Let’s assume that I’ve focused on an object in the centre of the frame. That point is in focus, with the focus falling off in front and behind.

What happens to the left and right (and above and below) the object? Does the point of focus continue in a flat plane? Is it curved, like a sphere?

I know in practical terms it’s pretty much irrelevant, and will vary depending on lens / sensor, just curious what the clever folk of STW think.

Depends on the lens type.

You can get tilt/shift lenses that can change the plane of focus.

Off the top of my head…

Does the point of focus continue in a flat plane? Is it curved, like a sphere?

You are focused at a specific distance – so that distance forms a sphere around you with a radius that is the distance from the target object to your image sensor.

(Assuming a normal lens, not a tilt-shift or some other contrivance)

It’s flat…….just like the earth. 😉

Yeah, assuming a ‘normal’ lens, such as that is. I always assumed a sphere, but probably so big as to not make much practical distance when combined with DOF.

Seems to depend on lens type. On a macro photography course we were told that macro lens had a flat plane of focus…..

It’s a sphere from the sensor, so that might make a difference in some situations (long macro lens on something very close) but you can generalise to flat.

Don’t investigate tilt/shift lenses, they’ll make your head hurt. 🙂

Just consider it flat.  Take a picture of a brick wall with a low aperture and it will be in focus left to right and up and down.  The corners won’t be a sharp but that is a different issue.

On a macro photography course we were told that macro lens had a flat plane of focus…..

I’m purely guessing, but I’d guess that with macro photography it’s still curved but the tiny degree of curvature makes it flat for all practical purposes…?

It’s a sphere, you can focus on something in the middle of the viewfinder with a split prism (or half press for spot auto focus with a newer camera) , then frame it up with the subject off-center.

There’s probably some quirk with aspheric lenses, but those are correcting distortion near the edges so the normal rule should apply over most of the frame.

Also, think less of focusing on an object, think more about getting the object in focus and the stuff you don’t want out of focus. So determine the camera for hyperfocal distance for a given f-number and the subject can be anywhere past that and be acceptably in focus, or hypofocal and put the subject towards the back of that to get blurred backgrounds. If you focus dead on the subject and ignore the DoF then you’re relying on luck (and a fast lense) to get the background/foreground blured whereas you could setup so the subject was the first/last thing in focus.

Shirley, given the negative/CCD is a flat plane, it will be a flat plane that is in focus rather than a spere?

Aye I learnt about hyperfocal a while ago, I’ve never been much of a landscape photographer so it was actually quite interesting to learn.

The CCD / film IS flat, but the sharp focus point is based on distance from that point, hence a sphere. Plus the lens which does the focusing is curved, not flat.

“The PoF lies parallel to the sensor”

From http://www.photokonnexion.com/definition-plane-of-focus/

I’d say it’s flat, given ideal spherical lens elements, if the subject in focus is flat and the sensor is flat.  In this image, all light rays from any point on the line A-B (which is perpendicular to the axis) converge on the corresponding point A’-B’ (which is also perpendicular to the axis).

And if the subject in focus is flat, then this is another way of saying that the flat subject lies entirely in the focal plane which is therefore also flat.

The CCD / film IS flat, but the sharp focus point is based on distance from that point, hence a sphere.

Don’t think so.  Because the sensor is flat, the edges are slightly further from the lens than the middle.  Therefore the corresponding point in the focal plane is also slightly further from the lens.  Thereby creating a flat focal plane *because* of the flat sensor.

Not only that, but if it weren’t flat it wouldn’t be called a ‘focal plane’ would it?

Common sense at last!

Thanks Mollie 😀

It’s a sphere, you can focus on something in the middle of the viewfinder with a split prism (or half press for spot auto focus with a newer camera) , then frame it up with the subject off-center.

But in that case you are moving the axis of the lens.  So there, you are focusing on a point X distance away from your camera, and the focal plane is a tangent plane to the sphere described by the distance X from your camera.  Move the camera, move the plane.  Obviously, because you are moving the sensor.

Scheimpflug  can help you to  get apparent infinite depth of field but you need a camera with movements at the back

to tilt the film(sensor)plane

https://en.wikipedia.org/wiki/Scheimpflug_principle

Still waiting for affordable LARGE sensor for my view cameras

It’s curved, but you would never see it. You used to be able get flat plane lenses for copying work, but they were very expensive  and are now redundant with the change to digital scanning.

Still waiting for affordable LARGE sensor for my view cameras

Never going to happen, so stick to film or just do what I and countless others did and buy a Cambo Actus or similar and stick a Sony sensor on one end. (Be it in a Fuji, Sony, Phase, Hassleblad body).

except your old film lenses will not have the necessary resolving power and your gears/movements lack precision for critical focus.

All that talk about it being spherical me worried for a moment! It might slightly off a flat plane but certainly not equidistant to the focal point of the lens; as above, a wall would be pretty much all in focus.

Cool. Some interesting reading there, thanks all.

Speaking from practical experience rather than theoretical knowledge.

When I used to photograph large groups of people in poor light (so I had a fast lens wide open) if the line of people was straight the ones at the ends would be slightly out of focus. Arranging the subjects in a slight “v” shape with the ends closer to me would solve this.

When I used to photograph large groups of people in poor light

When I want to look at a large group of people in a poor light I just come to STW

That was kind of where I was coming from. This article is quite good:

https://petapixel.com/2016/12/21/field-curvature-tricky-problem-photography/

Speaking from practical experience rather than theoretical knowledge.

Ah.. well I said ideal lens. Practical experience doesn’t come into it 😉 I’m happy to acknowledge that a real world lens with its many elements could easily change things of course.

Good point about the ideal lens.  Also matters what focal length the lens is and what bearing that has, i.e. a 12mm lens has everything in focus due to the massive DoF combined with the way the lens is constructed/works.

FMC’s article says that the focal plane should.in theory be a plane, but the lens characteristics can change that. The example he shows is a lens with focus curved away from the camera at the edges not towards it.

So we’re all right.

That was kind of where I was coming from. This article is quite good:

Wee I didn’t know that, I always thought it was curved around the camera unless the lense was specifically planar.

Yeah, it was the wave distortion one that got me. Interesting stuff. Well, if you’re interested I guess.