The Sweet Spot - Lens Performance

[Raffinator]

OK, so I keep hearing about the "sweet spot" on a lens, the stop at which the lens best performs. I have been looking for a decent explanation - maybe I haven't looked hard enough. It seems that a stop or two down from wide open is generally accepted as where this "sweet spot" exists. Is this why so many dps choose to shoot close to wide open, or is it mainly a choice of depth of field? Also, does this apply to both primes and zooms? I hear that shooting wide open also gives you the worst lens performance, generally speaking. I generally shoot in 16mm with somewhat budget equipment, so if I can get the best image quality out of my limited choice of lenses it would really be beneficial to me. Your thoughts?

 

Many thanks,

 

Raffi

[Guest Ian Marks]

Actually, I think the "sweet spot" refers to the center portion of the frame, where the lens (generally speaking) produces the sharpest images. For example, when using still camera lenses on a movie camera, people speak of using only the "sweet spot" of the lens because so much of the image produced by these lenses is effectively cropped out.

 

VERY generally, still camera lenses will produce their best images at about two, maybe three, stops down from maximum aperture. A few very expensive lenses, like the Noctilux and Noct Nikkor, are specially designed to perform well wide open. Also, the minimum aperture is to be avoided because that's where diffraction sets in.

 

Fast fixed focal-length lenses for 16mm cine use (like the Zeiss Superspeeds) are a different matter, in that they are designed to perform well at every aperture, period. (They should - look at how much they cost!) Older, slower lenses like the Cooke Kinetals will benefit from a little stopping down.

 

My understanding is that older zooms like the ubiquitous 12-120 Angenieux perform poorly wide open and must be stopped down to obtain acceptable performance. I have no experience with the newer, super-expensive zooms, but I would expect them to be just fine wide open.

 

I do believe it's possible to put together a good lens kit on a budget - I've seen some really sharp footage produced by Bolexes with relatively inexpensive Switar lenses.

[John Pytlak RIP]

Really small lens openings (e.g., f/16) don't yield maximum sharpness due to "diffraction limiting":

 

http://medfmt.8k.com/mf/diffraction.html

[Raffinator]

Really small lens openings (e.g., f/16) don't yield maximum sharpness due to "diffraction limiting":

 

http://medfmt.8k.com/mf/diffraction.html

<{POST_SNAPBACK}>

 

Thanks for the info, guys. The diffraction that occurs at F11 and beyond really makes me marvel

at how guys like Gregg Toland and Welles achieved the depth of field that they did on "Citizen Kane" and other films, while still making it look so fantastic.

 

Raffi

[John Pytlak RIP]

Thanks for the info, guys.  The diffraction that occurs at F11 and beyond really makes me marvel

at how guys like Gregg Toland and Welles achieved the depth of field that they did on "Citizen Kane" and other films, while still making it look so fantastic.

 

Raffi

<{POST_SNAPBACK}>

 

Don't confuse depth of field with sharpness. Yes, more things are in focus at f/16, but even when they are in focus, they are limited in sharpness.

[David Mullen ASC]

When necessary, Toland replaced the iris in the back of the lens with an f/16 Waterhouse stop to reduce diffraction problems.

[Raffinator]

When necessary, Toland replaced the iris in the back of the lens with an f/16 Waterhouse stop to reduce diffraction problems.

<{POST_SNAPBACK}>

 

I take it that a "waterhouse" stop is more perfectly round than the adjustable iris on the lens, thereby reducing the amount of diffraction? Very cool.

 

"Don't confuse depth of field with sharpness."

 

Sharpness vs. Depth of field definitley seems a bit tricky, because great depth of field, while increasing the amount of distance that's in focus, certainly doesn't always read as being sharp. When I see very shallow, selective depth of field, it often looks as if the object within that depth of field is super sharp, but I wonder if this is mainly due to perception (the sharp object vs. the blurry background/foreground) rather than a truly "sharper image" (no corporate plug intended).

 

Raffi