Tony is explaining what you have to do to make similar pictures on a crop sensor and full frame camera. I don't agree with the way he presents it however. He is adjusting the aperture on the longer lens to compensate for the differences in DOF caused by the different ACTUAL focal lengths. It is important to emphasis that it is the FOV that is equivalent to a longer focal length but that the focal length doesn't actually change and neither does the DOF. As a demonstration of what I am trying to say do this. Take your D610 and the 24-70mm lens. Take a picture with the zoom set at 60mm. Now, without changing any exposure settings, take a second picture with the zoom set at 40mm. Now take the 40mm image and crop it so the FOV is the same as the uncropped 60mm image. You now have two images with the same FOV but the one taken with the 40mm is going to have a greater depth of field. You can also use the DX crop in camera to illustrate the same thing in reverse. Cropping an image, whether in camera, in PP, or due to sensor size does not affect DOF. Of course if you are comparing pictures with the same FOV taken with cameras with different sensor sizes then there are other variables such as resolution, pixel density, and circle of confusion values that have to be taken into account. In short, just because a 40mm lens gives you an image that looks like it was taken with a 60mm on a FF camera, it is still a 40mm lens.