[Joyce Keay]

Hi, I'm wondering if the sweet spot for lenses is pretty much the same for all samples of one particular lens. In other words, could I ask what the sweet spots are for Pentax 18-135, 55-300 and 100mm macro lenses or do I need to check this out for my lenses? If each type of lens has pretty much the same sweet spot across all samples, is there a site that lists the sweet spot for all the Pentax lenses. Thanks. Joyce Keay

[pschlute]

Unless you have a faulty copy the characteristices of a lens will be consistent across all copies.

Lens tests...look no further Pentax Lenses - Page 2 | PentaxForums.com Reviews

[Kozlok]

I think the sweet spot for the Macro is on the camera (lol)

[Joyce Keay]

Unless you have a faulty copy the characteristices of a lens will be consistent across all copies.

Lens tests...look no further Pentax Lenses - Page 2 | PentaxForums.com Reviews

Wow! Thank you for a very comprehensive link. I've only read the review for the macro lens but I'm sure reading the reviews for all three of the lenses will improve my use of them.

[pschlute]

Wow! Thank you for a very comprehensive link. I've only read the review for the macro lens but I'm sure reading the reviews for all three of the lenses will improve my use of them

I assume you mean maximum sharpness when you refer to sweet spot. As a rule of thumb you will find that most lenses are sharpest in the f5.6-f8 area

[sqrrl]

I find these reviews to be pretty useful;

SMC Pentax-DA 18-135mm F3.5-5.6 ED AL [IF] DC WR Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database
Pentax SMC-DA 18-135mm f/3.5-5.6 ED AL [IF] WR - Review / Lens Test

Etc.

As a rough guide, most zooms are best in terms of absolute image quality in the central third of their range (focal length) and maybe 2-3 stops closed down. Most primes seem to be best 2-3 stops closed down and used at their assumed range (focus distance) (a bit past their minimum and a bit inside their maximum).

[Alex645]

Hi, I'm wondering if the sweet spot for lenses is pretty much the same for all samples of one particular lens. In other words, could I ask what the sweet spots are for Pentax 18-135, 55-300 and 100mm macro lenses or do I need to check this out for my lenses? If each type of lens has pretty much the same sweet spot across all samples, is there a site that lists the sweet spot for all the Pentax lenses. Thanks. Joyce Keay

Although not as thorough as the ones listed above, DXOmark has done tests and in order of sharpness report:

Camera Lens Database - DxOMark

"Best results" = sweet spot:
100mm f/4 on K-50 but f/2.8 on K-3
55-300mm f/5.8 @ 300mm on both K-50 and K-3

From experience, I question their results here as almost always, the sweet spot are f/stops midway between the largest and smallest apertures. In most wide-to-tele zooms the sweet spot is around 35-50mm. Telephoto or wide angle zooms are more complicated: At the best performing aperture, they are usually sharpest at a mid-focal length, although some have been designed to perform best at their extremes and many will have unexpected resolution at other aperture and focal length combination.

You may also find anomalies based on focusing distances.

[mikesbike]

Good to read. You will find there are more than one kind of sweet spots for performance of lenses in general, which are given in terms of the best apertures and also the best focal lengths for zoom lenses. Typically, zoom lenses are not up to their best at the extremes of their zoom range at either end. There are numerous performance parameters such as- resolution (sharpness), lack of vignetting, linear distortion, flair susceptibility, chromatic aberrations (CAs), bokeh (a Japanese word pertaining to how pleasing the out-of-focus areas are rendered), and more. Your DA 18-135mm lens is especially good within the range of about 22-70mm. One practical aspect is it can keep to f/3.5-4.5 aperture wide open with good to excellent imaging performance from 18 to 70mm. This means it can let in more light than a lens that goes to f/5.6 can accomplish, which means you can get a little more shutter speed without have to increase ISO more. It is also one of the fastest and surest for AF I have used.

In the moon test of the DA 55-300mm f/4-5.8 against the much larger, much more expensive DA*60-250mm f/4, rather than shoot at 300mm and enlarge the DA* lens's 250mm, I would have simply shot both at 250, because even though the 55-300mm lens is still pretty good at 300mm, better than other brands of the same range, its performance is improved at 250mm. (see testing by photozone- now renamed OpticaLimits) It can also keep to f/4-4.5 from 55mm up to 200mm. And it is not internal focusing, so your picture is not reduced in focal length (magnification) at closer distances. You will get a larger closeup when set at the same 250mm. By the time you blow up the other lens's picture to compensate, the results may be quite close. The newer DA 55-300mm PLM lens does not have these advantages.

In the target shots, keep in mind these are very large blowups. For normal size viewing the differences might not be noticeable at all, or far less so.

Your lens selection is a very good one indeed- they are both favorites among my lenses, which includes some top-ranked ones.

[Des]

Some points to add to what @sqrrl @Alex645 and @mikesbike have said:

- Reviews need to be treated with caution. They are generally testing only one copy of a lens, which might be particularly good or particularly bad. The test might be affected by the camera body used. They might not test focal lengths or apertures that matter most to you. Or to put it the other way, the things that matter to a reviewer might not be the things that matter to you. And it's often hard to verify their methodology.

- For these reasons, I think you only get a clear guide to the metrics of a lens if there are three or more reviews which produce much the same results.

- The Photozone test of the 18-135 is particularly controversial, especially because - and here's an issue with all tests - it doesn't match the reality of a body of field experience with the lens. Even if correctly done, the metrics don't reveal the character of the lens - such as whether it gives a 3D effect, or how colours appear. If you looked at Photozone's comments on the 18-135, you'd think it was the worst lens Pentax ever made. In particular you'd think it was the worst choice ever for landscapes. Yet if you look on this site or on Flickr you'll find any number of very good shots taken with that lens. I'm constantly surprised at how well it does, even for landscapes.



- I recently bought the DA 20-40 Ltd zoom. I found the reviews - both professional and user reviews - to be absolutely polarized. Lenstip panned it. The PF review found its sharpness to be mediocre. DxOMark found it had "relatively low uniformity and peak sharpness". One experienced member here (whose views I respect) sent it back because he found it unusable below f5.6. Yet the tests by ePhotozine (HD Pentax-DA 20-40mm f/2.8-4 Limited DC WR Lens Review) showed outstanding numbers everywhere that matters to me: centre sharpness wide open (except at 40mm where it was just below excellent), and sharpness across the frame at f5.6-f11. Imaging Resources found much the same (Pentax 20-40mm f/2.8-4 ED Limited DC WR HD DA Review). Users who had a good copy generally loved it, and image samples I looked at were impressive. So I took the punt. My initial tests suggested at least that my copy is not obviously defective (better than the 18-135 or 12-24 in their common range) - as for the rest, time will tell. The point is, that you need to look at a range of information to see whether the lens is right for you, and where it works best.

- With the 55-300 (assuming you are referring to the screw-driven versions), at the long end it's worth stopping down to f8-f11 whenever conditions allow. Some examples at f8 or f9:








- If I am mistaken and you are referring to the 55-300 PLM, I haven't seen much in the way of tests, but here is one: https://www.ephotozine.com/article/pentax-hd-pentax-da-55-300mm-f-4-5-6-3-ed...e-review-30837 The patterns in this review are the sort of results you would expect with a very good quality zoom: very good wide open across the focal range, excellent stopped down one stop. I'd say from my more limited experience with that lens that there isn't the same imperative to stop it down as far as the screw-driven version. It's surprisingly sharp at 280mm f5.6 or 300mm f6.3, although there is still a gain from stopping down one stop. At f8 it seems to be similar to the screw-driven version, which is to say quite sharp (for a consumer zoom in particular).

- The DFA 100 is an exceptional lens. Every spot is a sweet spot. Wide open or stopped down to f16, whether the subject is near or far. The tests suggest that it is best at f5.6 (see http://www.photozone.de/pentax/129-pentax-smc-d-fa-100mm-f28-macro-review--test-report?start=1 and https://www.ephotozine.com/article/pentax-d-fa-100mm-f-2-8-macro-wr-lens-review-24103 ) but really its so sharp that even with a diffraction hit you can go to f16 with confidence. You don't need to select an aperture for optimal sharpness; select it to achieve the effect that you want or the degree of light-gathering you need.

[mikesbike]

@ Des- I agree 100%. Characteristics of contrast is another which test bench techniques do not have a way of measuring, so that leaves the images themselves to evaluate. Yes, the DFA 100mm f/2.8 WR is a wonderful lens. I love it greatly too. I also bought the DA 20-40mm Limited to go with my new KP when I do not need the extra zoom range of the DA 18-135mm. It is a little faster aperture-wise, and it fits the KP like a hand to a perfect glove. It has been providing outstanding results.

Lenses are tools, so you take whichever you have that will best address the particular needs you will have at the time. But for having only a 3 lens kit, for range, image quality, and performance, along with compact carrying and covering a lot of needs, it is hard to beat the DA 18-135mm, DA 55-300mm, and DFA 100mm macro!

[Joyce Keay]

Hi, Thank you all for your amazingly comprehensive responses to my question. I really appreciate the time you've all taken to answer what I thought was a pretty simple question. Guess I was wrong. I bought my Pentax K-70 in August of 2016 so I'm relatively new to the wonderful world of DSLRs. Yes, I was referring to sharpness sweet spots as that seems to be the hardest thing for me to achieve - tack sharpness. I am referring to the screw-down version of the 55-300. I will take notes regarding best apertures for the various lenses and look forward to better shots.

[normhead]

Sharpness and the Appearance of sharpness.

See the test charts on these pages to see how your lesnses will performa t various focal lengths.

For the DA 55-300, probably not the same as yours but they are not so much different as far s I know.
Pentax SMC DA 55-300mm f/4-5.8 ED - Review / Test Report - Analysis

For the 18-135
Pentax SMC-DA 18-135mm f/3.5-5.6 ED AL [IF] WR - Review / Lens Test - Analysis

For the 100 macro
Pentax SMC-D FA 100mm f/2.8 macro - Review / Test Report - Analysis

The concept you may be lacking here is depth of field. For all the lenses mentioned their sweet spot is an Aperture of ƒ5.6

But that may not give you enough Depth of Field to keep all of what you want sharp sharp. As you increase your Aperture the DoF. The distance between the closest point and furthest point in acceptable focus. So there are two ways to interpret sharp. One would be the sharpest possible part of an image.

The second would be everything in the picture is in focus. For that you need to use a much higher aperture. ƒ11, or ƒ16. Hence o a macro shot, you may want to use ƒ 16 just to keep more of your subject in focus.

Look at the 100 macro....


The lens records it's sharpest values a ƒ5.6

Taking a picture of a flower 2 feet away at ƒ5.6 the DoF will be a plane parallel to the camera lens .4 inches deep. AT ƒ 16 the DoF will be .8 inches deep. Hence going from ƒ5.6 to ƒ16 you have doubled your depth of field.

You can see looking at the chart above, at ƒ5.6 you Line Width /Picture Height (The number of paired lines you can discern on an image drops from 2194 to 1749, so the lens stopped down is capable for reproducing less fine detail. But the image can still look a lot sharper because more of the subject is in focus.

I often shoot multiple ƒ-stop because it is so hard to predict, with narrowing DoF what amount of DoF will look best.

Here are a couple of snake images....
Notice how in the first image, the snakes head is in sharp focus but every thing around it is soft. This one taken with a 200 mm lens at ƒ2.8.


In the second one, I've taken an almost identical image at ƒ11.Now almost the whole snake with the exception of the tail is in acceptable focus.


The first image is probably sharper than the second in terms of centre sharpness but more of the second image is in acceptable focus, so which one is sharper? It depends on what you mean by "sharpness".

Another meaning of sharpness is "sharp edge to edge" or centre sharpness. If you look at the above charts one of the attributes of macros is they are designed to reproduce flat surfaces. So if you look at the values for the DFA 100 macro, ƒ5.6, it's sharpest value, it's 1995 lw/ph in the centre and 1980 on the edge. So it produces a very even image edge to edge.

Now look at the chart for the FA 77 ltd, a classic Pentax portrait lens.


Look at the centre sharpness at ƒ4, it's almost of the chart. But the edges are softer, the difference between the centre and edge is almost 400 lw/ph. Yet stop down one more stop to ƒ5.6 and you have excellent centre and edge sharpness. For shallow DoF portraits where the subjects head is near the centre of the frame, this lens will give you excellent centre sharpness at 2.8, but it will not be as sharp on the edges giving you nice smooth out of focus areas. Sharpness in out of focus areas looks odd. Hence the lens is great portrait lens opened up, but a great landscape lens stopped own. almost like having two lenses in one.

That relates to the DA 18-135. If you look at the chart for the DA 18-135 at 24mm and ƒ 5.6 you have a great landscape lens, with excellent sharpness edge to edge. AT 85mm through to 135mm you have a great portrait configuration, with excellent centre sharpness at ƒ8 and ƒ5.6 but soft edges to blur the out of focus areas.

So there are many combinations 1. Premium centre sharp, but softer edges, 2. not as centre sharp as #1, but still excellent and excellent edges as well. 3. Not really sharp anywhere (mostly older third party lenses), and lenses like the DA 70 2.4 that are pretty much sharp everywhere.



What you will consider to be "sharp" depends in large part on what look you are going for. The DA 70 is by the charts the sharpest lens overall. But personally, I would find the 77 more useful, in that it gives me the opportunity for two different styles in the same lens. I find the 18-135 the same. A great landscape lens at from 22-30mm, and acceptable portrait lens from 85-135mm. I'm a big fan of the 2 for 1 deal in a lens with both edge to edge sharpness and extreme centre sharpness with softer edges in the same lens. In your case, with your 100 macro and DA 18-135 you have both, with the DA 18-135 making up for it's lack of a wide aperture to blur the back ground with extremely soft edges for ƒ5.6 portraits, from 70-135mm and the 100 macro sharp edge to edge for more clinical images where the 18-135 has softer edges.

A lot of people don't understand why the 18-135 was designed the way it was. Don't listen to them, it's an awesome lens with a unique design.

For me those two lenses are an excellent walk around combination.

I know it's blasphemy around here but here's a DA 18-135 portrait.


Soft borders you say? It's better that way.

[BrianR]

Although not as thorough as the ones listed above, DXOmark has done tests and in order of sharpness report:

Camera Lens Database - DxOMark

"Best results" = sweet spot:
100mm f/4 on K-50 but f/2.8 on K-3

Their "Best Results" score is not sharpness alone, it's an amalgam of the various measurements. I've no idea how they weight this, but as with most DxO single number summaries it's best to ignore it and head to the individual measurement. By their measurements, the 100mm macro is sharpest at about f/5.6.

Anecdotally I'd tend to agree with f/5.6 being the sweet spot for sharpness and this macro, but I'd also say the lens is 'sharp enough' that from a sharpness standpoint I don't stress too much about using anything from f/2.8 to f/16. Selecting an aperture for the appropriate DoF is more important in this range (as Norm is getting at). I also tend to avoid f/2.8 if there are high contrast edges (fringing can be a bear under the wrong conditions). Past f/16 I start to weigh diffraction loss a little more heavily against DoF gains, but it's still a trade off that can be worth it (unless focus stacking is an option, then f/5.6 or f/8 here I come!).

[normhead]

Anecdotally I'd tend to agree with f/5.6 being the sweet spot for sharpness and this macro,

Falconeye once went through a mathematical proof as to why the absolute best lenses are shaprest at ƒ2.8, and the next bunch like the 31 ltd and 77 ltd. being sharpest at ƒ4, and your standard every day kit being sharpest at ƒ5.6, I know enough to know he may be on to something, but not enough to repeat the proof. Apparently some ZIess lenses are sharpest at ƒ2.8. There are enough lens reviews around on Photozone you can check this out empirically if you choose.

Having your lens sharpest at ƒ4 or ƒ2.8 makes for some really nice smooth out of focus areas on portraits, without having to open to ridiculously wide apertures and killing your depth of field.

At least I assume it would. Those lenses are out of my price range.

[Des]

Norm's answer is very comprehensive. I'd just add a couple of things about the DA 55-300 (screw-driven versions). I didn't really explain why it's worth stopping down to f8-f11 at the long end. Here are the results from the Photozone test that Norm linked to. Let's assume they are reasonably indicative.



At 300mm (the last chart), going from f5.8 to f8 hardly affects border sharpness at all (the maroon column), but you get a significant increase in centre sharpness (lilac column). At 300mm, mostly you are looking for centre sharpness because the bird or whatever you are photographing will be placed roughly in the central portion of the frame.

Stopping down to f8 also makes a big difference in vignetting (where the edges and corners are darker than the centre):


You'll notice with this lens, at 55mm, 100mm and 200mm you need to stop down to f8 or f11 to get reasonably even sharpness across the frame (e.g. where you want the whole of a building to be sharp), and you never really get it at 300mm (not that that usually matters, as I said). But if your subject is in the centre of the frame (like a person or a flower) and border/corner sharpness doesn't matter, it's very good wide open at 55mm and 100mm (good for portraits), and pretty good at 200mm, although in each instance you will get more resolution by stopping down one stop.

Norm has nuanced the meaning of "sharpness". The number one reason for photos that are not sharp when you want them to be is not the deficiencies of the lens, but the failure of the photographer: missed focus, camera motion or (unwanted) subject motion blur. It helps to know the characteristics of the lens, and for their implications to be instinctive, so that you can concentrate on getting the focus right, the shutter speed sufficient, and getting the camera steady.