In general there are five thing that affect image quality or iQ if you like: the lens, the sensor, the processor, image stabilisation and the most important of all you the photographer. Hmmm...
You bring vision and knowledge. Oh, so there are actually six things that affect iQ: the lens, the sensor, the processor, image stabilisation, your vision and your knowledge - oh and light.
So there are actually seven things that affect iQ then: the lens, the sensor, the processor, image stabilisation, your vision, your knowledge and light - oh and composition.
Thus there are actually eight things that affect iQ then: the lens, the sensor, the processor, image stabilisation, your vision, your knowledge, light and composition - oh and emotion.
OK, so there are nine things that affect iQ: the lens, the sensor, the processor, image stabilisation, your vision, your knowledge, light, composition and emotion - oh and the subject.
Ah ha, so there are ten things that affect iQ: the lens, the sensor, the processor, image stabilisation, your vision, your knowledge, light, composition, emotion and the subject - oh and exposure.
Oh dear, so there are eleven...
Hang about, hang about, what exactly do we mean by iQ in the first place anyway? And therein lies the problem... it can mean so many things to so many different people at so many different times. Its meaning gives a context to discussions we have together which in turn make these discussions themselves meaningful or indeed not.
Probably iQ can be separated into two distinct sub topics those of technical quality and aesthetic quality. Whist the two do interrelate one is not a necessary nor sufficient condition for the other.
Many aesthetically pleasing photographs have been made on technically poor iQ equipment (indeed the poverty of the technical iQ may have contributed to the great aesthetics of the image) and huge quantities of aesthetically awful images are snapped on kit that has great technical iQ all the time in these digital days.
Where then does this leave us the viewer of images when it come to their "quality"?
Well manufacturers would have us believe image quality is a purely technical thing that is measurable with numbers and that the bigger the numbers the better so they can differentiate their products - biggest number of pixels, highest ISO settings, widest dynamic range etc. They compete on technical iQ. Probably most consumers and enthusiasts are taken in by and make purchases based upon technical iQ too, chasing ever higher theoretical excellence.
Fine art photographers are more interested in aesthetic quality and whist some technical iQ remains interesting to a degree it is only in the context of it helping them to attain their art and they are slow to change and indeed many still use film in large format for example. What maybe considered as imperfection in iQ by modern technology standards may well be used to great artistic advantage, such as grain in film or spherical aberrations in old lenses that give a glow to images in portraiture, such things being greatly prized by the artist over modern clinical, characterless and uniformly boring technology.
In the end the true quality of an image when viewed by the beholder in real world and by a machine on a test bench is a different thing. It is qualitative not quantitative. The numbers only hold true in a very restricted and controlled test environment they tell us nothing about the real world, the perceived image quality of the equipment in the real world is often very different to that of the test bench and the qualities of the image as envision by the photographer in the real world and as perceived by the viewer of the image in the real world are different again.
The terrible truth is that iQ in the real word is subjective, a matter of perception and open to interpretation. Beauty, as ever, is in the eye of the beholder.