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I am insterested in buying a new Macro lens for my sony a7s II. The only thing is that the lens I want Rokinon 100mm macro for the emount is around $500 and the one for the K-mount is around $350.

What will happen if I buy an adapter to the K mount lens and use it that way. Will the picture quality be affected. If so, by how much. What would I be losing or risking by doing that? I am using the lens mainly for closeups in videos.

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If you have your heart set on that particular lens, go for it! That's exactly what adapters are built for, after all. I have a Pentax 50mm F1.7 that I use with a K-Mount to EF-Mount adapter on my EOS 80D, and I love it.

However, there are a couple of considerations.

Image quality

The image quality should be largely unaffected. However, the adapter you use is an additional piece of metal/plastic between camera and lens that the manufacturer did not plan for. This might ever so slightly reduce the cameras speed, but in practice, I don't think you will even notice that. Another consideration is that even the smalles of adapters functions like an extension ring. This means it might slightly shift the focus range towards the camera. This might reduce the minimal focusing distance. In turn you might lose the ability to focus on infinity.

There are a couple of might's in there, as it all depends on the specific lens and mainly on the quality of the adapter. The first cheap adapter I found claims to allow lens focus to infinity, so it doesn't seem to be much of a problem in pracitcality. I wouldn't worry about those things too much.

Technical limitations

Depending on your filming habits, this might be a larger problem for you. Most (if not all, not quite sure) adapters you will find are purely mechanical, meaning there is no electronic communication between the camera and the lens. This has a few effects:

  • No autofocus (mostly). You might have to focus manually. My K-Mount to EF-Mount adapter has an autofocus confirm chip. With this, my camera will confirm whether my selected focus point is in focus when I press the AF-button. So I can manually focus and then check if it's focused properly by pressing the AF button. I only use that lens for photography though, manual focus only will probably be more of an issue if you're using the lens for filming.
  • Manual only. If the camera can't communicate with the lens, it can't control the aperture as well (nor know what aperture you set the lens to). So you have to manually set the aperture. This might also throw off your camera's auto exposure, so you will mostly be limited to using your camera fully manual.
  • Missing or wrong EXIF data. No communication means all the lens-dependent parameters such as lens model, focal length and aperture won't be recorded into your EXIF data / metadata. Some adapters have an EXIF-chip that present the camera with static info. My own adapter tells the camera that a 50mm lens with f/1.4 is mounted, independent of what lens I use it with or what aperture I set it to. This means, for example, that I won't be able to find out what aperture a particular shot was taken with afterwords. Depending on your editing needs and habits, this might either not be a problem at all or utterly disrupt your workflow.

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