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03-12-2009, 02:54 PM   #1
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Sigma 30mm 1.4

This may have been discussed before but I could not find it. I seem to be having focus issues with the Sigma 30mm 1.4 lens. It seems to be a bigger problem closer to wide open in lower light.

Has anyone had similar experience and if so, what was your solution.

Thanks

03-12-2009, 03:13 PM   #2
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I had this problem with my Sigma 24-60 f2.8, but my 30mm 1.4 has been spot on from day one. It's easily my favorite lens.

I sent the 24-60 in to Sigma USA and they re calibrated it and it's been fantastic ever since.

Is your 30mm searching a lot, or are you getting blurred shots? If it's the later, you do have to remember that this lens has a very thin depth of field when wide open.
03-13-2009, 12:59 AM   #3
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When you say "in lower light", do you mean indoor, by tungsten?
If this is the case, you'll have to know that tungsten lighting offsets the AF system, and results in front-focusing. This has to do with the light spectrum shifting toward the reds, and as the AF module is calibrated for green, oups!

If you are familiar with lenses, you'll see that some have a little red mark under the distance scale, slightly off the main mark. See below, right between 4 and 11.

This is the focus indicator for infrared, and shows quite prettily the impact of wavelength on focus.

Anyway, a body focusing perfectly outdoor will be a little off indoor.
03-13-2009, 10:35 AM   #4
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QuoteOriginally posted by dlacouture Quote
When you say "in lower light", do you mean indoor, by tungsten?
If this is the case, you'll have to know that tungsten lighting offsets the AF system, and results in front-focusing. This has to do with the light spectrum shifting toward the reds, and as the AF module is calibrated for green, oups!
My FA50/1.4 shows exactly the same behavior. Outdoors, I can autofocus and shoot wide-open with no problems. Indoors, I have to dial in -9 in focus compensation on my K20D. Stopped down to f/2.8+, there's enough DOF that it doesn't make much difference either way.

I have mostly gotten used to it at this point, though I wish there were an easier way to switch among focus compensation values without having to drill into the "Custom" menu every time. I'd be especially happy if it automatically compensated based on white balance, but anyway . . .

03-13-2009, 10:37 AM   #5
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QuoteOriginally posted by dlacouture Quote
When you say "in lower light", do you mean indoor, by tungsten?
If this is the case, you'll have to know that tungsten lighting offsets the AF system, and results in front-focusing. This has to do with the light spectrum shifting toward the reds, and as the AF module is calibrated for green, oups!

If you are familiar with lenses, you'll see that some have a little red mark under the distance scale, slightly off the main mark. See below, right between 4 and 11.

This is the focus indicator for infrared, and shows quite prettily the impact of wavelength on focus.

Anyway, a body focusing perfectly outdoor will be a little off indoor.
So what is the proper lighting for testing focus indoors?
03-13-2009, 10:38 AM   #6
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get a daylight balanced bulb.
03-13-2009, 10:41 AM   #7
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I did not know that about tungsten light but I did some test with a tripod to make sure there was no camera movement and you are correct.

I went back to some other shots I had taken on an airplane and they are perfectly focused. I do not know what type of light is on an airplane but it seems to make a difference on the AF.

Thanks for the educations.

G
03-13-2009, 06:57 PM   #8
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"If this is the case, you'll have to know that tungsten lighting offsets the AF system, and results in front-focusing. This has to do with the light spectrum shifting toward the reds, and as the AF module is calibrated for green"


I thought AF sensors only saw in monochrome, interesting hypothesis.

03-13-2009, 07:26 PM   #9
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well dlacouture, I tested your hypothesis. and it appears that you are wrong. I tested it using my Pentax 16-45mm f/4 ED AL ( I removed the polariser I had on it, they mess with images displayed on LCD's) using this PSD file which was displayed on my 28" LCD screen. The PSD file has three layers, one high contrast pattern that any lens can lock on to, and a colour pattern that uses colours that a monochrome sensor wouldn't be able to see(it would just see uniform grey)...if the sensor actually saw colour then it would AF on the colour pattern no problems. But mine couldn't achieve a focus lock...the last pattern is what a monochrome sensor would see...it's simply a desaturated version of the color patern which I used as a control to see if the lens was focusing on the pixels on my screen.

the PSD file I used for this test is in this zip file.

Last edited by Digitalis; 04-07-2009 at 06:05 PM.
03-13-2009, 07:42 PM   #10
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QuoteOriginally posted by Digitalis Quote
"If this is the case, you'll have to know that tungsten lighting offsets the AF system, and results in front-focusing. This has to do with the light spectrum shifting toward the reds, and as the AF module is calibrated for green"


I thought AF sensors only saw in monochrome, interesting hypothesis.
Ummm...is that red monochrome or green monochrome...

Think about it.

Steve
03-13-2009, 07:50 PM   #11
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QuoteOriginally posted by stevebrot Quote
Ummm...is that red monochrome or green monochrome...

Think about it.

Steve
well if you looked at how I tested it...the sensor is unfiltered.
03-13-2009, 07:52 PM   #12
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QuoteOriginally posted by stevebrot Quote
Ummm...is that red monochrome or green monochrome...

Think about it.

Steve
Sorry about the quick quip. In reality, I think the spectral balance vs. focus accuracy issue is probably a little more complex that a simple red/green scenario. One that most of us are unlikely to have good comprehension of unless we were an optical engineer of some sort.

Steve
03-13-2009, 08:04 PM   #13
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QuoteOriginally posted by Digitalis Quote
well if you looked at how I tested it...the sensor is unfiltered.
I don't have Photoshop, though I think I understand what kind of images are on your layers.

There are two issues here that are unrelated. The first is the plane of focus for different wavelengths of light. Most modern lenses are pretty well corrected such that the visible spectra converge pretty much on the same plane. The second is the spectral sensitivity of the AF sensor. My expectation is that if the sensor was most sensitive to green light and the subject illumination was primarily on the red end of the spectrum, precision would suffer rather than skewing the accuracy. (The scene would "appear" more "dim" with inherit lower contrast.)

Now since I truly don't know what I am talking about, I am going to shut up now!

Steve
03-13-2009, 08:20 PM   #14
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To the best of my knowledge there aren't any lenses below 100mm that are truly Apochromatic (which means that optically all three wavelengths of light are brought into focus at exactly the same plane) But the colour shifts with non-APO lenses aren't as obvious at normal focus distances..It's only when you're focusing closely at wide apertures when those colour shifts can inhibit maximum sharpness. Generally, lenses with Large apertures seem to suffer from it more than lenses with smaller apertures.

first image is of the Voigtlander Lanthar 125mm f/2.5 APO, second image is from the sigma 50mm f/1.4 - to demonstrate the difference from APO lens designs against non-APO lenses- note the red/cyan fringing in the out of focus areas on the sigma.

Last edited by Digitalis; 05-08-2009 at 12:31 AM.
03-13-2009, 09:53 PM   #15
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I clicked through this thread expecting a discussion of my very favorite lens, and wound up learning something about how light wavelength can effect autofocus! Much love PF!
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