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12-01-2011, 01:55 PM   #1
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pentax k-r

I am chilean and I a new fanatic of pentax cameras .Two weeks ago I buy my camera and I very happy whit this

12-01-2011, 01:58 PM   #2
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12-01-2011, 02:46 PM   #3
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12-01-2011, 05:17 PM   #4
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I am enjoying mine. I don't understand over at the "other" forum telling me that my K-r, like theirs, is troublesome and unusable in tungsten light. While it is true that most of our lighting is CFL and fluorescent, in our one room that has tungsten, I seem to have no problems that I can tell. Also, all my lenses, which are the two DAL kit lenses and DA lenses, 35mm f2.4, 18-135mm, and 16-45mm, give no problem that I can tell. Perhaps I am not skilled enough to detect problems or just blissfully ignorant. I use the viewfinder indoors 90% of the time. Does this affect every single K-r or is it only some? Granted I am not a professional and probably not as skilled as the serious amateurs.

12-01-2011, 09:54 PM   #5
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The maximum aperture of the kit lenses isn't really that large, so the front-focus issue tends to be masked by the depth of field in many practical situations. That said, if you use maximum aperture and a relatively close subject under tungsten illumination, you might detect it even with the kit lenses. "Unusable" in tungsten light seems to me to be a little bit of hyperbole if we're talking about the roughly f/4 aperture of the kit lenses, but I don't have a f/1.4 lens so I can't test it out myself. Maybe a really fast lens like that is unusable.

The issue is apparently due to a wavelength sensitivity in the optical path of the focal system and the heavily red-shifted spectrum of tungsten illumination, compared to most fluorescent phosphors or to daylight. People claim that changing the white balance setting can help it. Maybe, maybe not. The wavelength sensivity is inherent to the materials used to focus light on the focus sensor, which can't be changed by altering white balance. It is possible that the camera's firmware could read a manual white-balance setting and introduce an offset to the "in-focus" signal from the focusing sensor in an attempt to compensate for the aberrated optical path, but I have a hard time believing that they would actually go to such lengths. If true, it would have to be from a manual WB setting - automatic WB can't be set until after the image has been acquired, at which point it is way too late to change focus. If you poke around the forum you can find a lot of discussion but not a lot of controlled experiments. (which are hard to do, I'm not trying to diss anybody's efforts, just pointing out the obvious.)

You can mitigate it by getting more light onto the subject, using brighter lighting or flash, so that the lens stops down and increases the depth-of-field. (Or use Av mode and set a smaller aperture, or Pv with a program-line shift to a smaller aperture - if you can tolerate the increased shutter time and/or increased ISO and noise.) If flash helps, it is because the extra light allows the lens to stop down a bit - the focusing still has to be done in tungsten light before the mirror lifts. By the time the flash fires, the focus has been set and frozen long before. Note, however, that mixing flash with tungsten light tends to produce unflattering white balance unless you correct it in post-processing. (An interesting experiment to try is to mount the camera on a tripod, in a room with tungsten illumination, focused on a neutral-colored subject, with the flash forced on and auto WB. Take a series of photos in Tv mode, varying the shutter from maybe 1/15 to 1/180. Assuming the room light is reasonably bright and the subject has average reflectivity, the camera will do a good job of maintaing the correct exposure. However, you will see a pronounced shift in color balance as the camera tries, but fails, to cope with the shifting proportion of scene illumination that comes from the room light and the flash.)

Also note that, if you use live-view mode with contrast-detection focusing, this focusing method is immune to the front-focus effect regardless of lens aperture. However, it is decidely slower than the phase-detection method that is used when you are using the viewfinder, and lots of people just don't like using the screen to frame and compose the image.
12-01-2011, 10:45 PM   #6
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Thanks for the further explanation, Jdbosma. Probably since my widest aperture is f2.4 and my house is either CFL or fluorescent except one room, I escape most of the problem. Yesterday I was shooting my 16-45 (F4) in the tungsten room just because that was what I had on my K-r at the time and could not induce it but that was obviously too small an aperture. But I did notice as far as focusing, it focused extremely quickly, with either viewfinder or live view. That is not a typical room, however.
12-02-2011, 06:45 AM   #7
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jdbosma, your post here has a more technical opinion of what's causing this issue than I've read elsewhere. Can you provide any links to further reading, or citation of any kind?
12-03-2011, 12:03 AM - 1 Like   #8
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I should point out, as I tried to suggest, ("The issue is apparently due to ...") that I'm not 100% sure what the problem it. It's a conjecture, but I think a plausible one.

I got the idea from this thread on dpreview:
A Theory about Pentax K-R Front Focus Errors: Pentax SLR Talk Forum: Digital Photography Review
I also know just enough about optics to be dangerous, and the suggested explanation made some sense. However, he talks about the "wavelength sensitivity of the sensor" but what's more likely going on is chromatic aberration in the lens array that focuses light on the phase-detection sensor, not in the sensor itself. All known refractive materials have dispersion (the index of refraction varies with wavelength) and one reason that lenses have multiple elements in them is to use different materials with mutually compensating dispersion to reduce the chromatic aberrations of the lens overall. It adds a lot to the cost of the lenses, but there really isn't any other way. It would make sense for Pentax to design a less expensive, but also less well-corrected, optical path for the focusing system, which works well enough most of the time but could well have some sensitivity to the strong IR components of tungsten light. The upshot would be basically a calibration error that creeps in when the spectrum of the incoming light deviates from what it is designed for. (And it's also quite plausible that one of the added costs in a K-5 is a better corrected lens - I don't know the difference between "SAFOX IX" and "SAFOX IX+," but apparently they have the same number and arrangment of autofocus points, so there must be something else going on. It could be as simple as a coating that filters out the aberrated wavelengths, as suggested in the dpreview post.)

You can get a visual picture of how this looks in a nice graphic on this Stanford site:
Autofocus: phase detection

and while I'm at it here's another discussion on the same topic:
Understanding Camera Autofocus

to find more - a good search phrase to start with is "phase-detection auto focus"

The Stanford graphic shows the two light intensity peaks coming nicely together at focus, but that's an idealization and would require precise matching of the path lengths in the focus-sensor and image-sensor light paths - expensive, and not likely for a consumer product with strict cost targets. What almost certainly happens in production is that the camera maker assembles the components, then there is a calibration step to figure out what separation between the positions of the bright spots corresponds to focus. That gets turned into a number that is then recorded in the camera's calibration memory. (On the K-r and K-5, you can tweak it with options in the setup menu.) So that's why I wrote that perhaps the firmware could tweak the focus depending on the manual WB setting - it would be just a little bit of adjustment to the "in focus" separation between the light peaks. But whether they actually do that or not is pure speculation. I'd be a little surprised if they did, because it's likely to spoil more shots than it saves, but I don't have real information one way or the other. (if you take pictures with the wrong WB, that can be fixed, but if the wrong WB ruins your focus, that image is lost. And you are a mad customer.) On the other hand, some of people who have discussed this issue on these forums claim that they could detect a difference with a manual WB setting. (but then some claim they can't detect a difference, too.) On the third hand, I did some experimentation myself and what I mainly convinced myself of is that it is pretty tricky to get consistent results, and when people are looking for an answer that is buried in noise, sometimes they see what they want to see.

Anyway, whatever the cause, the surest cure is to get more light on the subject so that you can stop down the lens and improve DOF. No speculation required there.

12-03-2011, 06:50 AM   #9
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The reason I asked about it is mainly I was wondering if it was consistent in all K-r's or just in some. I bought mine in August, knowing about the FF complaints. I have an older Nikon D70 and a D50. So, here was how I looked at it: If I had a problem, I had choices: I could use the aging Nikons with the 50 f1.8. I could use live view even though it is slower and not ideal indoors. I could change my lighting (tungsten light bulbs are easy to replace with CFL's. We have already gone over to CFL's.). I was pleasantly surprised even with the 18-55 kit lens and even more so with the 35mm f2.4. I have shot enough indoors with several brands to know that a lot of my missed indoor shots are my own fault. Every single year regardless of the camera brand, my Christmas photos of the tree and decorations are not the best. My outdoor into the evening candids of our neighborhood parties are not the best. My people photos are at times good and at times hit and miss. And I have to take a lot of photos of my cats to get ones where they are not moving. I am an outdoor photographer. Still, I was pleasantly surprised that my bumbling indoors was no worse than with my Nikons. And in my opinion, the K-r aces the Nikon cameras and lenses I own outdoors. Finally I have the true color of skies, the right greens, I have very good detail for such an inexpensive camera. Now I wonder if the buy the 50mm FA f1.4 if I will induce the problems everyone speaks of. I guess I will find out.

I should qualify that by saying that even though I don't always take perfect indoor photos, I have not experienced the specific problems of FF or the indoor focusing problems up to this point. I understand better some of the conditions that might cause it to happen, but apparently I am not operating in those parameters at the present.

Last edited by Darlene; 12-03-2011 at 07:00 AM.

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