Need help with focusing issue.

Started 2 months ago | Questions thread
Truman Prevatt
Truman Prevatt Veteran Member • Posts: 9,702
Re: Can’t say I agree...

Jerry-astro wrote:

Simon Elwell wrote:

This is typical! You use the focus and recompose method - a tried and trusted approach for many photographers. All but one of the responses so far have told you to change your technique - and use *their* approach - also tried and trusted.

But why should you have to?

You don't. Focus and recompose is not only a perfectly acceptable way of focusing and framing - but it works just fine in Fuji cameras - as one poster pointed out - you have to get the settings right.

Except for one little problem... that’s not necessarily true. Portraits are often taken at large apertures with very thin DOF in order to isolate the subject. At those apertures, even a slight focus miss can throw your image OOF. The camera focuses at a fixed range from the camera/lens, which means that the precise focus point is not linear, it’s actually circular — an arc at a fixed distance from the camera. With very thin DOF, moving the camera even a little after establishing focus can throw the location you’ve moved the camera to outside the zone that is in focus. Granted, this may be unlikely unless you’re shooting at very large apertures, but it can definitely happen... and it’s happened to me in the past.

This all true for highest quality flat field lenses. I would not place the 16-80 in that category. In reality there are very few lenses that are truly flat field. It is difficult to design a lens and eliminate field curvature. Focus recompose actually is the best way to focus on a spherical surface.

A single lens will focus all objects from the same distance - independent on the incidence angle. That is the focal field will be the surface of a sphere and not a flat field (plane). To create a flat field lens would require a lens with different focal lengths for different angles of incidence on the lens. That requires a lots of expensive glass to do so. The only true flat field lenses are copy lenses and enlarger lenses. Macro lenses are next. However, macro lenses are big, heavy and slow - all a result of the flat field design requirements. Wide angle lenses suffer more from curvature than longer lenses which is why most macro lenses are telephoto.

However, even the Fuji X80 f2.8 as big, heavy, slow and expensive as it is has some detectable field curvature.

Eliminating field curvature is a a tough nut to crack. The real solution is a curved sensor.  Sony has patented a medium format curved sensor and Nikon has a FF curved sensor. That would really simplify the lens design and result in smaller and less expensive lenses.

Corner softness wide open is the prime indicator of a curved focal field. If one has to stop a lens down to 5.6 or 8 to increase corner sharpness, the field is not flat.

While if one has the time, it might be better to place the focus box over the face in a portrait. However, I don't expect the 16-80 has a sufficiently flat field to eliminate focus recompose especially at f5.8 and f8 which is where these images were taken at a pretty "normal" focal length.

I don't think focus recompose was an issue with the OP's images. The images for the most part soft all over.

Optical field curvature.

Examples of field curvature.

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