Haroon - this is primarily a still photography website, so I would suggest you might find more information on a video website.
What you are looking at here is a video technique that I believe is called "pulling focus" and is done with a geared follow focus setup on the camera and lens. This allows a smooth manual change in focus. You start with a shallow depth of field lens and focus on one part of the image while the camera is running and end up focusing on the second part.
Here is what they look like:
https://www.bhphotovideo.com/c/produ...ma_camera.html
One problem with trying to use any focus changes with still camera lenses on DSLR camera is the very short throw between close-up and infinity focus. There simply is not enough distance to make the transition smooth and even.
Although the tool that Manfred suggested is really good, there are some other, less expensive, focus aids that also assist in smooth focus changes using the still camera lenses. Here is one of them.
This allows a longer focus shift using the extension rod. The focus shift can appear smoother than trying to use your fingers on the lens' focus ring.
NOTE: I am not recommending this specific Adorama product. I am simply showing this one as an option...
Last edited by rpcrowe; 6th December 2016 at 05:26 PM.
I have a version of this rig but, I have never used it. I got it as part of a freebie offer from Adorama when I purchased my 100-400mm f/4.5-5.6L IS II lens.
Note: Sometimes the freebie offers are just a come-on, aimed at including some trashy products in order to boost the price of the main product.
OTOH: When working with a product like a Canon lens which Canon USA will not allow a company to discount, the freebie offer simply is aimed at you purchasing the unit from the company offering the freebie. The price was the same from companies that were not offering the freebie kit as well as the came price from Adorama whether or not you included the kit.
The kit included a lens cleaning kit, a really neat folding lens shade, the focusing extender (like the one above) and a set of 77mm UV, CPL and ND filters. I sold the kit of filters for $20 on eBay...
Hey haroonifti, as everyone else has mentioned - this is called "pulling focus". You can actually do this with any lens - given:
1. You can open the aperture up enough to compress your focal plane OR get your subjects far enough away to achieve the same effect (it's certainly easier to do on a telephoto).
2. Go manual focus all the way. Lay a piece of board tape or masking tape on your lens and mark the point at which each subject is in focus. Practice the move over and over.
3. Shoot it (sorry, had to add that).
It takes some practice, but is a lot of fun once you nail it!
Happy shooting!