I'm going to start my career in photography. I have a canon 70D plus prime lens which is 18 to 135mm. For portrait shot I'm not getting more blured background. So can you suggest me which lens is suitable for the candid and potrait shots 35mm, 40mm, or 50mm canon lens
2An 18-135mm lens is not a prime lens, it is a zoom lens. A blurred background is still attainable with this lens. See How can I get dramatic shallow DOF with a kit lens?– osullicNov 22, 2018 at 16:20
The higher the focal range, the more background compression you get. It also allows you to be further away from the subject, which seems kind-of essential for candid photos. I might not look too natural if you hold a 24mm prime right in my face. That's why I'd say 50mm. The Canon 50mm STM is a really great lens, especially combined with the 70D's really fast auto-focus. (Which is also a huge plus for candid since you wanna act and shoot fast)– confettiNov 22, 2018 at 20:04
18-135mm is not a prime lens. A prime lens has a fixed focal length.
The focal length itself depends highly on your taste but in any case you should buy a lens with a wide aperture (low f-number). This makes it easier to get blurry backgrounds.
I used to love taking portraits at 200-300mm (on a crop sensor like yours) which often resulted in images where the subject didn't even notice you, which is the most natural expression you can get. Now I'd recommend a 70-200mm/f2.8, which is expensive but you get a nice background blur and the quality is really good.
You will need 35mm or 50mm if you take a picture of the whole body or group photos and not only the face. Plus, you get more context if you use wider angles, because the viewer will understand what the scenery was and not only see a face with a blurry background.
I would not recommend 35mm AND 50mm. Buy one of those and use your feet.