocal lens is a zoom lens that maintains the same focus distance as it is zoomed in and out.

Most lenses used by anyone other than professional videographers are not parfocal. When you focus a non-parfocal zoom lens at one end and then zoom it, the focus distance will change and must be adjusted again after zooming to be correct. There are some fairly cheap lenses that are effectively parfocal - that is, they lack sharp enough resolution or a wide enough maximum aperture to be able to tell the difference between focused and slightly out of focus.

That being the case, the answer to the second part of your question is that your lens is not made to be able to do that with any degree of accuracy.

The answer to your first part is a little tougher. It may be that you are just now noticing something that has always been the case. It is not uncommon for zoom lenses to be better on one end than the other, although it is usually the wider end that is a little better. Or it may be that your lens is drifting (or has been knocked) out of alignment and needs to be adjusted. If the lens is a cheaper one that does not cost as much as it would take to get it properly aligned, then it might be time to think about a replacement or upgrade lens.