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02-13-2015, 12:59 PM   #1
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645Z and D FA 55mm and CA

I have been plagued since the beginning, July 18, 2014, over the amount of CA on the 55mm w/o on the 645z body. The main reason for getting the 55mm was to use it wide open.
I've just picked up my replacement body and lens and before leaving the store I tested all combinations. No matter which lens on whose body, I was always ending up with strong green and magenta CA at 2.8.
I can obviously fix all that in ACR/LR but not if the model is wearing a garment of the same colour.
It has proved a mild PITA for a number of advertising projects.
I also tested the 45-85 at 55mm wide open at 4.5 and granted it only 4.5 but this lens is a good 20 years old, nothing; clean with no CA what so ever.
I would expect a new modern lens, even from a smaller manufacturer to be CA free for the most part.
Pentax has been exceptional in trying to help me. My body and lens are on there way to France or somewhere to be checked over but I'm sure they will come back and tell me everything is well within tolerances.

Has anyone else seen this CA, wide open on their 55mm?

Thanks
Rob


Last edited by rcdurston; 02-13-2015 at 02:45 PM. Reason: phew glad that was caught, might have been real trouble with CA otherwise
02-13-2015, 01:20 PM   #2
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QuoteOriginally posted by rcdurston Quote
I have been plagued since the beginning, July 18, 2014, over the amount of CA on the 55mm w/o on the 645z body. The main reason for getting the 55mm was to use it w/o.
I've just picked up my replacement body and lens and before leaving the store I tested all combinations. No matter which lens on whose body, I was always ending up with strong green and magenta CA at 2.8.
I can obviously fix all that in ACR/LR but not if the model is wearing a garment of the same colour.
It has proved a mild PITA for a number of advertising projects.
I also tested the 45-85 at 55mm w/o at 4.5 and granted it only 4.5 but this lens is a good 20 years old, nothing; clean with no CA what so ever.
I would expect a new modern lens, even from a smaller manufacturer to be CA free for the most part.
Pentax has been exceptional in trying to help me. My body and lens are on there way to France or somewhere to be checked over but I'm sure they will come back and tell me everything is well within tolerances.

Has anyone else seen this CA, w/o on their 55mm?

Thanks
Rob
You keep saying w/o...that's short for with out and makes no sense
02-13-2015, 01:39 PM   #3
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Wide open? Like enoeske said, w/o means without.
02-13-2015, 01:48 PM   #4
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w/o - definition of w/o by The Free Dictionary

02-13-2015, 03:25 PM   #5
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yeah I'm sure he meant wide open. But probably best not to abbreviate it w/o as it is in fact without
02-13-2015, 03:39 PM   #6
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Any answers to the main issue?
02-13-2015, 03:42 PM - 1 Like   #7
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QuoteOriginally posted by rcdurston Quote
Any answers to the main issue?
give people some time, we've only just deciphered what you typed
02-13-2015, 10:31 PM   #8
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the Pentax 645 FA55mm f/2.8 isn't apochromatic - not even close, so LOCA and PF are going to be visible if you use the lens at f/2.8 all the time. The pentax 45-80mm f/4.5 isn't apochromatic either - but it is a slower lens, it is easier to design and correct a slower lens. It is harder to design and optically correct a fast lens - that is why lenses like the Zeiss OTUS Distagon 55mm f/1.4 are so expensive and heavy and lenses like the Zeiss Planar 50mm f/1.4 are lighter and cheap* - it is all about the level of optical quality and correction for colour aberrations**.

QuoteOriginally posted by rcdurston Quote
It has proved a mild PITA for a number of advertising projects.
Exactly what are you doing that requires you to use this lens wide open? For commercial work I avoid using my lenses are their widest apertures if it is at all possible. And I work with some of the best lenses ever made.

* Less lens elements make for a simpler optical design, reduces flare, keeps weight and reduces the size of the lens and keeps cost per unit down. The Zeiss OTUS is heavy, has nearly twice the elements of the Planar 50mm f/1.4 but offers perfectly sharp images at f/1.4 when compared to the Planar lens design. The Zeiss OTUS is a classic example of cost-no-object Optical design.

**But bear in mind even the Zeiss OTUS isn't perfectly apochromatic, LoCa is still visible - but it is nowhere near as bad as other 50~55mm f/1.4 lenses.


Zeiss Otus : 12 lenses in 10 groups
Zeiss Planar: 7 elements in 6 groups.


Last edited by Digitalis; 02-13-2015 at 10:48 PM.
02-14-2015, 01:00 AM   #9
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Buy the DFA 90mm macro and step back a meter or so.

Problem solved.
02-14-2015, 01:10 AM   #10
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QuoteOriginally posted by 2351HD Quote
Buy the DFA 90mm macro and step back a meter or so. Problem solved.
This solution is grossly ignorant, the D-FA 90mm f/2.8 macro also has CA at f/2.8 - in fact, just about every fast f/2.8 medium format lens below 100mm has it - as well as a few non-apochromatic lenses above 100mm. But be thankful that the LoCa and purple fringing isn't anywhere near as bad as the majority of 35mm lenses.
02-14-2015, 01:16 AM   #11
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the zeiss master primes for cinema work are a classic example of cost no object design. the zeiss otus is an example of "the best you can get for twice the cost of the nikon canon offerings"

I don't find the 55mm 645 lens very usable at 2.8 myself. mine looks okay at 4 and real good at 6.3
02-14-2015, 02:12 AM   #12
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Perhaps I should have elaborated.

Stop down the 90mm to f4 and it will not only be cleaner and sharper, but will have equal DOF and OS.

It's not an ideal solution, 55 is 55, but obviously the OP needs to throw out the background and have clean bokeh with little fringing.

The OP is complaining about something that should have been tested prior to dumping the cash on an expensive system that could have been achieved with a D810 and 55 Otus with similar but not quite the same results.

Any amount of online research such as Diglloyd.com and GetDPi would have shown him that this is what to expect of the 55. It's an $1100 kit MF lens.

I wish we had a good answer for him but there just isn't one inside this Pentax system.

---------- Post added 02-14-15 at 07:16 PM ----------

Oh, and I have seen the type of shots the OP is looking to create at f2.8, staged advertising shots of models with backlighting etc.

I think the 90 would handle this much better and I am sure that for at least 80-90% of the shots he could find some space to move back as it is all set up and staged.

This is all off having seen one or two of his shots on the same subject of the CA of the 55. It could be much more diverse and that only he can answer.

Rcdurston takes some lovely shots and I understand his frustration.
02-14-2015, 04:42 AM   #13
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Thanks everyone. I was looking for more real world experiences and examples. I'll need to rethink how I'm going to approach my current project instead. Might go back to 4x5 to get the look I'm going for, that won't be very spontaneous
I'll ask for a loan of the 90mm as well as see how it does.
Thanks again.
R
02-14-2015, 09:10 AM   #14
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I was actually surprised at how much pretty much all Pentax lenses exhibit LoCA and in some cases purple fringing, but then I remembered that I was dealing with a 51MP camera, my old systems never broke 21MP so I was often ignorant of the fact that they actually had a lot of CA too, just with a lower magnification. In testing this out, I discovered that the 55/2.8 fringes about as much as my Canon 24-70 2.8 mkII, which I never though to have much fringing, but I never looked too hard into its IQ. For a relatively inexpensive, small and light lens, I really didn't expect much to begin with, but it can resolve the sensor for the most part at smaller apertures.

Unless you use some super "over-designed" lens, you probably won't get a razor sharp image, but the Pentax system doesn't yet seem to have any such lenses, although many people say the 90mm and 28-45mm qualify for that description. The 120mm macro is probably one of the most well-corrected lenses from the "old" glass, and even that has LoCA wide open.

Rather than making a set of super sharp and fast primes like Leica, Pentax is going the other way by first releasing various zooms, the 28-45mm is already out, so it leaves the 45-80mm and 80-160mm models, and maybe after that they'l begin refreshing primes, or perhaps bringing out new ones, but it'll take a few years to get there.

QuoteOriginally posted by rcdurston Quote
Thanks everyone. I was looking for more real world experiences and examples. I'll need to rethink how I'm going to approach my current project instead. Might go back to 4x5 to get the look I'm going for, that won't be very spontaneous
I'll ask for a loan of the 90mm as well as see how it does.
Thanks again.
R
It's kind of hard to compare large format glass to anything else, especially if you can utilize the newer digital-optimized lenses, which you can cut steel with even wide open. It's one of the reasons why there are still people who would dish out 2-3x the price of the 645Z for a digital back with the same sensor in it that can be used on a tech cam.
02-14-2015, 11:05 AM   #15
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The interesting part, is that there's a photographer who put out a post on the 645z and 55mm lens

Pentax 645z review - CCD vs CMOS and Pentax Lenses Chris Giles Photography Blog

55mm D-FA (Fast and silent, no CA anywhere)
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