From a recent thread, I professed to being the asker of daft questions. Always aspiring to be a man of my word, here's another couple of gumbies for you..... We often make comparisons between cameras to determine which might better suit our 'needs'. Two areas that we often focus on are lens speed and low light performance. Hence, I wondered on whether objective measures underlie our thinking here? 1. Lens Speed: so when you want to better light gathering or bokeh (OOF) capabilities we all seek a larger aperture. So f2 is faster than f4. I know this has to do with how 'open' the aperture is, but I wondered is this measure dependant on the relationship to sensor size or is it a constant measured in, for example, 35mm terms? Therefore, is "f2" different when you are talking in terms of a 1/1.7", APS-C or FF sensors? Accordingly, if you use an old 35mm f2 lens on a sensor half the size - e.g., m4/3 - does it remain f2 or is it twice as fast - maybe f1.4? 2. Low Light Performance: Again we all must confess to comparisons on DPR and the like looking at how cameras perform at ISO 400, 800, 1600 and up to 51200 (jeez, I'm hard pressed finding a film of this rating). So, again, is this an objective measure? For example, is ISO 1600 the same between manufacturers - i.e, Olympus vs Sigma vs etc.? If so, how is each ISO rating baselined? I know these are Digital Photography 101 type questions, but not one's I know the answers to, nor one's that my Google searches are helping with, I'm afraid. So really appreciate your insights.