I merely wanted to confirm that a 100mm macro lens is just another 100mm lens, with a really small MFD.
But comparing the 100mm macro with the 135mm at the same framing, DoF should be less with the 100mm macro.
+1 but at the same aperture.
Focal length: 135
Focus distance: 10 meters (to get 2X magnification)
Total Depth of field: 0.97m
Focal length: 100
Focus distance: 6.75 (to get 2X magnification)
Total Depth of field: 0.81m
Your math is wrong. 135mm at 10 m will have a wider FoV than 100mm at 6.75 m. For the same framing as 135mm at 10 m, the 100mm lens would be at a distance of 7.4 m [i.e. 10 m / (135mm/100mm)]. When you plug those numbers into your DoF Calculator of choice, you will find that for the same aperture the DoF is identical.
Here's the way it works: when comparing on a given sensor size (so we can ignore the circle of confusion), three factors determine DoF.
- Aperture: wider = shallower
- Focal length: longer = shallower
- Subject distance: closer = shallower
When you are talking about the same framing
, focal length and subject distance have equal and opposite effects, and thus they cancel each other out. When comparing lenses of different focal lengths for the same framing, DoF is determined only by aperture. So at f/2.8 for the same framing, there's no DoF difference between the 100mm Macro and the 135L, but the 135L can open up to f/2 meaning it can achieve a shallower DoF for the same framing.