[rankinelo]

s82011190 CHing,

Your method is incorrect for large aperture lens...


Take a simple example, consider a right-angle triangle with length of edges 3m, 4m and diagonal 5m...

when you focus on the eye, you are using the focal distance of the diagonal of the triangle of your "final composition", that is 5m

after your recomposition of your photo, your correct focal distance is the edge 4m, but you are still using the "5m" focal distance... this is the "back focus"
So Marksman01 moves back 1m to resolve this issue to match the focal plane with correct focal distance...

[Marksman01]

s82011190 CHing, Rankinelo C Hing explained correctly.

With consideration of the DOF, your method can be applied on shooting distance with 3 meters or further (take 50/1.2L @ f/1.2 as an example), since the DOF will cover the geometric change during "focus and recompose" process in a long focus distance.

If you shoot within 3 meters to MFD, the DOF will be too thin to cover the above process, image that when we use 50/1.2L or 85/1.2L shoot at f/1.2 in MFD, the DOF is thin like a paper.

[edwardchantk]

thanks~~~rankinelo CHING & Marksman01 CHING
i alway have the problem of (back focus)
i'll try to solve this problem with ur advise

[Marksman01]

Edward C Hing, it can only apply "baby don't move", taking moving baby in MFD is nearly impossible on wide open (faster than f/2) :-)

[s82011190]

If 我將部機於同一垂直面移動以作構圖，即可保持focal length不變，這樣是否可行？

[rankinelo]

s82011190 CHing,

>>同一垂直面移動以作構圖
All direction of recomposition will have the same issue... the triangle in the example above is in a 3D world...

You can draw the triangle on a paper... 3m, 4m and 5m (just for the example for easy understand).
the diagonal (5m side) connecting the model's eye and your camera, the 4m side is perpendicular to the focal plane and your camera.

The Center of your recomposition and the Eye of the model are on the same focal plane

From TOP view:

(Center of your re-composition) -- 3m --- (Eye of model)
|
|
|
4m
|
|
|
(Your camera)

[rankinelo]

From TOP view (after recomposition)

(Your Focal Plane without adjustment)
|
1m
|
(Center of your recomposition) --- 3m --- (model's eye)
|
|
|
4m
|
|
|
(Your camera)

------
So you can see why your image is back focus if you don't move backward as suggested by Marksman01 CHing

[s82011190]

How about?

(Centre of recomposition) --------------- (model's eye)
I I
I I
I I
I I
(Camera to shoot) <-----------------------(Camera focus)

[rankinelo]

s82011190 CHing,

If you use 1-pt (center) to focus on model's eye, and model's eye is your center of composition, then no adjustment is needed. Otherwise you may move yourself / your focal plane in order to make the focal plane = focal distance.

Hope that it helps.

[Marksman01]

<<If 我將部機於同一垂直面移動以作構圖，即可保持focal length不變，這樣是否可行？ >>

Yes, it is okay too, I sometimes do it.

You guys have wonderful graphic !!!! All correct presented !!

[Marksman01]

In actual shooting, just a little bit backward, say 1-2cm to several cm.

The graphic shown is for show concept.

[edwardchantk]

so complicate ~~~
@_@
just like PHYSIC lesson

the most important thing is.............try to keep focal length not change??
or i make misunderstand?

someone told me to turn the focus ring a little bit after composition to adjust the focus
but i feel it much more difficult in fact.......

[rankinelo]

edwardchantk ching, ur method is correct as well... Practise makes perfect...
Enjoy shooting!

[Marksman01]

should be - keep the focus distance not change

shoot wide open on a super fast lens is a challenge ^_^

[edd813]

新手

http://lh4.ggpht.com/_p2jlRDaLdi4/TA0Q9KfiI9I/AAAAAAAAIVs/0P-fQCTFUw4/s576/20D_2010_05_15_0092.JPG