OK to keep the extension tube?

Hi all,
My current set up is a Nikon D7000 with an APS – C sensor. I use a Nikkor micro 55 mm lens with a 13 mm extension tube. This seems to work OK but I’m wondering if that extension tube is lowering the quality of my image. Without the tube I can’t get the whole negative in the frame and in focus. Would it be wise to replace that 55 mm lens and the extension tube with a 105 mm micro?

Thanks.
Rich

Extension tubes have no optics so they put nothing into the light path to further degrade IQ.
OTOH, the lens itself only delivers a certain number of lines of resolution, and macro shooting simply spreads the total image circle over a much larger area, so when you capture a PORTION of that image circle (thereby capturing the object at a higher magnification) you capture only a portion of its total delivered image. (That is also why the apparent brightness is reduced at the focal plane, the total photon count is spread over a larger area, too!)

Moved to the Digital Cameras sub-forum.
The extension tube won't be what might lower your image quality. The factor that has a potential to lower the quality of image is the fact that your lens is designed to image to a larger sensor. In addition, using the lens with a smaller sensor may result in your using the lens for a magnification outside the range it is optimized for. If there is a bad effect, the 105mm macro would likely suffer from the same.
I wouldn't be too concerned. Any problems would be at least partially offset by the fact that the smaller sensor only uses the centre of the image formed by the lens.

Both the 55 micro and 105 micro are optimized to perform well at very close focus distances, so neither should inherently have an advantage over the other (apart from one maybe delivering a tiny bit more total resolution than the other...but you need to see resolution figures to know which has any advantage, if any)

Thank you all for the help. Given the advice here I am inclined to leave things as they are. I’m happy with the scanning results and I don’t see any distortion at the edges.

Thank you all.
Rich

That lens is designed to work well at macro distances up to 1:1 (on 35mm) and to use an extension tube to achieve it (one would use the 27.5mm extension tube to get all the way to 1:1). You're basically using it at 1.5:1 if you're copying a 35mm negative onto APS-C sensor. So well within the design range. Don't worry about it.

The advantage in use of a longer focal length macro lens like a 90 or 105 is that you get a greater working distance, which is useful if you're trying to light a subject like a small item, but doesn't matter as much for a copy or scanning and rig, and the longer focal length lens makes your rig significantly bigger.