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06-06-2011, 12:03 PM   #1
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1.4 lenses and CA

Hello,

How much CA is to be expected with lenses that can go to F1.4? I know there is a few things to take into consideration, like brand and model etc.

The reason I ask, is because I just bought a Sigma 30 mm 1.4 and basically, so far, all pictures taken with F1.4, seem to have CA, unless I tell my K-5 to take in less light IE lowering some EV stepps... like -2.

Also, the IQ don't seem much better than what you can find on a zoom lens, but I imagine that 1.4 makes it alot harder to make pictures sharp due to shallow DOF.

I hope I'm just suffering from a post purchase regret syndrome...

Any guidelines would be appriciated for me to keep in mind, while "testing" this sigma lens.

06-06-2011, 12:11 PM   #2
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I expect to recieve my sigma 30 soon, i will share my thoughts then.
Anyway, i used to have a FA50 1.4 and i didn't notice too much CA, but if you ask me about purple fringe at 1.4, thats is another history.
06-06-2011, 12:18 PM   #3
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Not so much that you would notice it without zooming into 100%.

You won't be shooting that often at f1.4. Stop the thing down to f1.8 or f2 and you will be much happier with the results.
06-06-2011, 12:19 PM   #4
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Well the performance off this lense isn't so impressive wideopen. CA is at all apertures to some degree present.

Sigma AF 30mm f/1.4 EX DC (Pentax K) Review / Test Report - Analysis

06-06-2011, 12:21 PM   #5
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@Sekul: It's like purple fringing, but just white instead. I can't upload a picture now, but I will do so tomorrow.

Last edited by mikknu; 06-06-2011 at 12:29 PM.
06-06-2011, 12:37 PM   #6
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QuoteOriginally posted by RonHendriks1966 Quote
Well the performance off this lense isn't so impressive wideopen. CA is at all apertures to some degree present.

Sigma AF 30mm f/1.4 EX DC (Pentax K) Review / Test Report - Analysis
I did actually read that before buying, but in the end, the user reviews on this site weighed in more, also, the only other lens at 30mm I could find was the Pentax FA 30mm, which is a whole different price range.

I don't understand the charts well enough, to know what a picture will look like from those stats, but maybe I will now
06-07-2011, 04:15 AM   #7
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You have to be careful with focusing if you shoot at f1.4, because dof is quite shallow. Also make sure to check for FF/BF and fine tune your AF if needed.

There is some CA at edge of high contrast areas (usually in backlit photos), but you have to pixel-peep for that. I get much more CA and PF (noticeable even without pixel-peeping) from DA* 16-50 and from FA 77 Ltd.
06-07-2011, 05:31 AM   #8
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post an examples of shots you are unhappy with and some folks will be able to tell you whether it's normal or expectable or a dud lens....

06-07-2011, 11:03 AM   #9
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Thanks for your replies so far guys. Went out in a well enough lit forest today and took some test shots. My conclusion so far is, that the CA is at an acceptable level for this lens, when looking at the price.

However, my K-5 autofocus, didn't quite seem up for the task of landing a well focused shot, which has happened more than I'd like lately.

At first I though this was due to the shallow DOF, but even at F4, it still couldn't focus well. Out of 80 shots, around 8 of them came out in focus.
I could beat or match every auto focus shot with a manual focused shot.
The ones it did get right, was those taken close up...

I would blame the lens, was it not because I have had these same problems with the Pentax DA* 16-50 and the Tamron Dii 17-50. Also, last weekend, my Sigma 70-200 HSM II seemed to struggle.

Only lens that haven't really seemed to struggle, is my K-x 18-55 kit lens.

It's driving me crazy, I can't figure out if it is the camera, me or if I just have very bad luck with lenses these days.

In short, I can produce a good sharp image in MF but not with AF...

Also, I'm uploading a picture of the CA in the sigma 30 mm below:

06-07-2011, 11:30 AM   #10
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QuoteOriginally posted by mikknu Quote
However, my K-5 autofocus, didn't quite seem up for the task of landing a well focused shot, which has happened more than I'd like lately.
Generally, you don't want to be using a lens wide open. The exception to this rule is the rather slow DA limiteds, which are built as if they were pre-stopped down. The FA 50 is heads and shoulders better at f2, and it's generally best to bump the ISO one than to resort to f1.4 except in extreme cases.

Here's where 1.4 is useful - a bowling alley. In fact, it was "disco bowling", the lights were off save some coloured decorations and blue lights. The image quality is not spectacular, but when you are toting the K20d (and don't want to blast people in the face with a flash), you're thankful you come home with any memories at all!

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There was enough light with this shot so I could actually shoot at 1/50. The detail in the distance is nice and sharp, with *some* bleeding due to CA, but nothing that really effects the photo.

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This one is pretty blurry - it was taken at 1/15. I was really pushing it with this one, but it's good enough for what it is. With a slower lens, this shot would have been impossible.

I converted to B&W to offset the rather nasty colours that show up when you shoot under blue lights with neon accents. Ick.

---

The 50 1.4 has some shortcomings wide open, but if you stop it down, it becomes monstrously sharp, and can go toe to toe with any of the FA or DA ltds. with respect to detail rendition.

The FA 50 is a pretty good deal. Thankfully I think it's because it comes in such ugly housing. But as we all know, it's inner beauty that counts .
06-07-2011, 12:23 PM   #11
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From my research on the lens, one issue that people had with it was Sigma's quality control.

I would say 50% of the reviews were good, and 50% were similar to your experience.

The copy that I had (in EF mount) performed well.

From looking at your example, my copy was superior. Not even close to the amount of CA / fringing, and definitely sharper - than what you're experiencing.

The fact that 10% of your shots are in focus is pretty brutal. If your lens is still under warranty, I would suggest looking into getting it re-calibrated. Or check for ff/bf'ing as simico recommends.


Unfortunately, I sold the lens a while back, but I'll dig around and see if I have any examples to upload for you to compare.
06-07-2011, 01:43 PM   #12
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QuoteOriginally posted by c5k0 Quote
From my research on the lens, one issue that people had with it was Sigma's quality control.

I would say 50% of the reviews were good, and 50% were similar to your experience.

The copy that I had (in EF mount) performed well.

From looking at your example, my copy was superior. Not even close to the amount of CA / fringing, and definitely sharper - than what you're experiencing.

The fact that 10% of your shots are in focus is pretty brutal. If your lens is still under warranty, I would suggest looking into getting it re-calibrated. Or check for ff/bf'ing as simico recommends.


Unfortunately, I sold the lens a while back, but I'll dig around and see if I have any examples to upload for you to compare.
A lot of the shots taken didn't have fringe or CA on them. I think CA and fringing was mainly on the backlit shots, though it was cloudy outside today.

The back/front focusing is my biggest concern just now. I'm gonna feel a bit stupid if I walk down to the store again, to tell them that my third lens in a row from their store, seems to be faulty... That's why I think it might be the camera.

I know only of one other 30 mm'ish lens and that is the Pentax FA 31 1.8 ltd lens and that is a very pricey one in comparison. If I get the chance for a swap, I might consider the sigma 50 mm 1.4 instead, but I'm not sure, 30 mm is a nice focal length.
06-07-2011, 02:00 PM   #13
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QuoteOriginally posted by mikknu Quote
A lot of the shots taken didn't have fringe or CA on them. I think CA and fringing was mainly on the backlit shots, though it was cloudy outside today.

The back/front focusing is my biggest concern just now. I'm gonna feel a bit stupid if I walk down to the store again, to tell them that my third lens in a row from their store, seems to be faulty... That's why I think it might be the camera.

I know only of one other 30 mm'ish lens and that is the Pentax FA 31 1.8 ltd lens and that is a very pricey one in comparison. If I get the chance for a swap, I might consider the sigma 50 mm 1.4 instead, but I'm not sure, 30 mm is a nice focal length.
Wow, now im scared to recieve my 30mm, its almost 50% chance to get dud.
If mine comes wrong i think i will swap for da 55 1.4, because i have bought this for low light primarily.
I have discarded the sigma 50, because its huge, also if my coming 30mm fails to deliver i dont think i will trust in the sigma brand.
And the fa 31... im not worthy for such fine lens, not just yet
06-07-2011, 02:22 PM   #14
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QuoteOriginally posted by Sekul Quote
Wow, now im scared to recieve my 30mm, its almost 50% chance to get dud.
If mine comes wrong i think i will swap for da 55 1.4, because i have bought this for low light primarily.
I have discarded the sigma 50, because its huge, also if my coming 30mm fails to deliver i dont think i will trust in the sigma brand.
And the fa 31... im not worthy for such fine lens, not just yet
Well in some occasions DA55 can have CA when light-contrast is big enough. And you should check your lens just for FF/BF and correct that in camera body (K-7/K-5 is the easiest way to do that).
06-07-2011, 06:34 PM   #15
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I wish I was more familiar with Pentax lenses. I'm a Canon user who finds joy in using manual focus lenses with adaptors. So, my only recommendations would be in the Canon EF world - or manual focus, which I'm guessing you're not looking towards.

But, I can say that this lens is plenty sharp.

Here's another example I found.

ISO 100, 1/1600, f/1.4 with Rebel t2i




5184 x 3456 size

Hope this hyperlink works. There's the original size.

Photo is basically untouched, except for -1 exposure.

Funny, only now I'm noticing that this lens does have some average control of CA @ f/1.4. I have an f/16 version of the same shot which fairs slightly better.
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