The higher the magnification the more apparent or observable this becomes, until with very high magnification it can totally blur the view.
Perfect seeing (totally still atmospherics) occurs very rarely, and there will generally always be an element of this heathaze effect. This is why magnification is of lesser importance when choosing a telescope.
Poor "seeing" has a strange effect on you! You fine-focus your eyepiece, and observe. A moment later you think "I'm sure I can focus better than this", so you try refocusing, and you think it's better. You continue observing and once again you think "it doesn't seem clear enough", better refocus again.
It's not you - it's the seeing playing tricks.
Contrary to popular belief, predicting when seeing will be good is now a more exact science, and just as the weather can be predicted with a certain degree of accuracy, so can the seeing. The following paper provides some fascinating insight into this, with links to Internet resources that forecast seeing, and it also "gives hope" to newbie astronomers that their new Christmas gift will in time give excellent views of the night sky. It might look like a nice clear sky in the British winter, but the "seeing" accounts for a lot as to the extent of what can actually be seen!
Paper published by Damian Peach in the Journal of the British Astronomical Association: Predicting Astronomical Seeing in the UK (PDF format)
If you have read the previous sections you should by now realise that whatever your reasons for purchasing a telescope, your prime consideration should first and foremost be Aperture.
Put your hard earned cash into the Aperture of the scope as this results in more "light collecting" power.
You will be able to observe dimmer deep-sky objects. Another fact is that the higher magnification EP's (eyepieces) you use, the dimmer the light from the stars becomes, but if you are collecting more light from the night sky with larger aperture you will be in a better position to use higher magnification.
Other Topics in this XT10 Review: