OK, I admit defeat. I haven’t found a lens slow enough to prevent the AF1.4x from working on the K-3.
Today’s tests: [Note 1: in all of the following I used Liveview as an aide in finding the darkest rotation with the polarizer etc. but all of the test shots were done with Liveview off and AF performed with the rear AF button. AF mode was my default setting: AF-C Spot] [Note 2: the scene was lit with mid-day, cloudy, flat light. Photos taken looking at a frozen river with large patches of old snow alternating with areas of standing water with reflections of trees on the other side. Tree branch in the mid-ground, focus was on one twig of that branch.] [Note 3: Before the tests described below, which used the scene described above in Note 2, I did a few hand-held tests with just the river ice in the viewfinder; no sharply outlined tree branch for the AF to pick up on. http://photos.stanhalpin.com/p649772105/e37610a33 http://photos.stanhalpin.com/p649772105/e309f1694 No apparent difference in AF functioning between this scene and the one with the branch - I was getting focus lock on the reflections.] 1. I found an 8x ND filter in my father-in-law’s old stuff. 52mm thread. A step-up 49->52 allowed me to put this on my slowest AF with a front element in that size range: DFA-100/2.8 macro. The 1.4x TC gives me an effective max aperture of 4.0 if my math is right. Or is it 3.5? Anyway, with an 8x ND, the light reaching the sensor should be equivalent to an f/16 lens? I leave it to someone who knows this stuff to do the math. Whatever the number, the view got quite dark! a. Shooting with filter, the result in AV mode was a 1/320 at f8, ISO 200. http://photos.stanhalpin.com/p649772105/e3db88ce9 b. Shooting without the ND filter on, the result was 1/1600 at f8, ISO 200. http://photos.stanhalpin.com/p649772105/e3184f7a2 So, another opportunity for you who understand optics and math to do a calculation of the effective max aperture of this combination. An added complication is that the metering system apparently did not fully cope with the filter. I.e., on the filter shot, I needed to add .5 stops in the Lightroom Develop module to get the apparent brightness and the histogram display to match the unfiltered shot. So instead of 1/320, maybe it should have been shooting at 1/250? c. The AF search-and-lock was equivalent in the two cases. 2. Back to my FA 28-104/4-5.6. Zoomed to 105mm, therefore shooting at a max of 5.6 without the 1.4x, at a max of f/8 (?) with it. I stacked several filters including two circular polarizers and some grungy scratched UV filters. Fiddled with the rotation of the polarizers to the point where the scene was at its darkest on the Liveview display as per my uncalibrated eyeball and the indicated shutter speed was at its slowest. a. Shooting with the filter stack, the indicated exposure was 1/15 at f/8, ISO 200. http://photos.stanhalpin.com/p649772105/e29e628e4 b. Shooting without the filters, the indicated exposure was 1/50 at f/8, ISO 200. http://photos.stanhalpin.com/p649772105/e36a254ba [Note: I am not sure I trust the apparent EV difference here. I had to add 1.5 stops in Lightroom to the second shot to get equivalent histograms and appearance; the 2nd shot may also have been with filters at a different rotation setting. I shoulda taken better notes.] c. The AF search-and-lock was equivalent with and without the added filters. d. The AF was quite a bit longer than with the macro; it racked all the way in and out while searching rather than moving to an approximate position then making micro adjustments. This may be a function of how far out of focus I had each lens before engaging the AF. If I had an f/8 or f/11 lens I planned to take on safari to use with the 1.4x, I think I would test the pair first before I planned my trip around that combination. They would probably work based on what I’ve seen so far, but you would be living on the edge. Any lens 5.6 and larger would seem to have no problems at all. stan On Mar 26, 2014, at 11:24 PM, Stanley Halpin <[email protected]> wrote: > My slowest are the 4-5.6 zoom and the 5-6.3 zoom that I tried earlier. > Anything I’ve thought of to simulate a slower smaller widest aperture would > only affect the taking aperture, not the focusing aperture. Except filters. > I’ll play with a polarizer a bit tomorrow, that will give some (unknown > degree of) reduction in light available. I don’t have any ND filters. > Ken, I think you may need to bring your 600 by and try out the 1.4x yourself. > > stan > > On Mar 26, 2014, at 9:50 PM, Paul Stenquist <[email protected]> wrote: > >> Your right of course. I don't own a slower autofocus lens. >> >> Paul via phone >> >>> On Mar 26, 2014, at 9:22 PM, "P.J. Alling" <[email protected]> >>> wrote: >>> >>> Since the lens was wide open it wasn't really focusing at f22, it was >>> focusing at ~f5.6. Now the relatively slow Pentax FA 28-105 with an >>> effective aperture wide open of 8.0 with a 1.4x TC is getting to be a test. >>> There ate two things that effect auto focus, absolute aperture, and >>> absolute light level. I expect that in bright light that even f8 lenses of >>> a long enough focal length will have no great issue auto focusing. >>> >>>> On 3/26/2014 8:09 PM, Paul Stenquist wrote: >>>> I shot a frame at f22 last week with the 1.4x and 60-250. It focused >>>> quickly. >>>> >>>> Paul via phone >>>> >>>>> On Mar 26, 2014, at 7:01 PM, Stanley Halpin <[email protected]> >>>>> wrote: >>>>> >>>>> I’ll further explore that question tomorrow. >>>>> >>>>> stan >>>>> >>>>>> On Mar 26, 2014, at 2:59 PM, Ken Waller <[email protected]> wrote: >>>>>> >>>>>> I'm more interested in the smaller aperatures at which it fails to >>>>>> focus. It would be nice to be able to use it on my 600. I have both the >>>>>> A-1.4X-L & A2.0X-L but of course I lose AF with those. >>>>>> >>>>>> Kenneth Waller >>>>>> http://www.pentaxphotogallery.com/kennethwaller >>>>>> >>>>>> ----- Original Message ----- From: "Rob Studdert" >>>>>> <[email protected]> >>>>>> Subject: Re: Because inquiring minds want to know (more on the 1.4x) >>>>>> >>>>>> >>>>>>> Thanks for the report Stan, it does look useful, but I will have to >>>>>>> sell a K5 body to afford it :( >>>>>>> >>>>>>> >>>>>>>> On 21 March 2014 02:38, Stanley Halpin <[email protected]> >>>>>>>> wrote: >>>>>>>> I just did a couple of focus "tests" with my two slowest lenses. I >>>>>>>> won't post any results - I was just handholding on random twigs, >>>>>>>> turkeys, lawn sculptures, trees, etc. The images would tell you >>>>>>>> nothing about sharpness. All of the following using center-point >>>>>>>> focus, K-3. >>>>>>>> >>>>>>>> 1. Sigma DG 150-500mm f5-6.3 APO HSM + Pentax 1.4x TC >>>>>>>> With the Sigma at 150mm zoom (presumably f/5.0) >>>>>>>> - Sigma AF OFF: nothing happens - no AF. >>>>>>>> - Sigma AF ON: snappy AF, no searching or other bad >>>>>>>> behaviors >>>>>>>> >>>>>>>> With the Sigma at 500mm zoom (presumably f/6.3) >>>>>>>> - Sigma AF ON: Quite a bit of searching on low contrast >>>>>>>> targets (e.g., tree trunks.) Fairly quick "snap" to focus on a weed >>>>>>>> seed head 20 feet away against a backdrop of the river ice 150 feet aw >> >> -- >> PDML Pentax-Discuss Mail List >> [email protected] >> http://pdml.net/mailman/listinfo/pdml_pdml.net >> to UNSUBSCRIBE from the PDML, please visit the link directly above and >> follow the directions. > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
