The Third Variable

November 15, 2013  •  4 Comments

In the good old, bad old days of film photography we were taught two main shooting variables: shutter speed and aperture. One entered the 'given' (ISO or ASA) into the light meter and it did the rest, with or without some help from us.

In truth, film speed itself was, though not generally treated as such at the shooting stage, a semi-variable: one made an educated guess at the time of loading the camera as to which film speed would likely suit the anticipated shooting conditions and took it from there.

Very experienced photographers would treat ISO as a much more freely altered variable. A film could be pushed or pulled during processing, for example, or it could be swapped for one of different sensitivity by rewinding a half shot roll and replacing it as desired. But it was never a truly variable variable. You couldn't change it immediately and frequently at will, on the fly, with the same ease as you would change aperture or shutter speed.

It seems to me that this mode of working, this mindset, has permeated the design of digital cameras and that the time has come to change this, for good, much more comprehensively than any manufacturer has yet been willing to attempt.

A recap:

Shutter speed is varied according to whether one wants to show or hide movement, and to reduce or eliminate camera shake, especially with longer lenses.

Aperture is used to control depth of field or to enable a lens to be used in its 'sweet zone' of sharpness.

ISO is not varied for its own sake: in the great, great majority of use-cases it is desirable to shoot at the lowest ISO possible given the requirements placed on it by the primary needs of shutter speed and aperture. One will almost never choose, on purpose, to shoot at a higher ISO than one needs. There may be rare decisions to shoot high ISO for creative reasons but they are rare indeed and largely obviated by post processing possibilities. Most modern sensors can be effectively regarded as "ISO-less" (shooting at base ISO and then applying push and pull to the processing of the signal to achieve apparent ISO variation) and in practice this means that for every extra step of ISO the photographer is forced to use, a step of DR and colour fidelity are lost. Very good sensors can hide this quite well up to what would, historically, have been regarded as impossibly high ISO levels but it is nonetheless generally true.

So though ISO is the 'poor cousin' of the three variables in terms of its apparent creative flexibility, it is the 'enabler' for the other two variables and as such, a system that allows it proper levels of automatic flexibility benefits them greatly. The trouble is, no one has yet come up with an algorithm for controlling this that adequately mirrors the decisions a photographer would make.

Of course there is now a fourth variable, Image Stabilisation, which can affect the required formula and I will take that into account as I describe the system I would like to see.

First, examples of the best and worst systems currently known to me.

The best is in the Nikon D800/E. The photographer has the option, in setup and menu, to choose a maximum ISO and to decide a minimum shutter speed. This can be a fixed speed, such as 1/100th sec, or a relative speed rated as Slower, Slow, Normal, Fast and Faster. Normal equates to approximately the old '1 over focal length' rule, so if the mounted lens has a focal length of 50mm, the shutter speed will be around 1/50th Sec. But wisely, Nikon allows for the fact that a high resolution sensor generally requires two or three times that shutter speed if used without a tripod and if optimal results are to be achieved. But the Faster/Slower system also allows a photographer to take account of whether stabilisation is being used, and to make adjustments based on personal factors such as whether the shot will be well braced or not and whether the photographer has steady hands, or is shivering with cold or being buffeted by wind. So the system is pretty flexible, but to my mind still requires too much intervention: if switching from a stabilised to non-stabilised lens, for example, one has to delve into menus to make parametric changes.

The worst I can currently think of is that in the Olympus E-M1. This otherwise excellent camera holds one major trump card over its larger sensor competition: In Body Image Stabilisation. Doesn't matter what lens you have mounted, IBIS gives you up to five stops of improvement - and it works. However, the Olympus Auto ISO system insists on using a minimum shutter speed of approximately 1/effective focal length. So for example, shooting a 40mm lens, which equates to 80mm in the traditional sense of Field of View, will cause the camera to select a higher ISO whenever the shutter speed threatens to drop below 1/80th second. However, shots can easily be made with excellent sharpness at 1/10th of a second (often lower) due to the IBIS. In effect, this means that the camera will often shoot at ISO 800 when it could be shooting at ISO 200 - and this completely wastes the advantages conferred by the IBIS and results in too many shots taken at too high an ISO, in which range of operation the E-M1 rapidly becomes less convincing than cameras with larger sensors.

I have noticed with the E-M1 that the effectiveness of its IBIS varies with focal length: with wide angle lenses it really does offer up to five stops of stabilisation but with the Panasonic 100-300mm zoom mounted and shot at the longer focal lengths, one generally does need to shoot at '1 over focal length'. In other words, the camera defaults to the 'worst case' scenario when it could quite easily be a lot more generous with some focal lengths than with others. I have explained, in my recent review of the E-M1, how to work around these problems but the solution is clunky and IMHO should not be necessary.

The Leica M240 suffers a different problem: it will often choose the nearest shutter speed to the 1/focal length formula but is quite capable of 'rounding down' to the nearest shutter speed it offers, even though the high resolution sensor generally requires around twice the shutter speed indicated by the 'old rule'. It really is as if the Leica engineers have not fundamentally thought about anything for about forty years.

The Sony A7r does not initially look promising: its Auto ISO system apparently also works on a 1/focal length rule though I hear that requests have already been put in to Sony engineers to look at this again. My guess after limited shooting time with the camera is that it will generally require twice the focal length as a shutter speed, possibly three times because the mechanical shutter has a hefty clunk to it.

However, it is possible to shoot the Sony in what might be called 'Shutter and Aperture Priority'. In other words, the photographer selects Manual Mode and sets the shutter speed where required for stabilisation and subject movement needs. The desired Aperture is then selected and the camera will vary ISO accordingly, taking all the strain of variability and acting, as it should, as the enabler for the two creative variables. But, and this is absolutely key, the manual Exposure Compensation Dial can be used in this mode (unlike with the E-M1 which requires a clunkier and more time-consuming workaround) so that one can choose to have the ISO automatically adjusted taking desired compensation into account. Excellent.

This Sony system is as close to what I would like as I have yet seen. But I still think manufacturers could do better. The system I would like is as follows:

The photographer sets 'broad use' parameters in the menu system. These include Max ISO and optionally Min ISO, and then, crucially, 'multiple of focal length' (similar to the D800 but more explicitly) for TWO shooting scenarios: with and without Image Stabilisation. The photographer can then choose to have, for example, '1/focal length' to be the rule governing stabilised shooting and, say, three times that shutter speed for unstabilised shooting. The camera knows whether stabilisation is present or not, and adjusts the shooting rule accordingly. Then, without leaving Aperture Priority mode the photographer can shoot, with or without exposure compensation, and always have the camera select combinations of shutter speed and ISO that achieve the desired aims.

This system would work best for systems with lens-based stabilisation because those systems are more finely tuned to individual focal lengths. For IBIS systems such as that of Olympus, where stabilisation seems to offer fewer stops of assistance at longer focal lengths, further parametric or automatic systems could be put in place.

For now, I am looking forward to shooting with the Sony system: it seems a good way of achieving my aims. But I do wish that camera manufacturers would finally get over their 'film hangover' and, instead of offering me an endless array of 'Granny by Moonlight' or 'Snow on the Beach' modes, get Auto ISO right

Doing so would very often save a stop or more of ISO and would, in so doing, add more to IQ than another year or two of sensor development.

What do you think?


Jonathan Slack(non-registered)
Hi Tim - great article as always.

Pentax (and now Ricoh) have something called TAV mode (which allows you to set shutter speed and Aperture manually, and then the camera selects the correct ISO). You can do this with the Sony as well (and actually you could do it with the M9 too), but neither have enshrined it as a separate mode.

To my mind this is the obvious recognition of the importance of ISO as the third factor in exposure, and it's better to be acknowledged as a different mode.

As for Auto ISO settings - I quite agree - it should be possible to devise a proper way of working it out - IBIS is helpful in that it applies to all lenses, personally I favour relating it to the focal length rather than Nikon's 'normal'. So that it would be 1/4, 1/2, 1x, 2x, 4x chosen focal length etc.

All the best - I wait with baited breath for you evaluation of the A7r with the new Zeiss zoom. . . . .
Tim Ashley Studio
John, thank you. You can indeed do the same sort of thing on the d800 but IMHO the fewer times you need to change settings the better, which is why I'd prefer to see a 'set and forget' parametric system. That would allow something of the purity of shooting experience of an M camera with a lot of pre-decided stuff going on 'under the hood'... Otherwise, at least in my case, I would often times fail on changing lenses to also remember to change User Mode... Also, I would prefer the camera's innate awareness of whether stabilisation is in use to dictate the appropriate choice so that merely sliding IS to 'off' briefly on a lens, as one sometimes does, effected the parametric change automatically.
John King(non-registered)
Tim, very well-written article. As a footnote, the Nikon D600 provides two User Modes that can each be configured for Slower or Faster Auto ISO. It also allows for all of the Manual settings with Auto ISO that you describe for the Sony system.

I have my D600 set up with U1 in Manual Mode with AutoISO (Faster) and can use the exposure compensation dial to quickly modify ISO, as you describe. Presumably this configuration is also available with some other Nikon models.
Tim Ashley Studio
Chris, that is what I refer to in the article and describe in detail in the review I published of the E-M1 last week. However, it doesn't allow you to use exposure compensation easily. To do so requires you to set the lever to position one when you want to change aperture or shutter speed and position two in order to vary ISO so you can effectively set exposure compensation. Proper auto ISO implementation allows you to use exposure compensation.
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