sweet spot confusion

[Outdoorsman]

I'm a little mystefied about the sweet spot of a lens. I know that a lens' resolving power is supposed to be better at roughly the middle of its aperture range. I've heard many people say never to shoot wide open or stopped down fully. So why, then, are other aperture ranges besides the sweet spot even available? They want you to take bad pitcures? Personally I've not noticed any real difference. What's the deal?

[Sebastian]

Just because a lens gets better stopped down does not mean it is bad wide open, or that it takes bad pics.

Fast lenses allow more light in, so you can focus better manually, so the viewfinder is brighter, and most importantly, it lets you get the shot if it's dark, or drop off the DOF to seperate the background.

I shoot a lot wide open, sometimes they're soft, but at least I got the shot.

[paulnj]

well said seb

[another view]

Maybe - for example - f8 is the sharpest aperture on a particular lens, but that aperture might give you too much depth of field, not enough depth of field, too fast or too slow of a shutter speed.

Usually, lenses are pretty consistent from wide open to stopped all the way down - but may be just a little sharper at a moderate aperture. I did have an exception to this - Nikon's 300 f4.5 manual focus lens was very sharp wide open, just a little more around 5.6 or 8 - but horrible past 16 (yes, tripod & cable release, etc.). That small amount of increased sharpness can easily be negated by camera shake (what if you can't hold it at the shutter speed that f8 is giving you?).

This is a long ways down the list for me when I think about my exposure, and I never worry about it if I'm not on a tripod. It's a good thing to be aware of, but depending on your shooting style probably won't be something you'll need to be concerned with too often.

[Asylum Steve]

This is easy to explain. The idea of a lens "sweet spot" is a concept that is profoundly important only to a certain group of people, me NOT being one of them...

Sure, it's something that has value in the back of your mind, the idea that, all things being equal, a mid aperture such as f/8 will probably give your shot the best overall sharpness, but letting something like that hamstring you into confining your exposures to a single f-stop or two is downright FOOLISH...

Different folks have very different rules as to the "proper" way to shoot. Some will say any grain or noise is bad, some will say the greater the DOF or sharpness, the better. Some will profess that any blur or subject movement is a mistake. Some say only "accurate" color is acceptible. Some say keep the sun at your back. The list goes on and on...

One great thing about these forums is that you will almost always get a wide range of opinions on just about any subject. While this may drive a beginner nuts, in the long run it's the best way to learn, as you eventually decide what works best for you...

[Photo-John]

Enlarger lenses work exactly the same way that camera lenses do. In the darkroom a lens's sweet spot is really important. Open up too much and you get falloff and soft edges on your print. Stop down too much and the grain isn't sharp anymore. When I was printing I found that a lens's sweet spot is usually two stops down from wide open. I would try to always expose at that aperture. Turns out that it's usually f/5.6 or f/8.

But I agree with the rest of the guys that for most purposes it doesn't matter that much. People that worry too much about the perfection of their equipment are often missing the point. It's the content that matters.

[Asylum Steve]

In the darkroom a lens's sweet spot is really important...

I was actually going to mention that very thing, John. I was religious about using f/8 with my 50mm el nikor, but as you point out, enlargers and cameras are two very different animals...

[StillMrFitz]

I'm a little mystefied about the sweet spot of a lens. I know that a lens' resolving power is supposed to be better at roughly the middle of its aperture range. I've heard many people say never to shoot wide open or stopped down fully. So why, then, are other aperture ranges besides the sweet spot even available? They want you to take bad pitcures? Personally I've not noticed any real difference. What's the deal?

"Sweet Spot" The name or term applied to the f-stop or range of f-stops where a lens performs at its best.

Lens corrections are easiest and cheapest to perform at the center of the lens.
This is one of the factors which accounts for the extra cost of fast, high performance lenses.
It becomes more expensive and time consuming to correct for abbrations and distortions as you get closer to the edge of the lens.
The So called Sweet-Spot is not theory, it is a measurable qantity.
The importance of the sweet-spot is determined by the type and purpose of the photography you happen to be doing.
If you happen to need the maximum resolution and sharpness for the photography you are doing then you need to know which f-stops will give you that particular requirement.
For example; if you "need" large, sharp blowups, then shooting at f2 or f22 would not get you the best performance for that job.
If you need lots of DOF then you may sacrifice some sharpness and resolution to get the required DOF. Now you have a reason for shooting outside of the sweet-spot.
If you require shallow DOF, then you may find yourself shooting at f2, again, a reason for shooting outside the sweet-spot.
The advice I have always seen and heard is - use the sweet-spot - unless there is some "reason" not too!!
Low light levels is a reason - not too!!!
High speed film used during very high light levels is a reason - not too!!!
Requiring a certain DOF is a reason - not too!!!
That shot or no shot is a reason - not too!!!
It is not unusual to find a cheaper lens which very nearly equals a much more expensive lens, if both are used at f8.
This allows the photographer to get good quality images with a less expensive lens, as long as they can manage with the restrictions imposed by shooting at one f-stop.
There are many "reasons" for knowing about the "sweet-spot"
Take a detailed photograph, with the same lens, prime or zoom, at f2.8 then at f8 and enlarge both to 11x17 - You'll see the difference.

Fitz