Telescope T2-Extension Tube Kit for Cameras and eyepieces - Length 3mm 5mm 7mm 15mm 20mm 30mm - M42x0.75 on Both Sides

3mm tube is not very usefull like everyone is writing. all other tubes with at least 4mm long thread, connecteing these to 3mm tube, it will act as 4mm tube if you are lucky. If you are very unlucky 3mm tube and other connected tube may stuck forever if you force it to go beyond 3mm tube’s 3mm long female thread. Other tubes are O.K. except that I felt these thread have never been tested , you feel odd tightness when you connect these tubes in first time.5mm 7mm tubes are very hard to remove if you attach these to your equipment like astrophotography camera. they are too short to grab on to unscrew.

The thread on all of the larger rings are 4mm. In order for the 3mm to work correctly whatever you are screwing into the 3mm has to have 3mm or less thread length, otherwise it works the same as a 4mm ring. So if you need a 3mm ring measure the male thread of what you need it for. The other rings are fine and seem okay. I would not want to have to rely on un screwing these all of the time they do get stuck sometimes. I plan on using 422 thread lock and not unscrewing them....

These tubes are really thin and not the best for supporting much weight for astrophotography cameras. They are also anodized shiny instead of flat black, not the best to suppress stray light. The also seize easily when screwed together. I should have spent more money and gotten a higher quality product.

Save yourself the headache and avoid these. The threads will lock together with such force that you will not be able to get them apart. The metal is cheap and the paint used is not flat black on top of it. In addition the 3mm adapter does not even fit onto the 5mm adapter. It's effectively a 5 mm adapter because you cannot fully thread it on.One of these is stuck in my filter wheel and basically fell apart before I could get it out.Just an absolute horrid product. AVOID

Worked for me, allowed me to get the camera into prime focus on the scope

Good range of sizes, I may buy another set. You can't have too many.

This product is well made and allowed for a perfect length for my optical train.

I have a few observations:Each adapter is in it's own Labeled plastic bag - the fact that it's labeled is very helpful and makes me lean towards storing it that way after I use it.The 30 mm adapter shaft felt smooth and thin - however - the threading on the male side outside threading not inside threading) felt rough.The 20 mm was fine, smooth threads on both endsThe 15 mm has a few rough spots on the threading and no label on it's bagThe 7 mm was good both sides5 mm had a rough spot on the inner threading3 mm I thought had a small rough spot on the inner threading as well -So, the rough spots concern me - because the walls of these adapters are so thin - integrity of the threads is a concern. I don't have thread repairing tools so if it doesn't "correct" if and when I screw them together - I might as well not have purchased this...I chose 4 star because my guess is not every adapter off the line will have rough spots, perhaps those spots have no consequence what so ever - but man these things are not thick.Do what you will with this info - I'm looking forward to trying the longer ones for sure

Dopo poche prove per capire gli anelli che dovevo utilizzare, la filettatura si è rovinata. Le 3 prolunghe più corte non si filettano più e ho rischiato di rovinare anche la camera alla quale li stavo collegando.