June 23, 2009

K-7 as a movie camera -- PART II: Controlling video recording

In part I, we have discussed the technical implementation of the video feature in the Pentax K-7.

Now, let's see how to make good use of it. Again, I'm not going to repeat the specification or user's guide.

At first glance, it seems to be very straight-forward: Turn the mode dial to movie and press the shutter! Ok, done this, been there. Now, for the more serious fun: how do we control shooting parameters?

The official answer by Pentax is: you don't! There is almost no manual over shooting parameters. But as always, there are back doors :)

Things to know:

  1. You can set global parameters like pixel format, quality, shake reduction in the "video" menu (press the menu button when in video mode) and is not affected by all the nice settings you may have tweaked. E.g., forget about your Auto-ISO range ;)

  2. Autofocus, aperture and E/V-compensation can be set prior to a recording. All three are defunct while recording. Of course, the camera won't complain if you change aperture or focus manually (read: mechanically). The focus in video mode is contrast autofocus by default. But you can switch to phase autofocus which is faster.

  3. The exposure (with all its parameters) can be locked/unlocked before and during a recording, using the AE-L button.

  4. EV values in video and still image modes seem to be the same.

Controlling exposure

Knowing the video exposure response curve is key in determinig and manually controlling an exposure in a K-7 video. It isn't published by Pentax but I researched them to be this:

Assuming an aperture is preset to a given value:

Light value [EV] at f/ShutterSensitivity    [s][ISO]
0123456789underexposureblng red
14151617181920212223overexposureblng red

How to read the table: Look up the aperture in the upper left (e.g., f/5.6, marked in bold), search the measured light value in the corresponding column (e.g., EV 10) and look up shutter speed and ISO in the same row (e.g., 1/30s, ISO 100).

I didn't research much how aperture would be controlled if set to AUTO. But it will choose a combination from the diagonal line of constant EV in the table above. In AUTO, the aperture is controlled live and you can hear the aperture blades moving! IMHO, this is a cool feature for a dSLR with a legacy SLR lens!

(Update) I had another look at this.

In video recording with aperture set to AUTO, I illuminated the sensor with a torch and observed the lens aperture's reaction. This is what happened:

  • Without extra light: wide open (f/2.4)
  • With medium extra light (torch into lens): shut down (f/5.6) (*)
  • With full extra light (torch fully aligned and directly in front of lens): closed down (f/11) (*)

(*) estimated from diameter left open by aperture blades, as seen thru the front lens element.

There are really only these 3 steps. With a f/5.6 lens, this reduces to really only two steps. If it changes aperture, it does so by jumping 2EV, at least. Of course, it causes a visible jump in brightness in the video then being compensated afterwards. Therefore, manually shifting aperture creates a much smoother effect. (end of update)

Note: The above table may not be fully accurate. E.g., the sweet spot shutter speed (1/30s) may be shorter, like 1/50s, actually. Note that many videographers prefer a shutter speed of 1/30s or 1/50s (i.e., the motion blur from it) for 30fps footage to minimize a stuttering effect in panning action. Also note that all this is from my own research. Pentax doesn't disclose the information given above and the recorded video contains no useful meta information.

Now, using the response curve above, you can meter any subject, set the required aperture, use E/V-compensation to hit the required EV value, switch to movie mode (E/V compensation stays active!), lock exposure with AE-L and you successfully manually controlled your video parameters!

It is possible. But I agree that it is awkward in many if not most circumstances. If you need longer shutter times at daylight without wanting to stop down then you need to use a gray filter like you maybe would use for water still photography. Note that for video, you can stop fully down without loosing sharpness in the resulting video.

However, what I did find very easy to use is the following trick: In video mode, before starting to record, I observe the live histogram on the rear display when pointing to different subjects which will emerge during a scene. Then I point to a "typical" histogram and lock exposure. Eventually, I record using these parameters.

Controlling focus

Autofocus stays inactive during recording (it was enabled up to firmware release 0.20). However, it is so slow and badly implemented that it wouldn't be useful in actual footage anyway. If you need to refocus using the autofocus, the fastest would be to set live view autofocus to phase detect and stop a recording (press the shutter again and wait ~2s), press the AF button (~1s) and start recording again (~1s). If there is no time for a ~5s break, then you must control focus manually. Unfortunately, magnifying live view is inactive during recording as well (and the view finder stays dark, of course).

Fortunately, the quality of the rear screen with its 640x480 resolution allows an approximate focus. If the DoF effect isn't required, then shooting with fixed focus at hyperfocal distance is a viable option. The DoF calculations as obtained for an APS-C sensor do still apply. If you need to pixel peep, set circle of confusion to 0.015 mm.

If you need more control over the focus, using either a field monitor or an enlarging eye piece looking at the rear screen may be an option. The K-7 outputs live view and life audio via HDMI in 480p, 576p, 720p or 1080p. The image at the top of this article shows a K-7 hooked up to an HDMI 1.3 type C cable. However, the video data will always be 480p only, possibly enlarged to match the HDMI protocol.

K-7 videographing its own live view from falconeye on Vimeo.

The video above shows how the live feed from the K-7 behaves. In particular, you can assess the latency between reality and HDMI output. I guess it is about 1/3 s.

There are field monitors one can connect via HDMI and mount to the hot shoe or flash bar. The flash bar however, may be the better choice if a microphone is already mounted to the hot shoe ;) Because the feed is always 480p only, there would be no additional benefit in getting a 800x600 or 720p field monitor (this notice may not hold true for the Samsung GX30). A 5-6" 480p field monitor with HDMI input will do it. Avoid monitors with A/V analog input only.

Field monitors may have a headphone jack for audio playback as well.

And of course, for proper framing and smooth operation, you'll definitely want a video rig :)

Controlling shake

The K-7 features electro-mechanical sensor stabilization which proves very efficient in video capture. Note however that it is designed to work for still photography. So, it cannot compensate all the shake during a longer take. Because wide angle requires the anti shake to compensate less and is more stable in the first place, it is best to use wide angle (and tele lenses on a tripod). Or a rig again ;)

Note that a wide angle lens has a closer hyperfocal distance and produces smoother panning as well. So, I just adopted wider lenses as my standard when videographing.

Controlling audio

The built-in mono microphone is very sensitive to environmental noise like wind. So, using an external (stereo) microphone is a much better option (or use external sound recording and a take board). I tried the RØDE Stereo VideoMic connected to the flash hot shoe and it produces excellent results. The recorded quality is definitely more than sufficient for voice and sound. For music one may want to record externally.

Btw, there is no control of volume. But the recording level is relatively low and I didn't have any problems with either oversteer or noise floor.

Now, let's go out and have fun with video. I.e., it is time for part III.

-> Continue to part III


  1. Hi Falk,

    Thank you for your review. As you mentioned, K-7 record volume is relatively low, which I found lower than K-x. Any thoughts?


  2. @Tony, well, I use the RØDE Stereo VideoMic and even with the -10dB Switch I now had one recording (a live music performance) which clipped! The RØDE Stereo VideoMic has a sensitivity of 12.6 mV @ 94 dB SPL +/- 2 dB @ 1kHz without the -10dB, i.e., with 12.6 mV @ 84 dB SPL I already experienced clipping once.

    Therefore, I am really glad that the recording level is exactly as low as it is. A manually adjustable gain would be better even. All other alternatives (higher gain or automatic gain control) would be MUCH worse, though.

    I never experienced any problems in adjusting the level of noise during post processing. Most post processors / converters offer an automatic normalization of audio level which I recommend to use. The K-7 audio input has a very low noise floor. I never had an issue with noise after audio level normalization.

  3. Hi Falk,

    I was talking about the built-in mics. So in your opinion, low volume is better than high?

    I do notice that K-x records more treble and K-7 more bass if their volume is normalized to each other. But the original signal/noise ratio is higher in K-x.

    The video samples in dpreview seems to confirm my observations but they are not side-by-side comparisons. Would you mind to do an audio test or just simply make your original videos of camels available with sound recorded by built-in mics?

    Thank you very much!

  4. Updated information:

    A similiar exposure table for the Pentax K-x has now be made availabe here:

    Thanks to Coby for the great work! :)

  5. Hi Falk,

    Playing around with the video today I noticed that it allows exposure compensation.

    It does this without changing the aperture.

    There are only two ways I know of while keeping the aperture the same this can be done, changing the shutter speed or changing the ISO.

    It limits the range to +/-2Ev which is a pity.

    Have you done any analysis of what it changes to do this as it may be an easy way give some control one of the parameters at least?

  6. Never mind. I just read your back door description. D'oh.

  7. Hey Falk! Thank you for this review. I have my K7 since they released it and I'd love to make some video shorts with it, but I'm very jealous of those Canon users that have manual controls on their DSLRs. I read that 1/60 is the best for 30fps but how can I stablish that and mantain it as much as possible while recording? I'd also love to have my ISO under 1600 because i'ts too noisy for a decent movie and don't want postprocessing because i'd have a leak of quality and sharpness.

    This is one test I made with it:

  8. Hi rodrigo,

    you can watch the exposure settings in LV (still mode) and when switching to video, point at a direction were you have known (EV) settings. Then you can use +/-EV compensation and AE-Lock to select the required recording EV and lock it.

    Note that shutter and sensitivity are always provided by the camera based on aperture and EV situation (which is why I created the table above). But you can control aperture and the EV value the camera thinks it must shoot at.

    There is a thread in the video subforum at explaining how you can actually lock a KNOWN EV situation in LV mode (so you can see all recording parameters) and carry it over to video using a manual lens and USER mode. But note that while this works, the camera will still shift shutter and ISO to obtain a combination as listed in my table above.

  9. Thanks for your good advises!
    One question: it looks like the aperture cannot be controlled using the ring on a -M lens. Does it automatically default to the biggest aperture?

  10. @Arie,
    I don't know where you've read this. Certainly not at falklumo ;) With an M-lens, or an A-lens with the aperture ring set to non-A, the aperture can be controlled throughout the video and the lens will open and close accordingly. Combined with AE-L, this can give dramatic effects where the automatic exposure adaption is delayed. E.g., you can expose for the background, then foreground in the shade, by changing the aperture. Maybe, there aren't many other video dSLRs which would allow you to do THAT ;)

    It is true, with an M-lens the aperture stays wide open during live view, i.e., while the camera isn't actually recording. But that doesn't matter much.

  11. Hi Falk,
    You are right: an M-lens closes the aperture nicely during recording. Since auto focus is barely an option the superior M-focussing is fantastic. Once again a reason to enjoy the old glass!
    I'll certainly experiment with your tip to use AE-L and manual aperture to switch between differently lighted zones... with the 50-500 Bigma using it's aperture ring!
    Thanks again, Arie

  12. does all this work also with K-5?

  13. > does all this work also with K-5?
    AFAIK: yes :)


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