D70 TAv Technique

One of the interesting exposure modes available on the Pentax K10D is TAv – the “Shutter & Aperture-Priority AE” mode. Let’s start with Pentax’s description of TAv:

Automatically sets the sensitivity so that shutter speed and aperture give the proper exposure according to the brightness of the subject.

In TAv you set the shutter speed and aperture; the K10D then achieves proper exposure by selecting an ISO within your previously defined Auto ISO range. Some useful points to note are:

  • On the K10D you can define both limits of the Auto ISO operating range
  • “Proper exposure” is defined as whatever zeros the meter
  • EV compensation is available in TAv; dialing-in EV compensation biases the meter’s zero point
  • If the K10D can’t set an ISO that achieves proper exposure, both shutter speed and aperture start blinking in the viewfinder

When the K10D was released TAv fell in for its share of the limelight. I read plenty of online opinion commending Pentax’s forward thinking while castigating other camera companies for not including such a useful feature. Now, that’s actually not quite true. The D70 – almost 3 years older – can emulate TAv, albeit in a less polished manner.

Steps

To emulate TAv:

  1. Set the D70 mode dial to M
  2. Although strictly not required, reset EV compensation to zero
  3. Choose a shutter speed, aperture and ISO that meets your needs
  4. While metering your subject, dial in EV compensation until the viewfinder analog exposure display is zeroed
  5. Turn on ISO auto
    • Press the MENU button
    • Select the Pencil icon to enter the CSM menu
    • Select menu item 05 titled ISO auto
    • Select On
    • Select Done
      Do not enter the menu item titled A, AVP mode – it’s not relevant for M

And…you’re done.

So what exactly did we do? Choosing manual exposure mode in step 1 allows you to set all the exposure parameters (shutter speed, aperture and ISO). Although it’s not required, following step 2 is a good habit to develop – especially in M mode; unlike P/A/S, you can’t tell from the viewfinder analog exposure display that you’ve dialed-in EV compensation – it simply biases the meter; your only giveaway is the +/- symbol in the corner. In step 3 you establish your ‘base’ exposure, and zeroing the meter in step 4 alerts you to deviations from this ideal. By enabling ISO auto in step 5, you allow the D70 to choose an ISO value over its full ISO range.
The result is identical to TAv – the flexibility to set shutter speed and aperture, while leaving the D70 to achieve proper exposure by handling ISO.

And how does it all work? In step 4 you zeroed the meter – in effect, telling the D70 “This is what I consider proper exposure”; after enabling ISO auto the D70 changes the ISO to compensate for deviations from this zero point.

Limitations

Although workable, this method isn’t as polished as the K10D’s TAv mode. Notable limitations are:

  • Can’t set ISO auto limits – the full range (200 – 1600) is always used
  • Without a viewfinder ISO display you’ve no idea of the ISO is being used (an indefensible oversight that remains in the otherwise excellent D80)
  • There’s no in-your-face feedback when the D70 can’t set an ISO that achieves proper exposure; your only clue is an uncorrected deviation in the viewfinder analog exposure display

And finally, there’s the never-ending flashing ISO auto indication in the viewfinder. Notifying the user is laudable – distracting them is not.

Utility

I was excited when I first heard of TAv. “Finally,” I thought, “Someone realizes that ISO isn’t fixed and is just as much an exposure variable as shutter speed or aperture.” And then I tried to use the TAv emulation on my D70.

I was disappointed.

At least for my MO – with its intolerance for large areas of blown highlights, preference for wide, story-telling pictures and no creative need to control shutter-speed – TAv isn’t useful. But, that’s me, and the relevant question is: When will you find it useful?

TAv is useful if you have strict creative/technical reasons to lock down shutter speed and aperture and need to always guarantee the subject’s proper exposure at the expense of all other scenic elements.

Example

The only real-life TAv use I’ve found is in aviation photography – courtesy of a Pentaxian off DPReview. The job? Taking photographs of airplanes and helicopters in flight. The requirements:

  • Composition is mostly “plane against sky”
  • Fixed high aperture to provide the DOF to capture the entire plane
  • Fixed shutter speed – fast enough to freeze the plane, but slow enough for prop blur
  • Plane must always be properly exposed; sky is irrelevant

Given these circumstances TAv is perfect; with it, you prevent the sky from unduly influencing the meter and underexposing the plane, while minor light differences can be compensated using ISO changes. Both shutter speed and aperture have to be fixed for creative and technical requirements respectively. And, given the speed at which the situation changes, you don’t have time to track the plane, compose, watch the meter and change ISO manually. But these use cases are few and far between. Offhand, I can’t think of any other cases where you’d be content with properly exposing the subject (potentially) at the expense of all other scenic elements.

That’s it. If you’ve any questions, found any mistakes, or can suggest any changes, drop me a line!

NB 1: As soon as I get a video capture card I’ll try and get some LCD menu screenshots and other pics
NB 2: I have no idea how all of this interacts with flash