Surely a macro lens designed for FX would be more than 1 :1 on a crop sensor, I thought the figures they quote are for FX unless the lens is specifically for DX then I guess they would quote for crop only.
Or am I missing something.
Normally 1:1 stands for the size of the subject in reality being identical on your sensor at closest focus distance. If a bug is 1 cm and you are at max, it is also 1 cm on your sensor. That's true for FX and DX, or any length the lens has. But for DX, the higher megapixel sensors some have translate that same size into "bigger" when scaling on a computer.
If I use my 200mm on the DX or FX, the sizes remain identical yet the D3300 takes more magnified shots. In reality it doesn't.
The problem or advantage here is that on computers real world sizes don't matter and they're relative to your pixels count. Higher megapixels combined with smaller sensors translate into a perceived increase in magnification.
1:1 on DX or FX is a bit like this:
They're both identical shots lens-wise and real world/sensor-wise but because I saved the cropped version with the same pixel count, it looks as if it is magnified more.