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I just wanted to confirm my thinking, if I have the option of D700 or D300 for macro work, using the Tamron 90mm, the logical choice would be the D300 as the crop sense makes the 90 ~ 135 so gets me close to the subject (if thats my aim, and with macros, it usually is, but not always). If I'm using flash and tripod, and can bring the ISO down pretty low then I assume the D300 vs D700 noise becomes a non issue, so I can stick with the D300 and get more bang for my buck with the 90mm. Is my logic ok ?

Sounds sound to me! Unless you are looking at terrible light and not enough flash power you are correct that the noise issue is a non issue.

yepp. pretty much. you also get a sliver more dof from a crop sensor.

but it also depends on how you want to expose the subject. sometimes you want to balance ambient and flash so you bump up your iso to compensate. you can do that with a d700 more effectively. you can also increase the iso with a little more freedom to bump up shutter speed when you have no flash.

Craig,

You pretty much have it, but there are some other aspects you need to consider with macro that the sensor size effects (and doesn't effect).

The sensor size doesn't effect closest focusing distance, the lens determines that. So you will be able to get just as close with the D700 as you can with the D300.

The sensor size doesn't effect (true) magnification. At the closest focusing distance with the Tamron 90mm, you will get a magnification of 1:1 with both the D700 and the D300. That is, the image projected on the sensor will be life size.

The effective magnification (with respect to resolution) will be greater with the D300 than the D700. As the D300 and the D700 have approximately the same number of pixels, a subject shot at 1:1 with the D300 will be larger in the frame than the same subject shot at 1:1 with the D700. For example, a 1:1 macro of a 16mm wide flower would just touch the top and bottom of the frame with the D300 but would only take two thirds of the height of the frame on the D700. This is where you will see the most benefit with the D300 over the D700 and covers not just 1:1 but less magnification, too.

The point above means you can get the same effective magnification from a greater distance with the D300 than with the D700 - which you already considered as part of the crop factor. It also means that you can photograph smaller objects with the D300 and have them take up more than the frame.

For the same effective magnification and effective f/stop*, you will get more depth of field with the D300 than the D700. With macro photography, DOF is based on (true) magnification and (effective) f/stop. As you require less true magnification for the same effective magnification with the D300 compared with the D700 you will have greater depth of field. This will be of great benefit as you can increase the DOF without going for a smaller aperture and potentially introducing diffraction.

If you only use the one lens, the D700 might give you the edge for some subjects, when using on camera mounted flash. As you will generally be closer (for the same effective magnification) with the D700, your flash will be brighter (less distance to travel) and so you will have a greater flexibility for smaller apertures (to increase DOF or reduce the effects of ambient light). This could be easily solved by adding a 60mm lens to the arsenal.

In my opinion, the D300 makes a better macro camera than the D700. Note that this is because the total number of pixels are the approximately same. The D3x, which is full frame, would be better than the D300.

* Effective f/stop considers the effect of bellows extension on the f/stop calculation. Modern Nikon cameras display effective f/stop when using modern Nikon lenses (and equivalent). For example, at closest focusing distance, my camera reports the smallest aperture on my 60mm f/2.8D as f/54. At infinity, the exact same aperture is f/32. This is because the bellows extension (movement of the lens element away from the focal plane during focusing) changes the f/stop calculation (lens to focal plane/aperture diameter).

Yup! Macro photography has never been so easy since digital photography and crop factor came along.