Stacking two 25mm canon extension tubes

Hello,

I have a 300mm f/4 IS lens and a canon 25mm tube, this combo with the crop factor of my 40D give me a magnification of 0,59 (0,37x1.6).

My question are: If I stack another 25mm on that combo I will have a maginfication of 0,92, almost life size? I'm I right?

How does that combination works? Does the lens still autofocus? Does the image suffer a little with the 2 tubes?

I know from experience that the quality I can get with one tube on my 300mm is excelent, but wonder if with two I'm starting to have problems.

Does any one have opinions about this?

Paulo anjo

Hi Paulo

I carry three 25mm tubes !!! Quality is fine but suggest to focus manually, do keep it in AF but move past your subject then focus back and press the shutter at the beep. No problem with quality !!

Remember as you use more tubes the shutter speed will decrease ... can also use the tubes in combination with extenders.