I thought the lens design has elements to compensate for field
flattening. In any event, the predictably flat silicon focal plane has
to be better than the lottery of film.
Tony Sleep wrote:
> On 06/07/2007 Arthur Entlich wrote:
>> Does anyone know if there is a chart which shows depth of focus at the
>> film plan versus aperture of lens used?
>
> No, but the plane of focus itself is not flat, it's usually a section of a
> sphere that is only part corrected to flatness. This becomes an issue when
> focussing wideangles at wide apertures, especially. If you use a focus aid
> or AF at the image centre then re-frame to put it near the edge, it'll be OOF.
>
> I used to do enough of this that with a 24mm f2 that I bought a plain
> matte screen without any focus aids so I could focus as framed. It can be
> quite a handy property since edge of frame close objects can be in focus
> at the same time as more distant central ones, without having to stop down
> to provide as much DoF as expected.
>
> If you photograph a flat wall with such a w/a, you can see the problem;
> the edge-of-wall to lens distance can be substantially greater (nearer
> infinity) than the centre ditto. This would mean the lens needs to be
> racked in further for the edge image to be sharp, more extended for the
> centre.
>
> Constant subject-lens distance d implies a part-spherical plane of focus
> of radius equal to d. The back focus of the lens b is also a
> part-spherical surface of radius b. For longer lenses with narrower angle
> of view none of this is really noticeable, as the smaller section of a
> sphere is near enough flat and DoF hides the effect.
>
> We need spherical film or sensors - but the radius would be different for
> each focal length dammit.
>
> --
> Regards
>
> Tony Sleep
>
>
>
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