My photos lose resolution on change of lens?

Earlier I used two lens for my Nikon D3100, one which is the kit lens of 18-55 mm and another Nikon 55-200 mm. I sold my 55-200 mm lens and bought one Sigma 18-200. What I found next that my photos on my new lens has lost resolution of nearly 25% of what I would earlier used to have on my earlier two lenses. I suspect it is the obvious result of compromises I should be prepared to accept on a super zoom lens. Is there any way I can solve this?

Not sure you mean resolution - perhaps you mean sharpness?

All high power zoom lenses are optically marginal, and the higher the power, the more marginal.

Your only recourse is to stop down the aperture.

I own the Nikon AF-S 18-200mm, and when used wide open, I can see some slight blur; especially in the corners along the focal length. However, when I stop the lens down to f/8, those issues pretty much disappear.

You may not be able to obtain the same results with the Sigma, but try limit your aperture to f/8 and see if that helps at all.

You can't lose resolution on a digital camera. Resolution is a measurement of pixels in the photo and that won't change unless you physically change it in the menu.

A photo can lose viewing quality and that's probably what you're seeing.

Actually it's most likely not the zoom it's the Sigma. Third party lenses are cheaper because they of lower quality that lenses made by Nikon. Lower quality glass equals lowered viewable quality.

Well I'm sorry you sold your 55-200mm. My experience, like a lot of inexperienced photographers, was to fall for the same trap. I too purchased a 18-200 Tamron (not Sigma) lens. My result the (insert cuss word) lens was just too heavy on my neck and carrying around for more than 30 minutes was a pain of the first water" (an old time saying). My 11x14 test print was soft! I sold the 18 to 200mm and purchased Pentax's 50-200mm. My images are sharp as a tack and my neck doesn't hurt as the new lens is about the same size as my 18-50mm. Enough of off brand lenses for me!

My only way to solve was to just accept that I have to change lenses more often. My 11x14's & 16x20's are a great reward for my extra efforts!