Refocusing when using zoom lens & difficulty in taking video with 600d

[maisatomai]

Hi I have 2 questions.

1) If I use a 18-200 and focus an object at 18mm, will I need to refocus again at 200mm?
2) For example, when taking video (maybe the camera following my kid around) using my iPhone, everything will be sharp. If I use 18-200mm, I will have the problem that my kid face/surrounding area/other people face will be blurred. If I want everything to be sharp, will I be able to do it with the 18-200?

 

[Rashkae]

Hi I have 2 questions.

1) If I use a 18-200 and focus an object at 18mm, will I need to refocus again at 200mm?
2) For example, when taking video (maybe the camera following my kid around) using my iPhone, everything will be sharp. If I use 18-200mm, I will have the problem that my kid face/surrounding area/other people face will be blurred. If I want everything to be sharp, will I be able to do it with the 18-200?

The iPhone lens is a fixed focal length (3.85mm) lens with a crop factor of 7.64. That's super wide angle, your DOF is very very deep. For the 18-200, you will need to use a small aperture like f/16, but even then you may need to use AF-C. Not many lenses will maintain focus as you shift through the zoom.

 

[meganut]

Yes you will need to refocus if you are in manual mode.... if you are in auto focus mode then the camera takes a really long time to refocus (while you half press the shutter button) but if you ask me its just very slow, from in my experience...

BTW the canon auto focusing in video is not fantastic... Most will have to use manual focus in video for Canon DSLR....

For best results turn off AF and use MF when shooting in video... you will need to refocus after you zoom in or zoom out each time....

Hope this helps

 

[Octarine]

1) If I use a 18-200 and focus an object at 18mm, will I need to refocus again at 200mm?

That depends on the lens design. If your lens is a parfocal lens then it will retain focus. A varifocal lenses will not do so.

2) For example, when taking video (maybe the camera following my kid around) using my iPhone, everything will be sharp. If I use 18-200mm, I will have the problem that my kid face/surrounding area/other people face will be blurred. If I want everything to be sharp, will I be able to do it with the 18-200?

iPhone and smaller cameras have different lenses and sensors, resulting in much deeper depth of field up to the point where no focusing is actually necessary (early handphone cameras) because the depth of field ranges from 1m to infinity.
DSLR with their larger sensors and lenses of different focal length will have a much shallower depth of field (depending also on aperture) which is often preferred for object isolation and background blurring. The idea of 'everything sharp' is often used in landscape photography where a small aperture (f/11 +) is used. In portraits (like shooting a child) it's rather the opposite, one want to isolate the face from the background to emphasize it.