Image quality is frequently abbreviated "IQ" in photography forums. What exactly constitutes IQ? More and more, people are using IQ as a term synonymous with sensor performance. Some people take it further than that, equating IQ sensor performance tests in a lab without a lens mounted. Everyone from the NY Times' David Pogue to your salesperson at Ritz Camera is referring to IQ this way. DPReview is the most popular online review site and photography forum in existence. Here is a statement from their new review of the Sony NEX-5N: Without question, the reviewer is referring to sensor performance as "image quality". Is that what IQ is? Sensor performance? Rewind a couple of decades. In one hand, I have an Olympus Stylus Epic pocket camera (great camera by the way; still have mine), and in the other, a Leica M6 with the latest 35mm Summicron lens. Both are loaded with the same low-speed film, so in effect, their "sensor" is the same. These cameras therefore have the same IQ. Right? Some more questions: My camera - by lack of mirror slap, image stabilization, presence of a soft release, or better ergodynamics - makes it easier to hold steadier and thus use a lower ISO with a static subject. Does that affect IQ? My camera lacks an AA filter and can capture more detail. Does that affect IQ? I just used the same camera to take a picture of a patterned cloth and the subject is dominated by color aliasing due to the absence of that filter. Does that affect IQ? My camera often misfocuses a bit and I try to compensate by using some wide radius sharpening? Any effect on IQ? I want to capture a very narrow angle of view. My system has no available supertelephoto, so I'm just going to crop the heck out of the picture before I print it. Any effect on IQ? Perhaps my camera doesn't shoot RAW or the RAW files aren't supported in anything other than the crappy proprietary software it shipped with. Any effect on IQ? All my greens with a particular camera tend to come out a little bit sallow, and I can't find a good RAW processor or color profile which makes that better. Does that affect IQ? Most of the lenses available for my camera are sharp and yet render bokeh which is very much to my taste with soft falloff and lack of doubled elements. Does that figure into IQ? My only lens is super contrasty. Will that affect IQ? My only lens flares like hell. Any effect on IQ? I use great film, but the lab which develops it for me and scans it is awful. Will that affect IQ? My camera has a great sensor and a crap JPEG engine. I eschew RAW. Any impact on IQ? Maybe the answer to all those questions is "No", and IQ is simply a formula based on sensor resolution, sensor dynamic range, sensor signal to noise ratio, and sensor tonal range. What does IQ mean to you?* Once you've answered that question, here's the next one: Is IQ the major determinant of what makes a photo good? *We had a discussion about this four years ago at DPReview (of all places) where I did my best to address what IQ means to me: http://forums.dpreview.com/forums/read.asp?forum=1029&message=25545477. I left out some things back then, but my overall stance is unchanged. That is to say that in general, for the types of photography I do (people photography with moderate print sizes), the lens is far more important for IQ than is the sensor.