During yesterdays forum outage, Rik emailed me to point out that my calculations above make some invalid assumptions ("thin lens" assumptions). While my general point that you can't snap on a close up lens and retain focus still stands, the calculated minimum focus distance in particular is wrong.
I hope Rik posts what he sent me, it was instructive.
Sure. Here it is, with a bit of cleanup...
ChrisLilley wrote:The Raynox DCR-250 is a +8.0 diopter lens. With the Raynox fitted, your 90mm lens has a focal length of 52mm when focused at infinity. (I don't know if the focal length of the lens gets shorter when it focuses closer; many do, and that would affect the focal length of the combination when focused closer also).
Guessing that you are using the Tamron SP AF 90mm f/2.8 DI macro, whose spec sheet is here the closest focus distance of the lens is 0.29m (measured from the sensor in the camera, not from the front of the lens).
With the Raynox fitted, the new closest focus distance is 0.087m (87mm!). Recall that this distance is measured from the sensor; the Sony A200 measures 44.6 mm from sensor to lens mounting flange, and the SP AF90mm has a length of 97mm. A bit of that length is the mount, which goes inside the camera, but even so the distance from the sensor to the front of the lens is going to be around 135mm. So if the lens was set to the minimum focus distance and the Raynox DSC-250 was added, the new closest focus distance would be well inside the lens.
Sorry, but this analysis is not correct. It sounds like you're treating the lenses as "thin" and not separated. When they are thick and separated, as in reality, the behavior is quite different. Even within the thin-lens model, your calculation of "new closest focus distance" has gone astray. For a thin lens, the minimum focus distance as measured from sensor occurs at 1:1, at which point the lens is 2 focal lengths away from the sensor and another 2 focal lengths from the subject -- 4 focal lengths altogether. For a 52 mm focal length, that would be 208 mm from sensor, not 87 mm as you write.
As a different way of thinking about the same problem, imagine that you have a "thin" close-up lens while the Tamron is whatever it is. Set the Tamron to its closest focus distance. Obviously it focuses at a point well in front of the lens. Now, place the imaginary thin close-up lens at that same distance, so that the subject and the thin lens are coincident. In this configuration, the close-up lens has no effect on focus. Starting from that configuration, slide the close-up lens closer to the Tamron. As you do that, the focus point will gradually move closer to the Tamron, but it will remain on the outer side of the close-up lens, away from the camera. This relationship persists until you reach the limit of movement when the close-up lens touches the Tamron.
This is probably a good time to put in another plug for
WinLens, a ray-tracing program that I have found very helpful for understanding & confirming the behavior of lens combinations. The cool thing about WinLens is that because it's ray-tracing and shows you the traces, it's very hard to fall into miscalculations that seem plausible but don't actually work out. WinLens3D Basic is a free download for Windows systems. It's certainly not trivial to use, but I've found it well worth the trouble to work through.
Here are a couple of displays from WinLens that demonstrate what I'm talking about with the closeup lens. What I've done here is to configure a 100 mm "main" lens at 1:1, then added an 80 mm closeup lens at various distances from the main lens. The main lens is a triplet from WinLens's library; the closeup lens is modeled as thin.
Notice that when the closeup lens is very close to the subject, the magnification is hardly changed. The perspective is changed a lot however, in this case becoming "inverted" so that parts of the subject farther away will appear larger, not smaller as usual. (See the
FAQ: Stopping down a lens combo for more discussion of this effect.)
Adding the closeup lens with 70 mm separation moves the focus point closer in and increases the magnification to 2.533:1. This configuration also turns out to be almost perfectly telecentric -- notice that the centers of the ray cones are parallel to each other as they come off the subject.
Moving the closeup lens even farther toward the main lens further increases the magnification, to 3.158:1 and restores more normal perspective, with the central axes of the ray cones converging from the subject toward a point in back of the lens. I've labeled this "telephoto" perspective because it's the same as what you'd get shooting with a much longer lens from farther back.
I hope this is helpful. The effects I've described here are all "basic optics" by the standards of optics specialists, but for the rest of us they can get pretty mysterious and nonintuitive. Physical experiments combined with ray-trace modeling using some tool like WinLens can do a lot to help keep things straight.
--Rik