Pentax 50mm f1.2 K aberration monster or normal behaviour?

[sdekanic]

Hi all!

I am new to this forum, at least with posting, I have been reading it a lot for the last two years since I got hooked up to MF lenses...

I would appreciate a lot any help, advice or input from older and wiser members with experience with these types of lenses.

SO, I have recently acquired Pentax 50mm f1.2 lens in K mount, pre-M version. I have read a lot about the lens before I got it. Lens is in full working order. No fungus, scratches, balsam separation or haze. When looking through the lens in direct light glass elements are crystal clean.

I am aware of all kinds of aberrations and softness and stuff ("character") while taking pictures with lens wide open at f1.2, and I was OK with that. However, the amount of chromatic aberrations I am getting from this lens is just so overwhelming, that I am suspecting I have a lemon.

I am getting a lot of chromatic aberrations in wide apertures (pictures #1 & #3), and not so wide apertures like f4-ish (picture #4). Especially in tree branches against white sky. And a lot of purple fringing in top corners at apertures where I did not expect any aberrations, like in f8 (picture #2).

I have also made a closer inspection of the glass elements today, and when light is not coming directly through the lens, there seems to be a lot of tiny, very tiny bubbles in the glass element that is located just beneath the front element. I always thought these bubbles cannot affect the IQ. However, on the last picture you can see that these artefacts in the glass in that particular lighting condition look like a lot of tiny black dots. I repeat, these dots are completely invisible when looking directly through the lens in daylight.

Please, if you have experience with this lens, can you please tell me if this is normal behaviour for the 1.2 lens, or there might be something strange going on?

I greatly appreciate any response to this post!

Thank you very much in advance!




wide open




f8, purple fringing in top corners




f1.4




f4-ish

 

[Aoi Usagi]

Hi all!

I am new to this forum, at least with posting, I have been reading it a lot for the last two years since I got hooked up to MF lenses...

I would appreciate a lot any help, advice or input from older and wiser members with experience with these types of lenses.

SO, I have recently acquired Pentax 50mm f1.2 lens in K mount, pre-M version. I have read a lot about the lens before I got it. Lens is in full working order. No fungus, scratches, balsam separation or haze. When looking through the lens in direct light glass elements are crystal clean.

I am aware of all kinds of aberrations and softness and stuff ("character") while taking pictures with lens wide open at f1.2, and I was OK with that. However, the amount of chromatic aberrations I am getting from this lens is just so overwhelming, that I am suspecting I have a lemon.

I am getting a lot of chromatic aberrations in wide apertures (pictures #1 & #3), and not so wide apertures like f4-ish (picture #4). Especially in tree branches against white sky. And a lot of purple fringing in top corners at apertures where I did not expect any aberrations, like in f8 (picture #2).

I have also made a closer inspection of the glass elements today, and when light is not coming directly through the lens, there seems to be a lot of tiny, very tiny bubbles in the glass element that is located just beneath the front element. I always thought these bubbles cannot affect the IQ. However, on the last picture you can see that these artefacts in the glass in that particular lighting condition look like a lot of tiny black dots. I repeat, these dots are completely invisible when looking directly through the lens in daylight.

Please, if you have experience with this lens, can you please tell me if this is normal behaviour for the 1.2 lens, or there might be something strange going on?

I greatly appreciate any response to this post!

Thank you very much in advance!

The amount of chromatic aberrations seems about right.

I really can't see any of the defects that you describe in the last photo. The image is extremely dark. If the black dots you describe are specks of dust, it will not cause CA, unless its really dirty.

 

[xtam667]

I have no experience with this specific 1.2/50 but my much newer Canon EF 1.2/50 also has lots of chromatic aberrations wide open. My even newer Nokton 1.2/40 has it in spades too. It is only my newest Nokton 1/50 that can keep it in check, but for an exorbitant price.

So, I would say your lens is not seriously out of whack, if at all. #4 shows very decent performance in my view. #1 has monstrous LoCA indeed, but then the scene is a torture test for any lens.

The bubbles / dots are not out of the ordinary for vintage glass and have a negligible effect on image quality. I have several such lenses in two copies, one pretty bad and the other one clean, and cannot tell the pictures apart.

 

[sensiblename99]

...


wide open


f8, purple fringing in top corners


f1.4


f4-ish

...

I don't have fast Pentax lenses, because like Fuji and Nikon they are the Devil's lenses as they focus the wrong way, but I have several F1.2 50-58 lenses and they all exhibit the kinds of aberrations you are showing, but to varying degrees. The keyboard shot looks about normal, even for the Canon EF 50/1.2L. The LoCa that you see in the trees can be due to those branches not being in the plane of focus so they will have some colour fringing, green or purple. The lens might even have some Lateral CA which does not go away when stopped down although this will show as red/blue fringing on contrasty edges.

 

[fferreres]

I don't think it's caused by the tiny dirt and it'd be hard for a lens to have this problem, as it'd have to be a problem with the elements themselves. I don't own this lens, maybe someone does and will chime in, but it seems the normal kind of strong longitudinal (axial) aberration. It's not purple! It's a different problem and will only show in very bright parts slightly ahead and behind the plane of focus.

 

[ProfHankD]

I have recently acquired Pentax 50mm f1.2 lens in K mount, pre-M version. ...

I am aware of all kinds of aberrations and softness and stuff ("character") while taking pictures with lens wide open at f1.2, and I was OK with that. However, the amount of chromatic aberrations I am getting from this lens is just so overwhelming, that I am suspecting I have a lemon.

You are way too easily overwhelmed. ;-)

Compare these to a modern kit zoom and I assure you the kit zoom images will look worse. Any f/1.2 lens is optically quite challenging, and some qualities will have to be traded for others. Your lens has a fair amount of bokeh CA, but otherwise seems about as good as I think you have any reason to expect. Remember that the lens you are using was not designed for a sensor, but for film, and the different spectral sensitivity curves commonly reveal out-of-band fringing (i.e., purple fringe or PF) which is really just NIR and/or NUV light not being focused in the same plane as visible. BTW, those lenses have exceptionally good coatings, so you're probably doing way better on flare.

In sum, except for the bokeh CA, I'd say it's better than average for an old f/1.2 fast fifty. The bokeh CA isn't that much worse than average, but is disappointing.

 

[sdekanic]

Thanks for reply!

these are definitely not dust specks, but rather a very tiny bubbles in the glass of the glass element itself. I was surprised to see that in that dark lighting conditions they appear as black dots. They are invisible while looking through the lens at bright daylight.

 

[sdekanic]

Thank you very much for your insight! Thanks also to all other members who chimed in with their thoughts.

At 250 EUR this was the most expensive lens I bought so far. I guess I expected miracles. Many reviews I read, especially on pentax forum, never mentioned these chromatic aberrations. I guess that was common knowledge that slipped under my radar due to my limited experience.

No other way for me to become friends with this lens, such as it is, but to shoot a lot of pictures with it

I hope to share some good ones in short while!

 

[fferreres]

Thanks for reply!

these are definitely not dust specks, but rather a very tiny bubbles in the glass of the glass element itself. I was surprised to see that in that dark lighting conditions they appear as black dots. They are invisible while looking through the lens at bright daylight.

I don't think it has any significant effect and many older lenses have a few bubbles, depending on how they manufactured glass. The lens probably uses some very custom glass types, and was made in small batches. I would not worry!

 

[John Crowe]

I recommend shooting in RAW and reducing CA in post processing.

Ultrafast lenses are a challenge, and I doubt the perfect one exists. Some have smooth out of focus areas, some are extremely sharp wide open, some have limited CA, some may have superior contrast or control over flare. If you can get enough characteristics to make good use of the lens that will be a win.

I went through 3 lenses that had smooth bokeh, but were horribly unsharp wideopen. I finally settled on having sharpness wideopen but with onion rings in out of focus highlights, and slightly less smooth out of focus areas.

Use your lens in a variety of ways and determine how you can make use of it's strengths and how to limit it's negatives.

 

[sdekanic]

hi and thanks for the insight.

I already started to explore behaviour of this lens in different circumstances.

I am sure I will also learn a lot about photography too, just by thinking more while taking photos!