October 3, 2011

Wiesn Girls in dirndl

A girl at Oktoberfest 2011 München (aka Wiesn). Wiesn girl #1

A trademark of Wiesn (the original Oktoberfest) are girls wearing dirndl for showing off their (sometimes remarkable) décolletage also known as cleavage. Dirndl are a traditional dress in Bavaria. It used to be worn by poorer working girls in the past. But more recently, it became rather popular at Oktoberfest because of its figure-hugging qualities. It even appears that the dirndl currently makes new friends all over the world. A fashion trend made in Munich.

So, all of this theoretically provides for ample of photo opportunities like in a giant fashion show. Except that Wiesn is a very crowded party zone and nobody would want to make it easy for you. On the other hand, taking photos is a tolerated sports as long as they are serious and don't show up in the internet without permission (which is why I omitted the face of people I have no permission from). Nevertheless, good décolletage photos are rare (except from prominent people posing to be photographed) and I made a test of what can be achieved.

Equipment wise, I used a Pentax K-5 with the DA* 60-250mm/4 using AF and a Pentax K-x with a DA 15mm/4. I fixed the latter to f/8 and 1.4m manual focus with SR turned off to have sub-second reaction time to take photos in the crowd. No photo bag to keep a low profile. This turned out to be quite a capable combination.

Actually, the girl in the opening image is taken from a video I took with the K-5 at 250mm using manual focus while she walked up the staircase to the Bavaria monument at Wiesn. The manual focus was difficult to maintain (and is not perfect). But I still like the result as a photo I may not have been able to shoot as a still.

Below is this staircase in the background.
Bavaria monument

Below, I'll show impressions from Wiesn 2011. Note that many images are heavily cropped. Also, some post-processing was applied to reduce some hard shadows from the sunny day.

Wiesn girl #2

Wiesn girl #3 (note the cleavage)

Wiesn girls #739.347 to #968.567

Wiesn girl #4 ("Awaiting him while being looked at")

Wiesn girl #5

Wiesn girl #6

Wiesn girl #7

Wiesn girl #8

I'd like to know from any comments if you like the fashion trend at Oktoberfest. It definitely provides for some nice eye catchers. If you have any favourites in this mini series, post a comment. I'd like to know your taste too :)

Thanks for stopping bye and I hope you enjoyed your stay.


April 26, 2011

A hypothetical Pentax DFA* 500mm F5.6 ED(IF) SDMii

Kestrel © 2011 Falk Lumo

  Kestrel in the wild near the garden.
 The equivalent focal length is 2600 mm. Shot with a Pentax K-5 and 1.7x AF converter using a 500mm lens. Cropped to half size.

Kestrel in the wild near our garden.
 The 35mm-equivalent focal length is 2600 mm. Shot with a Pentax K-5 and Pentax 1.7x AF converter using a 500mm lens. Cropped to half size.

The Pentax system as great as it is does still lack a super tele lens option from Pentax which is in production. This made me wonder which lens exactly Pentax should do to both fill the empty space and to attract new photographers into their system.

Pentax once was famous for optically great and affordable super tele lenses such as the Pentax FA* 600mm F4 or gorgeous 645 600mm F5.6 etc. It is a shame they take so long to revive their tradition of great long glass.

To my big surprise, the answer which lens to start with was pretty easy:
A Pentax DFA* 500mm F5.6 ED(IF) SDMii.

Before I'll dig into details, let me explain why. First, shorter lenses don't make sense with a popular DA* 300 F4 or DA* 60-250 F4. After all, digital cameras are crop machines and need larger steps between lens options to make sense. But an even faster or longer lens becomes too expensive to be a smooth enough upgrade (as detailed below, a 600/5.6 would have to be 80%, a 500/4.5 100% more expensive). Same goes for a very long zoom lens which would become too heavy and too expensive too. Moreover, f/5.6 is really fast enough with the advent of great sensors such as the one in the Pentax K-5.

However, it must not be slower than f/5.6 (or maybe f/5.4) because then the Pentax AF system wouldn't work anymore in conjunction with an 1.4x tele converter. Yes, it does work with f/5.6 or f/7.8 effective. How do I know? Well, the above photo was shot with a Sigma 500mm f/4.5 APO manual lens and the Pentax 1.7x tele AF converter which is f/7.7 effective. The AF works flawlessly and fast! Heck, the K-5 even gives me a stabilized 850 mm autofocus lens this way :)

A 500/5.6 lens really is the sweet spot between long enough, fast enough, not too heavy and not too expensive. And it is the missing option from Sigma, Sony, Canon and Nikon! Their product suites are all so 2000-ish! Optimized to be fast, expensive and heavy rather than the versatile resolution super weapons modern sensors need so much.

Pentax DFA* 500mm F5.6 ED(IF) SDMii details

I compared a couple super tele primes from Sigma and Canon (Canon MSRP prices divided by 1.34 to meet Sigma MSRPs, e.g., for the 800/5.6). The following two formulas are the best description to predict weight and price:

Weight [kg] = D[mm]^2.20 /12000 (typical error is +/- 15%)

Price [MSRP in USD] = D[mm]^3.26 /1177 (typical error is +/- 25%, street price shouldn't be higher)

D is the diameter = focal length / f-stop number.

D^2 is the expected term describing how glass surface grows with diameter. 2.2 accounts for increasing thickness, I guess. Price should be proportional to weight but isn't. It has an extra factor D probably accounting for increasing rareness and hand-made production steps. Anyway, the above formula allows to describe the lens in fairly accurate terms:
  • Lens: Pentax DFA* 500mm F5.6 ED(IF) SDMii
  • Mount: Pentax KAF
  • Price: 1940 USD (MSRP)
  • Weight: 1630 g
  • Filter size (front): 91 mm
  • Length: 300 mm
  • Close distance: 4 m
  • Lens elements / groups: ~12 / ~11 (e.g., like the Pentax A* 645 600mm F5.6).
  • Stabilized: Yes (in body)
  • Autofocus: Yes (SDM, version 2 (ring motor); screw drive supported; focus limiter in firmware)
  • Teleconverter: Yes, optomized to support Pentax DFA 1.4x SDM converter (tba ;) )
  • Image circle: 43mm (full frame)
  • Weather-sealed: Yes
  • MTF characteristics: Blur widths below 2px in the center, similiar to DA*300, outresolving the sensor x2 ("made for cropping" (tm)).
  • Pentax DFA 1.4x SDM converter
    700 mm, F7.8, phase AF supported in daylight.
I am pretty sure that this lens would sell in a volume high enough to make it an economically viable option for Pentax. And drag new users into the Pentax system. And keep existing users happy as well ;)

Pentax, where is your pre-order page? :)

March 27, 2011

LumoLabs: Pentax K-5 low light focus with firmware upgrade 1.03

Caribbean sunset. © 2011 Falk Lumo
Fig.1: "Caribbean Sunset".
Why doing measurements is no substitute to taking photos (taken while I type) ;)

The current story is continued from the blog article Pentax K-5 low light focus after Pentax has published the version 1.03 firmware upgrade officially featuring an improvement of focus operation in low light.

When the 1.03 update was released, many users reported positive findings. So, I was optimistic to find that the low light focus issues are a thing of the past. Unfortunately, this isn't what I eventually found.

Please refer to the updated version of the complete paper for my findings:


Basically, I find no significant difference between firmware versions 1.02 and 1.03 when using my testing scenarios. This means that there is no significant improvement if:
  • The background is white
  • The target has high contrast
  • The AF assist light is disabled
Under the above circumstances, the K-5 auto focus starts to lock focus in a false front focus position at tungsten light levels below 0 EV for a fast lens (like f/1.8) or below about 2-4 EV for slower lenses. This did not change at all with release 1.03. Note that the above light level values would read 2-3 steps higher with a target like a Caucasian skin (18% gray).
Moreover, I found that at low enough light levels, the 1.03 firmware seems to rely more than 1.02 on color information outside the direct selected AF spot. With a uniform color distribution, this can lead to somewhat improved results in the vicinity of the focus shift transition. OTOH, the focus system is more easily fooled by features with a singular color. Overall, the advantages may dominate and the effect is small anyway. Version 1.03 still doesn't seem to make use of white balance information, either manually or automatic.

If I believe that Pentax improved the low light focus situation for many users of the 1.03 firmware, then the progress must be bound to any of the following:
  • AF assist light engages more actively (not tested by myself).
  • Focus improved in the presence of dark backgrounds (not tested by myself).
  • Focus improved with low contrast or dim focus features (not tested by myself).
As I don't have a version 1.02 K-5 anymore, I cannot find it out. But any progress brought by the 1.03 firmware upgrade is limited rather than universal.

As much as I applaud Pentax to having addressed the problem, as much I am a bit disappointed they didn't dig deep enough to address the root problem: that the K-5 AF subsystem locks focus even in a situation where the colorimetric sensor(s) fails to determine the light color. It simply shouldn't lock focus at all then. Or ask for user assistance like a priming shot. The light-sensitive AF system of the K-5 has too strong a color dependency to autofocus if the color of the focus feature remains unknown.

A better workaround than 1.03 in firmware is feasible and should stay on Pentax' agenda.

Please, read the full paper linked above if you have further questions and come back here to leave comments or questions. General comments about the issue should still go to the general (earlier) article while firmware release 1.03 comments should go here. Thank You.

February 23, 2011

LumoLabs: Pentax K-5 low light focus

Note: I updated the article to version 1.3.
2011, March 27.

Note: I updated the article to version 1.1.

I think, the feedback and experiments done after version 1.0 helped to understand the observed focus behaviour rather well and version 1.1 takes this into account.

An important (new) aspect is a description how low key studio photography, esp. when combined with an f/4 lens (or slower), can lead to inaccurate focus.

2011, March 07.

Accuracy of the Pentax K-5 phase detect AF vs. luminosity in EV. © 2011 Falk Lumo. The chart includes all measurements, i.e. various lenses, light colors, distances and apertures. The accuracy is measured as deviation of the focal plane from the sensor plane, in µm.
Fig.1: Accuracy of the Pentax K-5 phase detect AF vs. luminosity in EV. The above chart includes all measurements, i.e. various lenses, light colors, distances and apertures. The accuracy is measured as deviation of the focal plane from the sensor plane, in µm.

In modern times, each new release of a digital SLR camera seems to be accompanied by teething troubles. This applies to all makes across the board. Sometimes, they are fixed quickly by the vendor, like the “Green Line Syndrome” video issue in the Pentax K-7, the “Hot Pixel” video issue in the Nikon D7000 or the “String of Pearls” stain issue in the K-5. Sometimes, they aren't like the shutter-induced blur issue with the K-7 which LumoLabs succeeded to document in this blog.

Currently, there are two remaining teething troubles for the K-5 which are widely reported: wrong PTTL exposure with external flashes in some situations (confirmed in writing by 3rd party flash makers, even for Pentax' own flashes). And a systematic wrong lock of autofocus in low (tungsten) light.

Therefore, LumoLabs has decided to have a closer look at the issue. After careful evaluation and many hundred test shots we found the issue to be real. Pentax has unofficially reported to work on the issue. The pressure is on them to address the issue and the author hopes that our findings may contribute to their efforts. Buyers of the K-5 must be able to be confident that the issue is fixed sooner than later.

A preliminary copy of the paper was provided to Pentax earlier this week and the head of Pentax Europe officially receives a printed copy today. I have been told that Pentax engineering will receive a copy too.

The study

The results are too complex to be presented in the scope of a blog. Fig.1 above provides a first idea of our work.

Please refer to the complete paper for our findings:
You'll gain a deeper understanding of Fig.1 too ;)

In a nutshell, this is what we find:
  1. The K-5 as it presently ships indeed has a flaw in its phase detect autofocus module or software which causes it to front focus in low light below a lens-dependent threshold in EV.
  2. If it does, it seems to consistently focus ≈ 255 µm behind the sensor plane (although with a significant ± 75 µm scatter of results which is about twice as large as the normal scatter of result).
  3. Faster lenses seem to keep working in lower light but of course, are prone to more blur when the front focus does eventually happen. Slower lenses can already start to front focus at light levels metering as 4 EV or 6 EV even. A fast lens may work down to 0 EV in white light.
  4. Light sources other than daylight emphasize this problem as they simply appear darker to the AF module. Moreover, it seems to be moderately color blind for red which further emphasizes the effect in deep tungsten light.
  5. The effect is real and can negatively impact the daily work of a photographer.  On the other hand, it is possible to run into a low light tungsten situation without the problem.
  6. The paper clarifies conditions to hit or avoid the issue. White light (halogen is not white enough though) and a wide lens stopped down help to work around the problem. AF assist light typically doesn't help though. But an LED flash light does. ;)
It may be interesting to note what we did not find: There seems to be no strong dependency on the light color. Except that colored light causes the transition to happen earlier as it means less usable light for the AF module. It seems to have a somewhat low sensitivity esp. for red light. Moreover, there seems to be no dependency on actual aperture or distance.

Move focal plane by 0xFF µm? Yes, do it so! :)

This may describe what's going on behind the curtain: a µm-valued variable becomes 0xFF (255) and causes a false shift of the focus plane by 255 µm. I call it the +0xFFµm hypothesis. :)
- Is it likely? No.
- Is it possible? Yes.

Please, read the full paper linked above if you have further questions and come back here to leave comments or questions. Thank You.