The focus lies along which line?

Started Mar 24, 2013 | Discussions thread
apaflo Veteran Member • Posts: 3,854
Re: Blue line wins!

fireplace33 wrote:

Thanks for those quick answers! Looks like the blue line wins!

This is what I was assuming; you guys have just helped me "win" a little argument here at home is after all called the focal "plane".

You can't help admiring the lens designers that manage to get our lenses to focus like that, no wonder they are expensive!

You apparently didn't read the cited URL's close enough!

The curved red line wins almost every time!

The exceptions are lenses specifically designed to have a "flat field".   Note that is not the same as a "flat field design", which can be used to describe almost every lens in use with a DSLR today.

A flat field design has a flatter field than a simple lens.   But it looks a lot more like that red line than it does the blue line!  Most lens for DSLRs fit that description.

Most, though not all, macro lenses are specifically designed to be what is described as a "flat field lens".  They are not anything near flat like a straight line though!   Instead the far outer edges  move at least somewhat towards the camera, and there are "waves" at somewhere probably a little more than half way from the center to the edge, which will comonly extend both in front and in back of the blue line.

The trick with a very good flat field macro lens is that if the center is focused on that blue line even at wide open all except perhaps the extreme edges will be within the DOF.  Hence while it is not really flat, it is flat enough.

The trick with virtually all non-macro lenses is to remember that at the edges the field of focus is going to be closer to the camera than it is at the center, which with wide apertures will definitely be significant.  (For group shots, with say 3 rows of people, focus on the center row, but make the back row half as wide, and the middle row three quarters as wide, as the front row.)

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