Autofocus wavelength dependency in a Pentax K20D

I take substantial amount of my pictures in low light conditions.  However, the autofocus of my K20D has proven to be a poor companion under these circumstances. I usually have to fight with problems of getting the focus right. The problem is made even worse by the fact that the camera triggers the AF light of the flash far too late. There is some brightness in which the camera can’t focus quickly or not at all but doesn’t attempt to use the flash’s AF illuminator. I have remedied this condition with the development of several AF illuminator assistants, basically a second- manually triggered- AF-illuminator. Although I used pretty strong light sources in them, performance was still sub par.  After reading about the “Pentax yellow light focus problem” that Ricehigh described, I decided to do some tests myself.

Basically what is known from other Pentaxians is that under incandescent illumination a front focus of the camera can be observed. It has been speculated that this is related to the amount of infrared radiation contained in this light. Therefore I tested whether the Pentax AF reacts to near IR or UV light. The good news: no it does not at all. I tried UV LEDs and very near IR LEDs, just at the border of human perception, and with both I could not get any AF lock-on. I then testes a range of LEDs with comparable brigthness on a focus test chart (please note my excellent hand drawing skills ;-)). Here are the results (left is front, right is back, 45 deg angle, focus until stable, two pictures each, both where always exactly the same (every time refocused)):

Focus in white light.
Focus in green light.
Focus in red light.

What you can see is that there is very little dependency on wavelength. For none of the used wavelengths, IR, UV, red, green and blue (not all are shown) were there siginificant differences in accuracy, if the focus could get a lock-on (not for UV and IR) at all. Therefore each color should be able to do the trick in an autofocus assist light.

Then I did a second test: I again mounted the camera on a tripod and aimed it at a lamp in my dark living room.  Now I illuminated the lamp with various LEDs and tested how well the focus worked. Here are some examples:

Green AF assist.
Blue AF assist.
Red AF assist.

As you can see, each color works. Surprisingly, even the dim blue LED can get a lock-on (in terms of energy it is as bright as the red and the blue).  The surprising fact however was that the bright red LED, which even projects an image of its die onto any surface (really short focal length of the LED dome lens) caused the most problems. The focus is able to lock on but often it was not accurate. Just as it is really difficult to manually focus under red light- due to the low acuity of the eye for red- the camera had the same problem. Often the focus was slightly of to the front or back. The best results were obtained wth the green LED. Now I understand why the Fuji bridge camera of my friend Jonas uses a green LED :-) (besides the fact that in LiveView green is alwazs superior due to twice as manz green pixels)! I will try to test a green LED for my autofocus assist light as well! So please come back because I will present this neat little device shortly on my site!


What do we learn from all this? First, the K20D does not appear to necessarily show a problem with the autofocus for different wavelengths. At least not if the illumination is decent. This excludes a systematic error in the AF and makes a “yellow problem” unlikely. I have also never experienced one. For near threshold intensities it seems to be that green light can be best used for accurate AF. Lock on is fastest and most of the times more accurate than red light. Another advantage is, that green LEDs are still effective and bright and green light does not trigger the pupil reflex as badly as blue light.


This test is not really “scientific”  as there are no accepted standards for this kind of measurements. Since the spectral response curve of the AF sensor and filters in front of it are unknown, no equal luminance setup can be built. It is merely a test which just tests if there are any grave problems with the AF under different wavelengths and which of those could best be used for an AF assist lamp.

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Wednesday, May 13th, 2009 Photography No Comments

Review of the SIGMA 28mm F1,8 EX DG Asp. Makro

For my Pentax K20D I recently bought the Sigma 28 f2.8. I wanted to use it as a standard 50mm equivalent lens for standard photos and especially for low light portrait applications. Since there isn’t that much information about this lens out on the web I compiled this little review to help everyone out there who is wondering whether to buy the lens or not.

Build Quality

A definite plus of the Sigma. It is quite heavy for a prime but this makes it feel really solid. I like the EX finish, other people might not. Nothing is wobbly or cheap-made. The lens hood is also firmly attached to the front, reducing the risk of the objective being dropped during exchange. The lens cap however is not so well designed, because it is hard to get off while the hood is still on.


The autofocus for medium to wide distances is pretty fast for a lens on a Pentax. Only if the autofocus misses and goes all the way to makro you might have to wait a second or two until it is back at infinity. Autofocus accuracy exceeds that of my Sigma 17-70 DC, although in low light it can take a moment until the autofocus locks. Surprisingly it works until it is really dark. In the streets at night the focus works without any assist lamp being necessary. Under all conditions one might experience some “hunting” of the focus, it changes a few times the focus direction until it is really on-target. What is a bit disappointing about the AF is, that you lose the autofocus points at the very left of the frame. The lens does not work with those and focus will give completely back-focused results. The 9 points inside the inner AF-frame do work, but only the middle sensor works really well. Please note, that although my lens doesn’t show it, many users have reported terrible front or back focus! Considering the age of the design Sigma might not be able to adjust it. If you have a K20D you can circumvent this problem by using the build-in AF-correction since the lens’ ID is transmitted properly. In terms of movement the AF does extend the barrel only slightly, eliminating “pump action” dust suck up. The front element does not rotate. But considering the price for 77 mm polarizers it doesn’t really matter 😉


Sharpness is pretty good already at f1.7. It is a bit blurry, but for portraits and many other situations it is sufficient. I found my lens to reach optimal sharpness already at f2.8. At higher f numbers sharpness decreased again.

Scene for cropsScene for crops

Sharpness at different aperture settings. 100% crops at 6 Megapixels. Please not the flare appearing at f16 in the upper left corner.

Sharpness at different aperture settings. 100% crops at 6 Megapixels. Please not the flare appearing at f16 in the upper left corner.

What is remarkable, is the even distribution of sharpness. This is due to the DG design, which is intended for full frame sensors. So on the K20D’s APS-C crop sensor edge sharpness is good.  It by far exceeds the performance of the Sigma 30 mm f1.4, which shows terrible edge sharpness.


Contrast could be better, highlights tend to eat away fine structures like branches and the sun drowns quite a bit of the image in white.  A specific problem of the lens is it’s tendency to flares. At the open end flares are pronounced even with the lens hood. Stopped down the optimum is around f8, but stopped down further, another type of flares appears. In my opinion the lens hood is too small, it can be extended a bit with black paper which helps with flares without increasing vignetting.


Only visible wide open.

Chromatic Aberrations

CA’s are low and can be corrected very well. Purple fringing can occur wide open, I found it to be pretty light however. The Sigma shows some colored edges for out-of-focus objects with high contrast but I have seen worse. Here is what it looks like:

Overview of cropped scene at f1.7

Overview of cropped scene at f1.7

Crop from center of the frame

Crop from center of the frame

Crop from edge of frame with chromatic aberrations.

Crop from edge of the frame with chromatic aberrations.

The chromatic aberrations seen in the above pictures can be complitely eliminated by the Photoshop or Lightroom  CA filter.


Here are a few samples. The night time photos are all taken without a tripod at ISO 1600. Enjoy!

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Wednesday, April 1st, 2009 Photography No Comments