@HDNitehawk I think I wasn't being clear on where I got the factor of two. Let's assume that both my 70-200 and a 100-400 can resolve a feature (at whatever definition you like) at 2 pixels being lenses of relatively the same cost. Obviously 200 mm is going to frame differently than 400 mm - a factor of two for an infinite target (I'm being lazy). I could crop my 200 mm image to have the same boundary as the 400 mm image - but would have 1/4 the pixels (half in each direction). To match pixel counts I can double each pixel in the 200 mm crop shot in both directions. Doing so I double my blur from 2 pixels to 4. So even though both lenses might have 2 pixel resolution to start, the detail on the object that can be resolved will be half the size with the 400 mm frame than at 200 mm. That's where I got the idealized factor of 2x.