Some M-42 lenses are fully manual only. That is when you select an aperture smaller than the maximum. the lens will stop down to that aperture. This makes things great for mirrorless users but was a bit of a PITA when originally using the lenses on the film cameras that they were designed for. You needed to open to maximum aperture to focus and then manually close down the lens to the shooting aperture. In doing this you needed to either look at the aperture or count the clicks as you closed down the lens. Since we do not need to open the aperture to focus on a mirrorless camera like the Sony A6xxx line, these lenses work just fine... The next evolution was a preset lens. You could select the aperture on one ring and that would be your shooting aperture. You could then open and close the lens without looking at the aperture ring or counting clicks. The lens would not stop down further than the selected aperture. Selecting the aperture manually on a preset lens is also no problem... Then along came "automatic" lenses. This has nothing to do with automatic exposure, it's simply that you selected the shooting aperture and the lens would stop down to that aperture as the shutter opens and then open up to the maximum aperture for focusing the next shot. This was really the "cat's pajamas" and IMO paved the way for the popularity of the single lens reflex camera. The problem with an automatic lens in the case of a mirrorless camera is that the camera has no way to shut down the lens unless your lens has an Auto/Manual selector. If it does, simply select Manual and you are home free. If it does not, you are stuck with shooting wide open, no matter what aperture you have selected! Unless you work around it! The aperture is controlled on most M-42 lenses by a pin which protrudes from the rear of the lens. The camera will press down on that pin which will close down the lens. There are ways to modify a lens like this to be fully 100% manual but, they require physically unscrewing the back plate of the lens and modifying the aperture pin. Some folks on YouTube even advocate using Super Glue to keep the aperture pin depressed. These methods could possibly ruin any collector's value of the lens. The absolutely easiest way to obtain full manual operation is to purchase an M-42 to e-mount adapter that incorporates a flange which will keep the pin depressed and allow operation of the aperture with the aperture ring. One of these is the Fiodox adapter: https://smile.amazon.com/gp/product/B0054EORJC/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1 While another is the K&F Concept adapter: https://smile.amazon.com/dp/B07JFSQ...&pd_rd_r=8d5e4285-7075-11e9-bb1d-335c9152e117 My Kipon x0.7 Focal Reducer will also allow you to use the aperture ring to stop down the aperture... https://smile.amazon.com/Kipon-Bave...+Camera&qid=1557254698&s=gateway&sr=8-1-fkmr0 Note... I purchased my copy used on eBay for considerably less than the amazon.com price. It works great and removes the worry about crop factor. A 135mm lens will give you a 135mm focal length (give or take a few mm).