I assume the "QualShot extension tubes" don't have electrical contacts, worse yet, they don't have aperture coupling lever.
If that's the case, and if you use them with a lens that doesn't have aperture ring, the lens will be at its smallest aperture. That is probably f/22 or even f/32. Coupling with the extension tubes, the effective aperture can be something like f/64.
No wonder the viewfinder is so dark.
To use with lenses that don't have aperture ring, or have aperture ring set at A, you need extension tubes that have electrical contacts. In this case, the camera can control the aperture.
Extension tubes with electrical contacts can be hard to find. It's easier to buy teleconverters with electrical contacts and remove the optics to use as extension ring.
Or you can buy an inexpensive manual lens (e.g. Pentax-M 50mm f/2) to use with the existing extension tubes. In this case, you can open up the aperture, focus, then close down the aperture using the lens's aperture ring before hitting the shutter release.
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