# Focus and recompose related question



## rpt (May 26, 2013)

I have been doing focus and recompose since I started to photograph! I mean with my old Lubitel what choice did I have? Same with AE1 and the 300D. Now I always thought that the focus is done along the radius. Is my understanding incorrect? Is it done on a plane perpendicular to the ray of light going through the center of curvature of the lens? I have seen a couple of articles on this but there is no real explanation there. They just say that the focus goes off a bit. Very early on, I would shoot at f8 or f11 or even f16 so I guess I never had an issue with DOF...

If you could point me to some articles that would help.

Thanks in advance.

Rustom


----------



## neuroanatomist (May 26, 2013)

http://www.visual-vacations.com/Photography/focus-recompose_sucks.htm


----------



## rpt (May 26, 2013)

neuroanatomist said:


> http://www.visual-vacations.com/Photography/focus-recompose_sucks.htm


Thanks. So what I infer from this article is that selecting an AF point other than the center point the camera computes the angle to the subject and derives the focus plane from that. Is that a correct inference? See none of the articles talk of this...


----------



## neuroanatomist (May 26, 2013)

Sorry, incorrect inference. There's no 'computation of angles' on a Canon camera. Using an outer AF point eliminates backfocus if it enables you to focus on your subject without recomposing, reduces backfocus if it means you don't move the camera as much to get the composition you want. 

I state 'Canon camera' because some Hasselblad bodies use a gyro sensor to detect camera rotation during recomposition, and adjust focus to compensate.


----------



## rpt (May 26, 2013)

neuroanatomist said:


> Sorry, incorrect inference. There's no 'computation of angles' on a Canon camera. Using an outer AF point eliminates backfocus if it enables you to focus on your subject without recomposing, reduces backfocus if it means you don't move the camera as much to get the composition you want.
> 
> I state 'Canon camera' because some Hasselblad bodies use a gyro sensor to detect camera rotation during recomposition, and adjust focus to compensate.


See, the Hasselblad thing makes sense to me. Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.

Thanks for the inputs.


----------



## neuroanatomist (May 26, 2013)

rpt said:


> Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.



Why does there have to be? The point is, with a fast lens shot wide open, don't focus/recompose. Use an outer focus point so you don't have to move the camera. With narrow apertures and/or distant subjects, the deeper DoF masks the backfocus, so focus/recompose is ok.


----------



## Pi (May 26, 2013)

F&R is usually not a problem with longer lenses at not extremely close distances. It is a problem with 24/35 in most real world cases, and sometimes with 50mm.

One technique that works quite well is to use the rear AF button in AI Servo mode, and the outer AF points. This increases their precision tremendously, at least on my 5D2. The precision is similar to the center AF this way, with apertures like f/1.2-f/1.4.


----------



## rpt (May 26, 2013)

neuroanatomist said:


> rpt said:
> 
> 
> > Surely there has to be some computation of angles internally to identify the focusing plane in order to eliminate the back-focus.
> ...




Sorry, I don't doubt that it needs to be done. The programmer in me is curious about how it is implemented thats all.


----------



## Sporgon (May 26, 2013)

I know this is a little off topic, but I have always found the single plane sensitive AF points (as found on the 'outer' points of many SLRs) quite beneficial. Over the years I have often found situations where the x type sensor can become confused, yet find a vertical line ( or horizontal depending on the AF point you've chosen) and the camera can nail focus. 

So I'm quite happy that the 6D along with the 5D mk1 and mk2 has this type of outer sensors, and am quite happy to use them for accurate focus, and to avoid 'focus and re compose'. It just means you must choose the appropriate line to focus on.


----------



## alexanderferdinand (May 26, 2013)

Although the 5d2 had a great IQ, the not so good outer AF- fields and focus/recompose sometimes drove me nuts.....


----------



## neuroanatomist (May 26, 2013)

rpt said:


> neuroanatomist said:
> 
> 
> > rpt said:
> ...



Maybe I'm misunderstanding - are you suggesting that when an off-center AF point is selected on a Canon dSLR, the camera is performing some sort of algorithmic focus correction for the point being off-center? I don't understand...


----------



## chauncey (May 26, 2013)

That focus point represents a set distance...anything the same distance will be in focus...anything of a different distance will not be in focus.


----------



## ZoeEnPhos (May 27, 2013)

neuroanatomist said:


> http://www.visual-vacations.com/Photography/focus-recompose_sucks.htm



+1 Big THANK to you Neuro! Illuminating article! Enjoyed to read the content!/All the Best to you Neuro!/C


----------



## sanj (May 27, 2013)

When you focus and re-compose, you change the focus distance and focus goes off slightly.
Also, how does one use 'focus and recompose' when tracking focus? 

It is so important for all who photograph moving objects and large apertures to become totally at home with focus points and back focus. I am getting many more keepers now.

Regards


----------



## Singsling (May 27, 2013)

This Guys red line/green line theory is correct, but focus the intended area first i.e. her face and then recompose to include the dress. Result pin sharp where you want it to be on her face with the dress slightly out of focus. In other words put the cart before the horse.


----------



## neuroanatomist (May 27, 2013)

Singsling said:


> This Guys red line/green line theory is correct, but focus the intended area first i.e. her face and then recompose to include the dress. Result pin sharp where you want it to be on her face with the dress slightly out of focus. In other words put the cart before the horse.



Might want to re-read that article. Your 'method' is exactly what he's demonstrating. Focusing on the eyes is the red line, recomposing to include the dress is the green line. Result eyes slightly out of focus, dress pin sharp. In other words, your suggestion is why focus recompose sucks.


----------



## adhocphotographer (May 27, 2013)

Thanks for the info all, i never thought about this... I will certainly not F&R during portraits.


----------



## sanj (May 27, 2013)

Singsling said:


> This Guys red line/green line theory is correct, but focus the intended area first i.e. her face and then recompose to include the dress. Result pin sharp where you want it to be on her face with the dress slightly out of focus. In other words put the cart before the horse.



I checked several times but there were no horses or carts or cowboys in the article.


----------



## sanj (May 27, 2013)

And the example is of a camera on tripod. When handholding the movement of the photographer compounds the problem am sure..


----------



## rpt (May 27, 2013)

*Ting!* well looks like the penny finally dropped 

So I read a few more articles and all of them refer to a plane of focus. So here is my new (and hopefully correct) understanding. The lens is shaped such that the focus of an image on the sensor or film is done for a plane parallel to the plane of the receptor. Thus the focusing distance indicated by the lens is the perpendicular distances between the two planes. See, that was my problem - I always thought that things would be in focus on an arc whose radius was the focusing distance. So in that case F&R would have worked - and it did for me from '67 till 2000 as most of my outputs were either contact print of 120 film or 5 by 3 or 6 by 4 prints. 

Neuro, thanks for persisting 

Others, thanks for the inputs.

So I had better learn how to do back button focus and selecting the appropriate AF point...


----------



## neuroanatomist (May 27, 2013)

rpt said:


> The lens is shaped such that the focus of an image on the sensor or film is done for a plane parallel to the plane of the receptor.



Correct - the lens is designed to project a flat image onto the image plane. However, not all lenses are perfect - when a lens doesn't project a flat image, it's called field curvature. The 24-70/2.8L (MkI) has that problem.


----------

