Reversing ring with AF confirm chip

Hello,

I'm using Reversing Ring with Quantaray 28mm, it works great with a huge magnifying, but so hard to get right focus. They sell Af confirm chip for Canon very cheap (around $5), is it possible to attach that chip on Reversing Ring to confirm right focus?

Any idea will be appreciated,

Personally I would not bother, I don't use the AF confirm at all with the MP-E 65mm or any other macro lens, just judge it off the standard issue focus screen.

I used to use a reversed 28mm lens in my film days, my memory was this was hard to use with very limited working distance and that was with an auto-stop-down cable release, if you are trying to focus stopped down that will make it much harder. I don't know how much magnification you are getting but I would guess about 3:1.

If you can afford it look into the MP-E 65mm, it is a breeze to use in the field even giving 1:1 to 5:1. Alternatively get a ~65mm stack of tubes to use with your 100mm, this will give about 2:!.

Actually the AF confirmation lights do not work at all with the MP-E 65mm -at least not on any of the Canon bodies that I've used my lens on.

Another way to get better than 1:1 is to use Canon's EF-S 60mm macro lens. At minimum mag it's actually a 37mm lens, so you only need 37mm of extension tubes to get to 2x, and a full set of Kenko tubes will get you to almost 3 (about 2.8x). As an extra added bonus you can even use the EF-S 60mm on a full frame body as long as you add a 12mm extension tube to it. You'll lose infinity focus, but it's cheaper and lighter than the MP-E 65mm. I used it on my old 1D MKIII with no issues. The AF indicators will work, but I agree with Lester -you should ignore them and simply look for textures in the area that you want to be in focus. If you rely on the AF indicators you'll be centering the subject in your photos most of the time and your compositions will suck.

I use Vello Macrofier Reverse with 17-40mm, much more light come to viewfinder and magnitication much better (I do not know how many big I have) but it so hard to get focus, IQ not sharp, I think I need more light come to object like LED or flash light

A reverse 28mm is tough to use, i have a pentax k 28mm, the field is not flat so you have to look dead center to focus. Working distance is only a few inches and f8-f11 will be needed to get a proper dof in most cases. The pentax has a bayonet which can be pulled with your finger to open to f2.8. I used this to focus and then take burst shots inwards and outwards usually keeping only if the subjects eyes were in focus. You will absolutely need a flash also.