General notes on using adapted lenses
Using adapted lenses on your modern digital mirrorless camera is simple, cheap and easy! But there are a few points to bear in mind…
In general mirrorless cameras provide by far the most versatile platform for using lenses adapted from old manual focus SLRs (and many much weirder lenses too!). The is because the distance between the lens mount and the sensor (often referred to as the flange distance) is much less than the equivalent distance on an SLR. This means that the thickness of the adapter won't prevent you from focusing all the way to infinity. Having said that, if you use a DSLR instead there are still opportunities for playing with vintage lenses… in particular M42 screw mount lenses can be easily adapted to almost many SLRs, digital or film.
The adapters you use to mount classic manual focus SLR lenses on mirrorless cameras are really just simple metal rings with few moving parts and no electronics. The are often referred to as 'dumb adapters' as the don't communicate with the camera in any way. There is a huge range of dumb adapters, ranging from those made by the camera makers themselves and prestigious brand such as Novoflex and Voigtländer to inexpensive unbranded adapters purchased direct from small independent manufacturers in China on eBay. The former can run to well over £100, while the latter can cost less than £5 (but you'll have to wait a few weeks for them to be delivered from China!). In between there are a number of brands (e.g. K&F Concept) that make good quality but reasonably priced adapters that can be purchased from Amazon or eBay in the £20-40 price range. I've never had any problems with even very cheap adapters, so I generally wouldn’t pay more than £30 for a simple adapter, and I'm not shy about using inexpensive unbranded adapters, particularly for screw mount lenses.
One thing to look out for with adapters for screw mount lenses (either M42 SLR lenses ot L39 rangefinder lenses) is some way to adjust which part of the lens faces upwards. The lens will work just the same whichever part of the lens barrel faces upwards, so don't worry about this too much, but if the aperture and focus indexes point downwards it can be a somewhat inconvenient. Some adapters come with an alan key that lets this be adjusted.
Because dumb adapters are so simple they are economic for small independent companies in China to make in very low quantities, so even very obscure lens mounts can be catered for.
Some digital cameras are set up by default to lock the shutter button if there is no lens attached. Makes sense, right? What kind of idiot wants to take photographs without a lens?!
The problem is that lens adapters for old manual focus lenses have no electronic communication with your camera so you camera won’t know it has a lens attached. In fact some of the experiments described here do indeed involve removing the lens from your camera all together (I’ll just let that sink in for a moment…)
This means that while you're certainly not the kind of idiot who would shoot without any kind of lens at all, your camera may think that you are! Fortunately cameras set up in this way usually also have a menu option that tells your camera to keep its opinions about your idiocy to itself and let us do what we want.
In general, on all the cameras I have ever used, by far the most useful focusing aid for manual focus lenses is the simple viewfinder magnification mode. Again check Google or your manual to find out how to activate this, and then assign the magnification function to your favourite custom button to keep it easily available.
I generally leave my camera in the 'P' exposure mode, set whatever aperture I want on the lenses aperture ring and then let my camera worry about the shutter speed.
Over the last few years I have used Sony, Fujifilm and Olympus mirrorless cameras, and Pentax DSLRs. So I can offer a few tips on setting up cameras from those brands. For other brands Google will help you out!
Sony is one of the brands that requires you to use a special menu option to tell the camera to shoot without a lens. If you have a Sony A7 Mark III or earlier look in page 4 of the settings menu (with the cog icon). If you have a Sony A7 Mark IV or later look in the Shooting→Shutter/Silent menu. If you have any other Sony camera just Google [Sony model name] Release w/o Lens and change "[Sony model name]" to match your camera's model.
On a Sony cameras with built-in image stabilisation you can tell the camera the focal length of the lens to ensure the image stabilisation is optimised. To do this on the A7 MkII Find the camera tab (with the camera icon) in your camera's menu, go to page 7, and select the 'SteadyShot Settings' item. Then select the 'SteadyS. Focal Len.' option.
Fujifilm cameras also requires you to use a special menu option to shoot without a lens. On the Fujifilm X-E1 I owned a long time ago there was an option called ‘Shoot without lens’ in the Shooting Menu, which was the very first tab in the menu system. It may well be in a different location on more recent Fujifilm models, but it will always be called ‘Shoot without lens’. Again Google should be able to help you find the location. As usual Google can help: Try a searching for fujifilm [insert model number] shoot without lens.
Olympus cameras don't require you to set a special menu option to shoot without a lens. So you can just mount the lens adapter and lens and shoot, but there are a few settings that will help. Just like on Sony cameras, you can set the focal length the camera will use to optimise the built-in image stabilisation. The location of the 'Image Stabilizer' option varies with the model. On my older cameras (i.e. my Pen E-PL3 and OM-D E-M10) look for the 'Image Stabilizer' option in the second of the 2 camera menus (with the camera icons). On my more recent Pen-F you'll find it in the 'C Release/Drive mode/Image Stabilizer' submenu of the 'Custom menu'.
More recent Olympus cameras go further: on my Olympus Pen-F you can set up profiles for up to 10 manual lenses and which you can easily select when you change lenses. To do this go to the 'Custom Menu' (with the cog icon), find the 'K Utility' or submenu, and then look for the 'Lens Info Settings' item. You can enter not just the focal length of the lens (to optimise built-in image stabilisation) but also give the lens a name which will appear in the EXIF information of your photographs. I assign the 'Lens Info Settings' menu to one of the function buttons so I can change it easily when I change lenses.
Pentax DSLRs, like any other DSLR, are not particularly suitable for adapting lenses from other non-Pentax mounts. But of all the DSLR brands, Pentax has gone to by far the longest lengths to make their modern DSLRs work with their older manual focus lenses. You don't need any kind of adapter to use any Pentax K mount lens made since 1975 and your camera will automatically prompt you to set the focal length to optimise built-in image stabilisation. What's more your camera's metering will work perfectly.
With older lenses that don't have the 'A' setting on the aperture ring just put your camera in 'M' mode, select the aperture you want on the lens and then press the Green button. You camera will then stop down the lens, take a meter reading and automatically set the required shutter speed (and ISO if you have ISO set to auto). You are then free to change the shutter speed as you desire.
And because Pentax was such a big supporter of the M42 screw mount you can also get a similarly smooth experience using M42 lenses using Pentax's own M42 to K adapter.
With slightly more recent A Series lenses (introduced in 1983) that feature the 'A' setting on the aperture ring thingss are even easier: set you lens's aperture ring to the 'A' setting and you have access to all automatic and manual exposure modes and all metering modes. Your lens won't magically become an auto focus lens, but otherwise you can use your camera pretty much as if you had any modern lens on it, and focus confirmation will still work to make light work of focusing.
This is one of the reasons I love Pentax DSLRs! For even more information on using classic Pentax lenses on modern Pentax DSLRs see this article on Pentax Forums: Manual Focus Lens Choices for Pentax