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? :)


  1. Wow Falk! That sounds very good. Sigh! If only pentax coud take this advice!

  2. I do beleave in this product, and with that new 1.4 TC it makes sence as a good combo.

  3. Hope they decide to bring out a lens like this! This would become my primary lens and it would stay on the camera all the time!

  4. Clever thinking, Falk , I totally agree, fine combination with the TC. Hope this come true with next Photokina..

  5. Very nice post. I really enjoy the reading. I come here from the google while searching for some good article.Thanks

  6. I thank Tony Lang very much. He pointed out the bird in the title photo is a kestrel (Turmfalke, Falco tinnunculus). I originally assumed it is a young goshawk (Habicht, Accipter gentilis). I must say that a "Falco" is a much better match with my name :)

