Sweet-Spot

[Tommy49]

I have a 18-200mm lens and want to find the sweet-spot. I plan on photographing a page from the newspaper using each of the full f-stops with the camera mounted on a tripod then review the photos on my computer. My question is; does the lens' sweet-spot f-stop change depending on my zoom? Do I need to change my focal length and run thru the f-stops at each specific focal lenght? Help! Thanks

[Geoff F]

There are test results for a lot of lenses here http://www.photozone.de/Reviews/overview

This may give you a good indication of the best settings.

[xpatUSA]

I have a 18-200mm lens and want to find the sweet-spot. I plan on photographing a page from the newspaper using each of the full f-stops with the camera mounted on a tripod then review the photos on my computer. My question is; does the lens' sweet-spot f-stop change depending on my zoom? Do I need to change my focal length and run thru the f-stops at each specific focal lenght? Help! Thanks

Also, SLRgear.com.

For your own lens(es) I would shoot a spoke-type target (Siemens Star) with your highest resolution camera (to avoid obfuscation by moire). The sweet spot should become evident. You'd set aperture priority and AF - perhaps center-weighted. I've done it to compare lenses but it would be even better for just comparing aperture settings for one lens.

[tut] to answer the question - I would do it at max, mid and min focal length. I wouldn't try all the aperture settings, I would go between maybe 3.3 and 11 and include the 1/2 or 1/3 stops. Maybe your camera can bracket a range of apertures?

[Shadowman]

You'll test the lens on a target and then use the lens on what? Include something that will represent the lens actual usage, it doesn't have to be anything elaborate, just an eye or an egg.

[Black Pearl]

Why?

What possible use can you make of the results?

Are you only going to use your superzoom at its sweet spot to get its optimum performance for a minuscule set of parameters or are you going to go out and take pictures making the most of all of its capabilities?

[William W]

[U] I have a 18-200mm lens and want to find the sweet-spot. . .My question is; does the lens' sweet-spot f-stop change depending on my zoom?

Yes.
This may be quite apparent to you, if you are using full F/stop increments.
The spot is not usually a “spot” but a slowly moving “range” of a “sweet area”

***

Do I need to change my focal length and run thru the f-stops at each specific focal lenght? Help!

If you want to do this exercise ‘correctly’, then yes you do.
The “Sweet Spot” will also vary with respect to SD (Shooting Distance) – so you will have to choose a range of SD’s to make your test even more ‘correct’.

WW

[xpatUSA]

You'll test the lens on a target and then use the lens on what? Include something that will represent the lens actual usage, it doesn't have to be anything elaborate, just an eye or an egg.

Sorry, forgot to mention the Real World, tsk. I suppose the egg would hold still long enough for a series of tests, once balanced. Best use a flat egg though, to avoid contour confusion.

[revi]

Sorry, forgot to mention the Real World, tsk. I suppose the egg would hold still long enough for a series of tests, once balanced. Best use a flat egg though, to avoid contour confusion.

There is this little thing called depth of field. Somehow flat test targets seem to have a hard time showing it. So a real world subject could have some slight value in showing another aspect of the situation (hint: 200mm @ F5.6-8 in macro mode).

[rpcrowe]

The "sweet spot" of most lenses is approximately 2-stops smaller than the maximum aperture. I would guess that the sweet spot would be two stops smaller than the largest aperture at the focal length you are using.

Unfortunately wide focal range zoom lenses (of virtually any brand) such as 18-200mm traditionally are not as sharp as lenses with more moderate zoom ranges.

I would expect that the quality of your imagery might depend on which 18-200mm lens that you are using. There are a number of lenses of this focal length on the market from various manufacturers and the prices of these lenses vary greatly; ranging from (B&H New York prices in U.S, Dollars) $199 for a Tamron version to over $800 for a Nikon. Since the image quality of a lens is often directly related to its price; I would guess that there may be some great differences in quality between a two hundred dollar model and a lens that costs four times that...

Additionally, many zoom lenses are not at their best when shooting flat surfaces, especially from a relatively close distance.

[xpatUSA]

And there I was thinking that the so-called 'sweet spot" was when increasing sharpness due to decreasing diffraction was offset by increasing lens aberration. Now we learn that depth of field is factor. We live and learn . . .

[William W]

For clarity to the OP
I have never taken into account DoF when considering a lens's "sweet spot".
By definition the "sweet spot" is reckoned at the Plane of Sharp Focus.

WW

[Dave Humphries]

Hi Tommy,

I agree with Robin, I don't see this exercise as being helpful to your photography - you'll be better off just shooting subjects and learning the photographic craft. Good shooting practices; correct choice of shutter speed, aperture, plus good sharpening technique is going to have a greater effect on the quality of your images.

Especially as, and you guessed right, it will differ by focal length, etc.

I suppose if you have one of the cheap $199 models, it might make a big enough difference.
I have the Nikon version and on that I am going to choose my aperture to get the correct DoF, or shutter speed for the subject - not try to use an inappropriate aperture because it might be a tad sharper, while simultaneously blurring the shot because the shutter speed was too slow for the subject (or my camera shake).

It's a bit like buying a new car and obsessing about the oil in the sump when you should be enjoying the driving experience, or making sure you don't prang it

Why not post a picture or two and get some useful feedback, these may help;
HELP THREAD: How can I post images here?
How to Get Effective Feedback for your Posted Images
But please - no newspaper shots

Anyway, welcome to the CiC forums and I do hope I haven't put you off

[William W]

The practical application of doing something like a comprehensive “sweet spot” test on a medium quality zoom lens is self evident in this recent thread:

The 18-55mm kit lens. How sharp? How useful?

A useful fact of having done several tests, allows me, to state with confidence that the all of the seven kit lenses discussed in that thread, will be “at their best” if used between F/7 and F/10.

Now any person with a little knowledge of lenses, could make a reasonable educated guess where the range of sweetness might be for that particular lens

So really, the fact that I have actually ‘tested’ the lenses makes my comment perhaps more sound in science, but not the least bit any more useful as - advice.

The more “useful” comments (that I made) on that thread are letting people know that the Kit Lens will make very OK photos NOT ONLY at the sweet spot. And hopefully encourage people to go out and MAKE PHOTOGRAPHS – because that’s what it is all about, really.

If you will have fun doing these tests – then that’s great too.

I have a great deal of fun trying to nail a shot (to my satisfaction) at a Shutter Speed slower than 1/8s hand held with my 85mm lens – I haven’t made one good enough, yet: but I still have fun trying.

I don’t spend my every waking moment obsessing over testing lenses either – but those Kit Lenses I had a great deal of fun testing: mainly because there’s so much poo piled on them, I had fun doing tests and MAKING PHOTOS to defend the poor underdog.

I know a lot about the CA of those Kit lenses too . . . but again anyone with a bit of knowledge will guess: “I reckon there’s a bit of CA especially at the edges and when the lens is at the wide, more so, probably.” And that casual comment would be very close to the mark. And that casual guess would be in 99.999999% of instances just as useful as the detailed knowledge I have.

So that’s a roundabout way of saying - before you start on this endeavour, have a think about why you want to do it – if you are getting something out of it: then that is great.

WW

[revi]

And there I was thinking that the so-called 'sweet spot" was when increasing sharpness due to decreasing diffraction was offset by increasing lens aberration. Now we learn that depth of field is factor. We live and learn . . .

DoF isn't a factor in determining the sweet spot (as you very well know); it IS a factor in real life use of a lens (as you also very well know).

And I was responding to your "REAL WORLD" remark; thank you for removing the context of my answer.

[jcuknz]

I use a zoom lens all the time and simply close the aperture down a couple of stops for preference, I don't see any point in running a test If you take good intersting photos people are not going to be bothered if it is absolutely sharp and anyway there is editing to follow.

[Stagecoach]

From Tommy's initial post he does not say exactly 'why' he wants to undertake the test. I will not go into the technicalities (as I'm no expert) but from what I read above most areas are covered except one.

As someone who occasionally needed to use my gear to produce the highest image quality I can obtain for technical reference purposes knowing the 'sweet spot' of a particular lens was advantageous. I of course knew it would be around between 5.6 to 11 but being as close as I could would give me a better chance of achieving my aim as long as within ISO and speed limitations.
Technical subjects I have shot where this has been usefull knowing are such as cracks and breaks in engine compoments.

Grahame

[Dave Humphries]

Hi again Tommy,

Subsequent replies have made me realise that I do have the "educated guess"/experience to know that, most lenses will perform better one or two stops down from maximum aperture (as many have said) - if you don't know that (this being your first post we don't know how much knowledge you have already), then I hope this thread has been useful to you in getting across the following message:
Don't worry about it, go by the 'rule of thumb' that two or one stop down is better than wide open and use that IF all other things are equal, but don't be a slave to it; always put the choice of shutter speed or Depth of Field above the sweet-spot consideration.

Do I use the sweet-spot?
Well, I choose f/8 over wide open (f/5.6) on my 70-300mm if that doesn't give me too slow a shutter speed or too much Depth of Field. In really bright conditions, I might go to f/11 if the subject is well separated (by distance) from the background (e.g. white, sunlit bird against a blue sky).

Hope that helps,

[rpcrowe]

In actuality I choose my aperture based on the depth of field that I need (or think I need) for any shot. Obviously, if I need a deep DOF, I will lean toward the smaller apertures and when I need a shallow DOF, I will shoot wide open or close to wide open.

The advantage of using a top-line lens is that I can feel secure in getting decent quality imagery wide open and at each end of the zoom range!

The disadvantage of top-line lenses is that they are more expensive; often a lot more expensive

They also generally weigh more. Consider the difference in weight between the Canon 18-55mm + 55-250mm kit lenses (591 g. total) and the EF 24-70mm f/2.8 ii + 70-200mm f/2.8L IS ii lenses (3,293 g. total). I carry the 17-55mm f/2.8 IS and 70-200mm f/L IS whiich, although lighter than the preceeding pair, are still over twice as heavy (1,395 g. total) as the two kit lenses....

[xpatUSA]

The well-respected Roger Cicala at Lens Rentals has couple of good blog posts:

http://www.lensrentals.com/blog/2011...own-just-a-bit

http://www.lensrentals.com/blog/2011...e-or-the-other

Warning to Real World folks: there's scatter graphs with a MTF axis. No twigs or distant trees to be seen . . .

[Manfred M]

While every lens will have a "sweet spot", or technically it is the design point, it really has very limited bearing unless you spend all your time running around shooting test patterns.

Your image is going to be the culmination of numerous tradeoffs that you will make in taking the shot. As an example, if you know that you have a "sweet spot" at 100mm focal length at f/11, but have to shoot at ISO 6400 to get a proper exposure, I would be more than willing to shoot wide open at f/4 to get ISO 800 so that I am working in a range where there will be less noise and a higher dynamic range (assuming the same shutter speed).

In photography, you are controlling multiple parameters to get you the image you are trying to capture. In my workflow, the sweet spot never ever becomes a consideration, even though I know it (multiple ones for a zoom lens; they will vary by focal length, as per Bill's previous posting).

In my workflow, I usually go in the following order:

1. What ISO should I be using. I tend to shoot as low an ISO as I can as dynamic range decreases as ISO increases; noise increases as ISO increases. The lowest ISO you can get away with will get you the best quality capture.

2. Focal length - I will figure out roughly at which focal length I will shoot at and this will help determine my minimum shutter speed.

3. Do I need to shoot in a way that I want to fix depth of field (shallow for a nice, out of focus background or do I want everything in focus) or do I want to concentrate on motion (freeze motion with a high shutter speed or do I want to introduce blurring by shooting at a low shutter speed).

4. Once I figure the main parameter or shutter speed or aperture, the exposure triangle will dictate the shutter speed - aperture setting I need.

I've run out of parameters to worry about my lenses sweet spot. All of the other parameters have taken over the decisions. This does not mean I don't know that stopping down a couple or three stops is going to give me optimal lens performance, but this is meaningless if I can't pull off a sharp shot.

That being said, your 18-200mm lens is not going give you super sharp results and will have some interesting distortion patterns. That is just a fact of life with lenses that have such an extreme range. My wife has an 18-200mm lens and I have the two kit 18-55mm and 55-200mm lenses. These two cost me about half as much as my wife's single lens and combined weigh a lot less (and are not nearly as well built). If I am to believe the reviews I've read, they are sharper and have less distortion. On the other hand, she is shooting away, while I am busily changing lenses and missing some shots.