Possible to recompose without messing up focus plane?

[MTVision]

Dude...I think you're getting focal plane and dof mixed up..or you're letting dof mess with your understanding of focal plane..our something...

LOL! It was a joke. I just wanted to see what he/she was going to say!

 

[analog.universe]

Assuming the same distance, the corners on a wide angle shot would be 'behind' the corners on a normal or tele shot. That is why the corners aren't as sharp as the center. The corners are not on the same plane that the focus point is on.

There is really only one distance in which the corners would lie directly on the plane of sharpest focus, and that would vary from lens to lens. In all focus distances expect for that one, the plane of focus will never intersect the corners of the frame.

This is why I said, in cases of the LONG focal tele lenses, focal plane is CONSIDERED as flat since the lenses have narrow FoV. But for short lenses which have wider FoV, focal plane cannot be considered as flat.
Understanding that, one now can know why focus-recompose isn't messing up the focal plane. And it doesn't suck at all!

The focal plane is actually flat. Some lenses don't manage to achieve it perfectly, sometimes the sharp edges are farther away than the flat plane, sometimes closer, but lens designs generally try to be flat. (hence names like "planar", known for the flatness of it's plane)

You're likely never to find a lens where the area of sharp focus is equidistant from the sensor. If such a spherical equidistant field was the norm, there would be no difference between recomposing on a wide lens and tele lens. When thinking about focus and recompose, a flat plane of focus is the only geometry that allows you to make the proper compensations. (unless of course you know exactly how not-flat a specific lens is at a specific focal length and focus distance)

 

[bhop]

Dude...I think you're getting focal plane and dof mixed up..or you're letting dof mess with your understanding of focal plane..our something...

LOL! It was a joke. I just wanted to see what he/she was going to say!

I meant to quote 'AperturePriority'

 

[MTVision]

I meant to quote 'AperturePriority'

Well it could've gone for what I wrote too. I almost said something about you calling me Dude since technically it'd be Dudette

 

[dumeril7]

The focal plane is actually flat. Some lenses don't manage to achieve it perfectly, sometimes the sharp edges are farther away than the flat plane, sometimes closer, but lens designs generally try to be flat. (hence names like "planar", known for the flatness of it's plane)

You're likely never to find a lens where the area of sharp focus is equidistant from the sensor. If such a spherical equidistant field was the norm, there would be no difference between recomposing on a wide lens and tele lens. When thinking about focus and recompose, a flat plane of focus is the only geometry that allows you to make the proper compensations. (unless of course you know exactly how not-flat a specific lens is at a specific focal length and focus distance)

Thinking about this last night, the flat plane notion makes the most sense. Being one of the folks who was confused on plane of focus concept, please tell me if I have this right...

Hypothetically taking the design of the lens out of the equation, I would expect there to naturally be a sphere of focus. But it also seems to me that a sphere of focus is far less pragmatic than a plane of focus, since we tend to arrange our world in straight lines rather than circles and curves. So I would think that one important but difficult job of the lens is to try to flatten the focus space so we can take photos and have them be in focus for a majority of cases. (I.e. Its a plane because the lens tries to make it so.) And this is why it is considered a defect for a lens to have poor sharpness from corner to corner and why lenses that are sharp across the entire frame are expensive.

Do I have that right?

D7

 

[EchoingWhisper]

Let me add - try using Dynamic Area AF. If your AF system is good enough, dynamic area AF may be able to keep the focal plane the same.

 

[Demers18]

I think a lot of people don't really know exactly what a plane of focus is.

This is probably my most posted link. Hopefully it will enlighten a few people:

Why Focus-Recompose Sucks

That article has the quote of the year:

"the fact that depth of field at 50+ yards distance is usually at least a few feet; even an inch of error in that case is about as significant as a fart in a tornado."

 

[analog.universe]

The focal plane is actually flat. Some lenses don't manage to achieve it perfectly, sometimes the sharp edges are farther away than the flat plane, sometimes closer, but lens designs generally try to be flat. (hence names like "planar", known for the flatness of it's plane)

You're likely never to find a lens where the area of sharp focus is equidistant from the sensor. If such a spherical equidistant field was the norm, there would be no difference between recomposing on a wide lens and tele lens. When thinking about focus and recompose, a flat plane of focus is the only geometry that allows you to make the proper compensations. (unless of course you know exactly how not-flat a specific lens is at a specific focal length and focus distance)

Thinking about this last night, the flat plane notion makes the most sense. Being one of the folks who was confused on plane of focus concept, please tell me if I have this right...

Hypothetically taking the design of the lens out of the equation, I would expect there to naturally be a sphere of focus. But it also seems to me that a sphere of focus is far less pragmatic than a plane of focus, since we tend to arrange our world in straight lines rather than circles and curves. So I would think that one important but difficult job of the lens is to try to flatten the focus space so we can take photos and have them be in focus for a majority of cases. (I.e. Its a plane because the lens tries to make it so.) And this is why it is considered a defect for a lens to have poor sharpness from corner to corner and why lenses that are sharp across the entire frame are expensive.

Do I have that right?

D7

It's actually more a question of symmetry as it turns out. The focal plane is flat because the sensor is flat, that's just how the geometry plays out. It is more useful in practice, I agree, but the math happened to make it flat before we got that far. Sometimes it is the corrections for other issues (CA, nonrectilinearity, etc..) that causes the deviation from the flat field.

In large telescopes, where the aberrations in the optics are harder to correct for, often the sensor itself is a mild spherical section. This way the optics can be allowed some field curvature, which is compensated for on the other side of the glass by the sensor shape.

In the most basic hypothetical scenario though, a single element (most camera lenses have 10-20 elements) mathematically ideal lens will focus a flat plane if the sensor its projecting on is also flat.