Instruction: Answer the given question using the supporting document.

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Question: I have the Canon EF 40mm f/2.8 STM Lens.. would I be better off with the Canon EF 12 II Extension Tube or the EF 25 II Extension Tube for macro work?
Document: Awesome product. Every single one of my canon lens worked in auto focus settings without issues including my favorite nifty fifty Canon EF 50mm f/1.8 II. Yes I have full APERTURE control. I bought and returned the cheapo name brand and non-brand name Macro extension tubes including those that claimed to work but don't. One of those EF-Mount Body Mounting Rings actually got stuck on my camera. Definitely a good addition to my bag.
My current setup for macro shooting is the Canon T3i and Canon's 60mm macro lens. If you plan on getting into macro shooting, be aware that extension tubes require more light (especially when stacked), so invest in a ring flash and/or tripod for reduced shutter speeds.I also have the EF 25 II extension tube from Canon. Both the 25 and 12 stack great with no issues in regard to losing AF and aperture control.
Canon quality construction. The extension tube has all the electrical contacts to connect the camera to the lense (auto focus,etc). I don't use the auto focus but instead manual. If you are stacking images manual is an excellent way to go. I am using the extension tube with a Canon T2i and a Canon 5D Mk III with a Canon EF 100mm Macro f/2.8 L lense. The extension tube gets you CLOSER. I am very happy with the tube.
I bought this extension tube to make closer macro shots ,Works great with the Canon 100mm  f/2.8L macro.Add 2x multiplierYes, I would recommend it.
In my book this is absolutely fine as long as what you are photographing is not moving and you have the time to make the necessary positional, exposure, and lighting corrections. Photographing jewelry in a static and controlled environment would be a good example.5. The Canon tube, more like a thick ring actually (TheCanon EF 25 II Extension Tube for EOS Digital Camerasis a bit more tube-like.)
So this item is for use while taking macro photographs or nearby objects ONLY.I've also uploaded a macro image in the gallery of this product, showing how well this macro tube works!As a side note, if you want to get even CLOSER to your subject, pick up the OTHER Canon extension, theCanon EF 25 II Extension Tube for EOS Digital Cameras, and combine the 12mm and the 25mm into a 37mm tube! It works VERY well!
This canon EF-12 ll  extension  tube is great,work exacthly as I expected. I also bought the canon EF 25ll extension at same time. I love them. Thanks.
Pros:You can get much closerWorks with both EF-S and EF lensesAuto focus still functions perfectlyTakes up very little room in your camera bagItem looks about 3 times as large in photo using 17-40 Canon F/4 LensCons:A little tight to put on and take off of lensCan no longer focus very far away let alone infinityReview:I purchased this Extension tube so I could take better Macro photos without having to purchase an expensive Macro lens.
Wanting to dabble in macro photography, but not wanting to buy an expensive macro lens, this extension tube was theanswer for me. The 12 mm tube is not so close up that one still has a reasonable depth of field.The 24 mm tube would seem to be too restrictive in depth of field for many uses (unless you only want to see the eye of an insect in focus).Very happy with this purchase, and would recommend.
I purchased this extension tube so that I could take close-ups of food using the wide end of my T3i 18-55mm lens. That lens only focuses to 25 cm (9.8") and in auto focus only to 28cm. But with the tube in place, at the wide end of the zoom I have to bring the camera within a couple of cm to the subject, which is far too close.
Answer: The 25 will get you a larger( closer) image than the 12. It really depends on what you want the image to be. I'm using the 25, But mainly on my 100-400 telephoto so I can focus closer on small wild life. My lens only focuses down to about 6 feet. the 25 lets me get as close as 2, Not really macro work but it suits my needs. Hope that helps.