Does the camera lens "sweet spot" also apply to focusing distance?

[sereze]

We already know that most lenses give their best performance at two stops narrower than wide open aperture. And Middle range gives better quality for zoom lenses. So, does it also apply to focus range as well?

I have a Canon 450D kit lens minimum focus distance of 0.8 ft. Would it be wise not to have my focus at 0.8 ft. and practically having my focus (and subject distance of course) at lets say minimum of 1.2 ft. ??

[William W]

I believe I would be pretty safe in generalizing that one of the main reasons that these two "Sweet Spots" exist in zoom lenses, is because design compromises are made, so that focus CAN be achieved THROUGHOUT the focus range.

Specifically referring to the kit lens, because of its physical properties (plastic mould) and (assumed) manufacturing tolerances, I would not measure 0.8ft from the Film Plane marker to a flat Subject (like a page of newspaper on a wall) and then wind the Lens to minimum focus and rest assured that the subject would be in focus: what I mean is, minimum focus on your lens might be 0.9ft, because of the physical aspects of its manufacture.

However, if the red square indicates focus, and the viewfinder screen indicates focus I see no reason to assume the lens would be better focused at the minimum distance possible than at any other: assuming the lens / camera focus calibration is adequate.

Remember that these factors all have tolerances, and we are speaking in generalizations.

WW

[sereze]

Thanks... Ok

[William W]

And: if you focus on a subject at one FL, and then zoom in or out, do not assume the plane of focus remains.

WW

[McQ]

We already know that most lenses give their best performance at two stops narrower than wide open aperture. And Middle range gives better quality for zoom lenses. So, does it also apply to focus range as well?

A great question. Bill's right -- the minimum focusing distance is set based on design tolerances. Focusing anywhere between the minimum focusing distance and infinity should therefore achieve a level of focus within design tolerances.

However, just because a lens is optimally focused does not necessarily mean that it will reproduce the same sharpness for any focusing distance. Lens aberrations can be slightly amplified or attenuated depending on how the light passes through the lens elements. Since the lens elements change relative position when you focus at different distances, it is reasonable to assume that so too will the maximum sharpness. I've heard others make claims about this, but have not seen good examples to substantiate this claim in real-world use. However, lab room tests have shown that the lens MTF curve does indeed change depending on the focusing distance of the test chart. This is a difficult thing to test in a way that isolates the effect of focusing distance though.

Now...where is the optimum? I think it's probably reasonable to assume that it is somewhere between the minimum focusing distance and just beyond infinity (yes, such a distance can occur, believe it or not). Beyond that though, I really don't know. It's probably highly dependent on the lens -- the particular focusing mechanism, whether it's a zoom, etc.

The only situation I can think of where you might consider "sharpness vs. focusing distance" is if you are trying to photograph something like a painting, and have a range of options for how far away you can take the photo from (and still fill the camera's frame). This could happen if you were using a zoom lens, for example. In this case I would avoid taking a wide angle shot right up next to the painting, at the lens's minimum focusing distance. However, this is primarily because you want to be in the focal length sweet spot of the zoom lens -- not necessarily because of any focusing distance sweet spot. There might also be considerations based on viewing perspective as well.

On the other hand, this is really nitpicking at definitions. Does it really matter? If you need something in focus at 0.8 ft for artistic reasons, then you need it in focus at 0.8 ft. I would not redesign your composition in order to satisfy a barely perceivable increase in sharpness. Very close focusing distance can be an incredibly powerful artistic tool to exaggerate perspective -- especially with wide angle lenses.

[William W]

just because a lens is optimally focused does not necessarily mean that it will reproduce the same sharpness for any focusing distance.

Yes! I was not thinking of that when I scribed. Thanks.

Since the lens elements change relative position when you focus at different distances, it is reasonable to assume that so too will the maximum sharpness.

. . . which is also the rationale to explain why a zoom lens design is a compromise and has a Focal Length "sweet range", in the middle of the whole zoom range: becasue in a zoom lens the lens elements (usually) change postion, significantly, when zooming.

Now...where is the optimum? I think it's probably reasonable to assume that it is somewhere between the minimum focusing distance and just beyond infinity (yes, such a distance can occur, believe it or not). Beyond that though, I really don't know. It's probably highly dependent on the lens -- the particular focusing mechanism, whether it's a zoom, etc.

Ha! I like your guess "somewhere between the minimum focusing distance and just beyond infinity" . . .

As we are in the sub-science of guessometry (somewhere I am really uncomfortable):

I pictured in my mind's eye that we have all these intersecting graphs of sweet spots, like a big 3D Venn Diagram: Sweet Spots for: Aperture; FL; Acutance; CA; Sharpness . . . there is a big OPTIMUM intersection is somewhere . . . my guess was likely that firstly Focal Length and then Aperture will be dominant factors . . . then I got an headache thinking about it.

On the other hand, this is really nitpicking at definitions. Does it really matter?

No, but it was fun, and like you said, a good question.


WW

[crisscross]

If your subject stays still, why not try a range? I am usually aiming at flowers or calm butterflies at min range (used to have canon 18-55 kit with 350D, now Nikon 18-135 kit). Where it appears to be auto-focussing at near minimum, it may not actually be , so shoot another 50mm or so further back aswell. A well focussed crop is going to be better than a larger fuzz.

[sereze]

Thanks guys, I appreciate your detailed answers