This is an adjustable 48 mm extension tube that adds 8-12 mm of length. Thread is the standard 2" filter thread size of M48 X .75. The depth of the male thread is 9.5 mm and there is a thin (1.52 mm) threaded locking ring on this thread to adjust the spacing. The female thread is 4.95 mm deep. The body is 2" in outside diameter.
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It is a very well made product, very helpful when adjusting / dialing in fine adjustment on camera backspacing.
Product is well made and works very well for me. However, it is important to understand that the adjustment is made by changing the depth to which the male threads go into the female threads of the next object in your optical path. If the female thread depth of your next object is less than 8mm), then you cannot achieve the shorter end of the stated range (8mm).
I purchased every M48 extension tube Stellarvue has to offer in order to get the spacing just right for my EdgeHD SCT and I was amazed at the build quality. They are light weight and solid. Combining them allows me to get perfect images without vignetting on a full frame DSLR. This particular extender was critical to getting my 10.5 mm spacing right.
This is a great product if you need to finesse back focus spacing on your camera-which is exactly what I need it for. I recently purchased 4 different extensions, including this one, from Vic at Stellarvue. I've come to expect nothing but high quality coming out of Auburn--- which is what I got. I'll be giving this review for the other 3 extensions.