Lock exposure, focus and refocus

[Perspectics]

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One last example to try and make this clear. If you shoot a brick wall straight on wide open -are the bricks at the edges of your photograph in focus?

 

If your lens focused on a radius instead of a plane only the center of your image would be in focus and as you moved away from the center in all directions you would get progressively more out of focus. But that doesn't happen. With some lenses you do lose some resolution at the edges because the lens is not focusing perfectly flat but that is considered a lens fault and it is kept to a minimum with most modern lenses.

 

Perfectly correct. A test-shot with two rulers can been seen here, you´ll have to enlarge to see: L1000690 on Flickr - Photo Sharing!

The left ruler is on the (correct) plane of focus, while the right one is on the arc (at the "13" mark).

Thanks much, Hank!

[jaapv]

Hey guys! You're confusing the issue now. If one rotates the camera with the camera as a pivot any given point on the plane of focus will move along a curved path. That has nothing to do with brick walls.

[hankg]

Hey guys! You're confusing the issue now. If one rotates the camera with the camera as a pivot any given point on the plane of focus will move along a curved path. That has nothing to do with brick walls.

 

 

Not confusing it at all. All the illustrations I posted shows the effect of the plane of focus moving along that radius. The plane remains a plane rotated to a different angle. The brick wall illustration is to demonstrate that a lens focuses on a plane not along a radius or sphere. It does not need to be complicated.

 

And has been pointed out it's only an issue with a narrow DOF.

[ChrisC]

......One thing I've read completely diverging opinions on is how many modern lenses really have a flat focal field. Some people say most modern lenses are designed this way but others say it's actually quite rare due to focal shifts and field distortions.......

 

When I described two schools of thought [radial focus, plane of focus] I too had such diverging opinions in mind. But in truth, in 30+ years of making photographs no such doubt has ever hindered me when making a photograph.

 

................. Chris

[jaapv]

It does not need to be complicated.

 

And indeed it is not. But I fear we have confused a few

[fursan]

Thanks folks and excellent mental gymnastics for the mathematically and photgraphic

Tarzans.

 

Unfortunately I am mentally challenged ( not retarded mind you.. i did buy an m8! ),

I am not one for gymnastics either, maths..metric system only, photography..struggling

with it.

 

Now.. when you say recompose..what do you mean..no maths pls. english i speak but

don't understand..so please kind folks help out a lost soul see the tree sharp!

[Guest guy_mancuso]

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When I described two schools of thought [radial focus, plane of focus] I too had such diverging opinions in mind. But in truth, in 30+ years of making photographs no such doubt has ever hindered me when making a photograph.

 

................. Chris

 

Thank You. you guy's confused the hell out of me and I know better. ROTFLMAO

[guy]

Use the force, Luke.

[hankg]

Thanks folks and excellent mental gymnastics for the mathematically and photgraphic

Tarzans.

 

Unfortunately I am mentally challenged ( not retarded mind you.. i did buy an m8! ),

I am not one for gymnastics either, maths..metric system only, photography..struggling

with it.

 

Now.. when you say recompose..what do you mean..no maths pls. english i speak but

don't understand..so please kind folks help out a lost soul see the tree sharp!

 

One more illustration to add to the confusion (I think in pictures). You want the person in front to be in focus but you want him at the edge of your picture. The RF focusing spot is in the center so you have to point the camera at him to get focus and then move the camera to frame the picture the way you want.

 

Now I have to get back to work. But you can carry on without me.

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[jlm]

yet another twist:

 

using hank's picture; if you had a view camera:

1. you could focus on the near person, then, using a shift, recompose with no focus change.

2. if you focus on the near person and use a lens tilt to recompose, you would have to refocus either by moving the lens to film distance or by a tilt of the film plane