DOF question

[englishpremier]

Just a quick one.

For a given aperture at given focal distance, everything that is an equal distance away from the lens as the main focal point will be in focus right?

eg pretend i have a 50mm lens at f1.8 and i'm 3 meters away from a poster that i'm focusing on. Everything on the poster will be in focus as it's more or less the same distance away from the camera, while anything behind the poster, of objects in front, will be out of focus (hyperfocal range notwithstanding)?

 

[D.P.]

In a simplistic view yes but you need to be careful with small depths of focus. For one thing from the geometry the edges will be a larger distance than the center, which can cause problems. More importantly, the plane of focus is not actually a flat plain and is curved like a surface of a sphere. Large aperture lenses wide open often have a much greater degree of curvature, mean the edges may not be in focus. This is the reason why you should never focus and recompose.

 

[bigredshark]

This is the reason why you should never focus and recompose.

Well, I'd slightly disagree, you're absolutely accurate and correct and you absolutely should be aware of those facts and keep them in mind when making a decision about whether to focus and recompose. Know it's limitations and exercise some judgement, don't rule it out always because it's a bad idea sometimes.

 

[D.P.]

True that is can be OK, and is harmless at large DoF. Shooting any of the 17-55/24-70 f/2.8 lenses r faster primes wide open focussing within a few meters will likely cause problems if recomposing. Some lenses like the Nikon 17-55mm f-.8 have quite a pronouned feild curvature even when stopped down to f/8, one reason why many people find the Nikon 16-85mm sharper edge-to-edge for landscapes.

Unforunately the degree of curvature and DOF effects are non'linear so it is impossible to generalsie effects across all lenses and some trial and error is needed.