[MSL]

Then it is not focusing that is the issue.

But it can compound a focusing issue. I take your point about Depth of Field, but even that has a subjective aspect to it. There is still only one area that is dead in focus, but there will be a range on either size where the extent to which it is out of focus shouldn't be perceptible. However, if you have a narrow focus throw you could easily get your focus plane at the wrong distance and still not feel things are sharp out to infinity, if you ended up focusing just a bit to the near field.

[richard0170]

I managed to track down a M 28mm f3.5 at a reasonable price (£45 inc P&P, thank you Ffordes), which arrived this morning. Bright sunny conditions prevailing locally, I have spent some considerable time trying out the M 28mm f3.5, A 28mm f3.5 and K 28mm f3.5 on my KP and K3 cameras, both indoors and out. The KP has its original focusing screen and the K3 a micro prism collar screen. These are my observations:-
In terms of absolute sharpness, the K 3.5 is the winner, but this is marginal and only evident at the widest aperture. The A and M @ f3.5 seem about equal to me. Stop any of these lenses down to f5.6 and there is no issue.
The K and A seem equally good at controlling CA's. The M has CA's apparent even when stopped down to F8, but you have to look for them and for normal picture viewing it is not an issue. Even wide open all these lenses are acceptable to me.
They all render colours in a typically wonderful Pentax manner.
In terms of handling, there are some considerable differences, and these relate to exposure, size, weight, focusing and build quality as far as the lenses and camera combinations are concerned.
The K is the best built and smoothest operating, but also by far the heaviest. This is fine on the K3, but makes the KP nose heavy. Not a problem when actually taking a shot but I find it a pain to otherwise carry around. Both the M and A are commendably light and match either camera just fine.
All exhibit exposure inconsistencies when using the cameras' metering system tending towards over exposure when stopped down. This M is by far the worst (2 to 3 stops) rather than c.1 stop on the other 2. All shots I used in my testing were via the aperture ring and the green button on manual mode.
Nailing focus, unless using live view, can be an issue on my KP with all 3 lenses, with the A being the worst. This is I believe due to the focus screen being optimized for relatively slow AF lenses. Using the microprism collar screen on the K3 I have had no issues with any of the lenses regarding focus.
The last thing to consider is flare. All these lenses are designed as wide angles and capture light from a broad field of view, but we are discussing using them as standard FL lenses. I think a lot of the apparent softness issues are down to mild flaring. Attaching a standard lens hood alleviates the issue a great deal on all of these lenses. I suspect my F appears sharper quite often when compared to my A because it has a better coating. Memo to self, unless absolute compactness is required, stick a standard lens hood on any 28mm lens!
So what would I recommend based on my recent testing and longer experiences?
Well, I think that largely depends on what camera you have. If you have a KP, K70 or one of the smaller older bodies and do not mind manual focus and using manual mode the M f3.5 is a nice compact option and which still feels pretty decent on a K3. If you are going to use MF glass a lot though I do recommend getting a screen designed for MF. The A provides to convenience of automated exposure settings and is a nice upgrade with this in mind but I think without the aforementioned screen upgrade I would prefer the M. TBH though I would rather have my F on such bodies just for the AF convenience. If you have a K3 or similar though, there is something undoubtedly special with the K f3.5. And IMO this is due to the superior build quality rather than the marginal optical superiority.
Lastly a word on GUB's observations about DofF. Whilst it is true that you can set your lens to infinity and stop down a little and everything will be in focus, much of the time I for one am not operating anywhere near infinity. In addition based in the UK, we rarely experience really clear atmospheric conditions, and judging any lens resolving power at this setting is somewhat problematic. A few years ago I got to visit Canada and I noticed a huge difference in the quality of light between here and there!

[AgentL]

But it can compound a focusing issue. I take your point about Depth of Field, but even that has a subjective aspect to it. There is still only one area that is dead in focus, but there will be a range on either size where the extent to which it is out of focus shouldn't be perceptible. However, if you have a narrow focus throw you could easily get your focus plane at the wrong distance and still not feel things are sharp out to infinity, if you ended up focusing just a bit to the near field.

I think the issue is more with the way digital sensors render detail. I've been thinking about how small focus inconsistencies with film typically either aren't noticeable, or don't look unpleasant. While digital just seems to look bad if the focus is off even by a hair - even though smaller formats like M4/3 and APS-C should have a larger depth of field. I think digital sensors process the detail to pull out lots of contrast, and are seldom optimized for slightly off-focus detail. Most camera's processors do well with tack-sharp detail and with way-out-of-focus bokeh, but there's a kind of no man's land between those where things just sort of look ugly before they smooth away to bokeh. Like the camera is doing it's best to interpret the tiny bit of unsharpness as sharpness. Also, grain helps with film, it disguises minor focus inconsistencies. But I have a feeling it's the former issue that's primarily to blame.

[MSL]

I think the issue is more with the way digital sensors render detail. I've been thinking about how small focus inconsistencies with film typically either aren't noticeable, or don't look unpleasant. While digital just seems to look bad if the focus is off even by a hair - even though smaller formats like M4/3 and APS-C should have a larger depth of field. I think digital sensors process the detail to pull out lots of contrast, and are seldom optimized for slightly off-focus detail. Most camera's processors do well with tack-sharp detail and with way-out-of-focus bokeh, but there's a kind of no man's land between those where things just sort of look ugly before they smooth away to bokeh. Like the camera is doing it's best to interpret the tiny bit of unsharpness as sharpness. Also, grain helps with film, it disguises minor focus inconsistencies. But I have a feeling it's the former issue that's primarily to blame.

That's an interesting thought and perhaps I should go back to a few "disappointing" images and apply a very small amount of blur or noise to see if it makes the image more appealing.

[GUB]

I think the issue is more with the way digital sensors render detail. I've been thinking about how small focus inconsistencies with film typically either aren't noticeable, or don't look unpleasant. While digital just seems to look bad if the focus is off even by a hair - even though smaller formats like M4/3 and APS-C should have a larger depth of field. I think digital sensors process the detail to pull out lots of contrast, and are seldom optimized for slightly off-focus detail. Most camera's processors do well with tack-sharp detail and with way-out-of-focus bokeh, but there's a kind of no man's land between those where things just sort of look ugly before they smooth away to bokeh. Like the camera is doing it's best to interpret the tiny bit of unsharpness as sharpness. Also, grain helps with film, it disguises minor focus inconsistencies. But I have a feeling it's the former issue that's primarily to blame.

I am curious about what aperture you and the others with issues tend to run at.
I am on the K-1 and routinely shoot f4 or f5.6.
I am rarely at f8 or more.
And because the 28s are not much of a bokeh machine it is only bad lighting conditions that make me use them wide open.
Given that they seem to be all 5 bladed irises and they don't round off at all stopped down I wonder if you are bumping into extra diffraction/interference issues from the corners of the pentagons aggravated by stopping down.

[GUB]

I managed to track down a M 28mm f3.5 at a reasonable price (£45 inc P&P, thank you Ffordes), which arrived this morning. Bright sunny conditions prevailing locally, I have spent some considerable time trying out the M 28mm f3.5, A 28mm f3.5 and K 28mm f3.5 on my KP and K3 cameras, both indoors and out. The KP has its original focusing screen and the K3 a micro prism collar screen. These are my observations:-
In terms of absolute sharpness, the K 3.5 is the winner, but this is marginal and only evident at the widest aperture. The A and M @ f3.5 seem about equal to me. Stop any of these lenses down to f5.6 and there is no issue.
The K and A seem equally good at controlling CA's. The M has CA's apparent even when stopped down to F8, but you have to look for them and for normal picture viewing it is not an issue. Even wide open all these lenses are acceptable to me.

I dont have the K28 but I have Takm42s, M 3.5s , Tak Bayonets and As as well as one of each M28 2.8 mk1 and mk11
I put the Pentax A, the Bayonets and the M28 2.8 mk11 at the top
The M 28 3.5s and the Taks together just behind and the M28 2.8 mk1 is a dog. This is open.
There is a crispness almost a harshness to the later ones that set them ahead. But I wonder if that is what others are seeing as a flaw? The M28 3.5 is more like a usual Pentax image.

[richard0170]

I think the issue is more with the way digital sensors render detail. I've been thinking about how small focus inconsistencies with film typically either aren't noticeable, or don't look unpleasant. While digital just seems to look bad if the focus is off even by a hair - even though smaller formats like M4/3 and APS-C should have a larger depth of field. I think digital sensors process the detail to pull out lots of contrast, and are seldom optimized for slightly off-focus detail. Most camera's processors do well with tack-sharp detail and with way-out-of-focus bokeh, but there's a kind of no man's land between those where things just sort of look ugly before they smooth away to bokeh. Like the camera is doing it's best to interpret the tiny bit of unsharpness as sharpness. Also, grain helps with film, it disguises minor focus inconsistencies. But I have a feeling it's the former issue that's primarily to blame.

One theory I have heard are that digital sensors are made up of micro lenses and old film lenses do not necessarily focus properly on them, with the problem being worse with wide angle designs compared to telephoto ones. It seems reasonable, but there are plenty of old wide designs out there that perform to many peoples' satisfaction. I am inclined to think, focus issues aside, minor flaring is causing apparent lack of sharpness. Potentially you not only have flare from the lens itself but also flare playing on the sensor's micro lenses. Perhaps this is why Pentax have introduced newer coatings on old lens designs, such as the HD ones on the FA primes.

Another theory which also sounds reasonable is that, especially with old wide angle lenses, dust accumulates on the rear element. As these lenses have a relatively large depth of field the dust is disproportionately effecting the image

---------- Post added 04-20-21 at 02:48 PM ----------

One thing I forgot to mention from today's tests is that the M 3.5 did seem to provide more punchy, contrasty B&W images than the others at the same camera settings.

[richard0170]

Having now convinced myself that the cause of most complaints about the A 28mm f2.8 is down to user error (including myself), I have decided to give the lens an extended run on my K3 in real world situations. From today......

[trixtroll]

Having now convinced myself that the cause of most complaints about the A 28mm f2.8 is down to user error (including myself), I have decided to give the lens an extended run on my K3 in real world situations. From today......

I reached the same conclusion and ditching the UV filter seemed to make a huge difference for me. I probably could have tweaked the AF a little to compensate, but the option to do it for a single lens seems to be unavailable when using the A lens.

[GUB]

Great work guys.
I had a look at the distance scale on the A and now I commiserate with you!
The first digit down from infinity is 3 metres and it is only about 5mm from the stop.
I guess I am just used to it. And the use of Live view and focus peaking.
I checked and all my other Pentax 28s are the same except for the takumar which is about 8mm
And looking at off brands they are all similar.
I guess it is a function of the constant pitch of a helicoid and the need to get all focusing done in one rotation. And the rest is pure physics. Have a finer pitch helicoid and you are lengthening your closest focus.

[Jonathan Mac]

Having recently been using a K series 28mm f/2 I can say that it's certainly the sharpest wide open, and by some margin, when compared to either the Komine or the Kiron 28mm f/2.

[richard0170]

Having recently been using a K series 28mm f/2 I can say that it's certainly the sharpest wide open, and by some margin, when compared to either the Komine or the Kiron 28mm f/2.

You are indeed most fortunate to own such a rare thing! I just looked on the EvilBay and only one is available at $1250 plus P&P and any taxes. Ouch!

I would suggest that the K f2.0 is not a practical option as a walk around "standard" lens on a crop body. Too big, Too heavy, expensive and virtually unobtainable. Never the less, I am quite envious of you and your acquisition!

[Jonathan Mac]

You are indeed most fortunate to own such a rare thing! I just looked on the EvilBay and only one is available at $1250 plus P&P and any taxes. Ouch!

I would suggest that the K f2.0 is not a practical option as a walk around "standard" lens on a crop body. Too big, Too heavy, expensive and virtually unobtainable. Never the less, I am quite envious of you and your acquisition!

I wouldn't say it's too big or heavy for a walk-around. I doubt I'd do any travelling with it as I'd need to carry it around all day, but for going out for a wander with it's perfectly fine.

[bdery]

The FA28 f/2.8 is the one Pentax lens that I sold and very much regret selling. Slightly 'romantic' wide open, but sharp as anything from f/4. I have the both the K and the M 28 f/3.5s. Everybody says the K is better, but to be honest I can't tell the difference. They are both very good, and fun to use.

For those wider lenses, I'm always a sucker for starbursts, and the FA28 isn't good for that. If it had been, I'd have kept it. I hear the F28 is better but I've never tested it.