What impact do long macro lenses have on DOF?

Question

When using any 1:1 macro lens, my understanding is that the framing or field of view remains the same at 1:1 magnification and all that changes with the focal length is the distance at which the you take the photo (i.e. the working distance).

That said, it seems to me that using a long macro lens like the Tamron SP AF180mm F/3.5 Di or the Sigma APO Macro 180mm F2.8 EX DG OS HSM should provide an increased depth of field compared to using a typical 50 mm F2.8 1:1 macro (provided the apertures are the same).

Is my intuition here correct or does the working distance not matter at all?

Answer 1

The "working distance" as you put it cancels out. Think about focus and the opportunity to be out of focus as a cone of confusion. The cone comes to a point at where the picture is in focus, which in the case of a 1:1 macro will be 2x the focal length from the effective aperture of the lens. The cone goes from a point in focus to the aperture. Anything inside that cone will add something to one particular image point.

Given that view, consider what happens as the focal length is doubled. The distance from the cone point to where its outer edge is defined is doubled, so that would make the cone narrower. However, you said the f-stop is held constant, so the aperture diameter is also doubled. You are now back to the same cone of confusion as it leaves the object, just that it is longer before it gets to the lens. Since the cone angle is the same, objects in front of or behind the focus point will be out of focus by the same amount. Therefore whatever threshold of blurriness you decide is still in focus will occur at the same distance from the object being photographed, so depth of field is the same.

Answer 2

If you double the focal length and double the working distance, the DoF stays the same along with the field of view. Also note here that it's still a different perspective.

If you fix the focal length and change the working distance, the DoF changes along with the field of view.

Likewise, if you change the focal length and fix the working distance, the DoF changes along with the field of view.

Answer 3

It turns out things cancel either other out. I did the same exercise when choosing between a 35mm F/2.8 and 100mm F/2.8 macro, both at 1:1 magnification and the DOF was nearly identical.

That happens because your working distance is further with the 100mm which means more depth of field but the 35mm is wider so has more depth-of-field. So at 1:1 magnification, both give the same DOF.