Why "crop factor" is so pervasive
ETL Moderator- Staff Ornithologist
03-22-2012, 10:06 AM
Subject line is a little unclear- what I mean is why do we continue to read posts mentioning the importance of crop factor in reach for distant subjects? The explanation that crop factor is irrelevant and that it is in fact pixel size, pitch or density which is the relevant metric, is often given by Roger et al. but it seems to need repeating. This is not just a BPN phenomenon, it's all over the web.
There are several reasons and I'm sure the fact that a crop factor camera trims off the edges of the image (in relation to FF) and makes the subject appear relatively bigger in the frame must be compelling, however, I think there is another reason, illustrated in the attached image.
I took sensor data published on his Roger's web site or elsewhere and picked some current DSLR camera models and a modern point 'n shoot (PNS) as an outlier. The graph shows pixel size (pixel area or pixel pitch squared) against sensor size (sensor area or crop factor) and illustrates a simple relationship I had suspected for some time. As sensor size increases pixel size increases, in other words in modern cameras, larger sensors tend to be sampled more coarsely than smaller sensors. Coarsely sampled sensors provide less reach than finely sampled sensors but because of the positive relationship between pixel size and sensor size, the two variables get confused and reach is assumed in error to be determined by sensor size.
The line on the graph is fitted to the points from sensors smaller than FF and shows that the D4 is right where is "should" be but that the new D800 is way off the trend as are most of the newer FF cameras. I guess the D3x was way ahead of its time!
Last edited by John Chardine; 03-22-2012 at 01:51 PM.