To begin with, I tested the camera in a dark room, selected an object with a flat surface on which there was clear contrasty patterns, and then projected pure Red light (using the spotbeam projector of an external dedicated Pentax flashgun) and Green light (using the built-in AF assist light) onto the target and did the AF. The results were: NO problem for both, the AF was fairly accurate!
I then tried the Blue light. But since I could not find a pure Blue light source for projection. I used pure 6000K White light source and then put on a 80A (Blue) filter and did the same test. Again, the AF had been *accurate*! :-o
As such, I re-thought again.. What lights actually a tungsten bulb contains? It is Yellow light, which should be somewhere between the Red and Green for the wavelength in the visible light spectrum.
But however, since the above investigations shown that it is not the problem with neither Red nor Green and thus it is safely derived that Yellow light should cause no problem neither. So, actually what is the real *source* of problem? Could it be the Infrared? Which was just invisible, as tungsten does also emit very strong Infrared, as a heat source more than a light source actually.
Some time later, I picked out my old Pentax F35-70 AF lens, which has two Infrared focusing marks on the distance scale. I then tried to shoot under a *pure* tungsten environment that was actually fairly bright, at 260 Lux:-
The camera was set up like this and the shoot target is also shown below:-
(Firmware in Camera is the Latest, i.e. V1.11; Focus Fine Adjustment was Disabled.)
Below are the test results obtained:-
| AF Method and Focal | Record of the Focus Scale for where AF was Measured | Result Image (Central 100% Crop) | Result Image (Large and Uncropped) |
| @ 35mm, Optical Phase-matching AF |  | View | Download |
| @ 35mm, Live View Contrast-detection AF |  | View | Download |
| @ 70mm, Optical Phase-matching AF |  | View | Download |
| @ 70mm, Live View Contrast-detection AF |  | View | Download |
After all, what do you observe above? Indeed, Live View CDAF always measured correct focuses, whilst front focusing *had to* be resulted with PMAF under tungsten. Now, as seen from the above, it is confirmed that amount of errors and the shifts in distances from the correct focus points are very close to the indicated derivation with the IR marks from the central focus mark for normal (visible) light. This holds true for both 35mm and 70mm respectively! That's say if we shoot Infrared film with our Pentax K-r cameras, then the results *should* be mostly correct! :-o
It should be further noted that the effect and amount of error is lower for tele than wide angle, i.e., the magnitude of the distance error is larger as focal length becomes shorter. Indeed, this is what shown on the Infrared marks on the above lens, already.
But since that all Pentax DSLRs are 1.5X cropped with the APS-C sized sensors inside, a shorter focal length is always used for the same Angle of View. As such, the effect of this error is magnified! >:-(
Now, I just could never imagine that after twenty something years, the Infrared marks of my old lens have been useful again, but they are used in another way, i.e., instead of to put the focus to a closer distance, the lens should be manual focus to a farther distance! :-o
After all these, it seems that the main cause of the problem is about the existence of Infrared light in the shooting environment, which is received by the AF module and ultimately onto the sensor and then the whole focus measurement is flawed! The fact is that the those SAFOX IX Pentax DSLRs had to focus on IR instead of visible light when IR is existent and they are rather too sensitive to it! Really damn it! >:-(
In fact, the old AOC Pentax engineers knew about the adverse effect of the Infrared light to the AF system when the original SAFOX (I and II) was designed and created, see this diagram (Credit: Bojidar's Pentax K-mount Page):-
Where the component #5 in the above is just the Infrared Filter!
I am 100% sure that neither my SFX (the first Pentax mass-production AFSLR) nor my MZ-S (which is the second last Pentax 135 film SLR) that uses the older SAFOXes did suffer from this tungsten front focusing issue. There was no nightmare of this issue not until I acquired my *ist D, the first Pentax DSLR, back to January 2004!
So, am I going to resolve this tedious and super annoying problem, which is simply a latent design flaw and defect of the K-r (and many of the Pentax DSLRs) created by Pentax? We shall see in the next/coming part of my report.
Continued on the Next Part.
Previous Related:-
Pentax UK and Pentax Germany Admitted K-r Tungsten Front Focusing Issue (But there is No Solution, as Both Clearly Told)
SAFOX IX Tungsten/Yellow Light Front Focus Issue Fully Investigated