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Each camera supports a standardized scale of ISOs, shutter and aperture speeds, adding or removing one stop of light at each value. Some cameras also support 1/3rd stop increments which adds a bit more of a scale.

But why can't the speeds be controlled more precisely, down to (say) a single ISO percentage (101 ISO) or a single aperture point (f/7)? It would obviously be too tedious in manual mode, but wouldn't the automatic exposure modes benefit from having more graduated controls?

JonathanReez
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1 Answers1

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They can. But it is extremely expensive to do so with such precision. If you need such accuracy there is plenty of scientific grade photographic equipment available on the market as long as you have financial resources the size of a small country's entire military budget to work with.

For creative photography, there is no need for such scientific precision in the cameras we use. 1/3 stop is as close as most of us need to be able to hit. Often, 1/3 stop is more accurate than the equipment we actually use.

Many lenses are not quite as wide at maximum aperture as their specifications claim. Most lens manufacturers round down to the next nearest full f-stop. The same is true of focal length: Telephoto lenses are usually rounded up to the next "standard" focal length and wide angle lenses are usually rounded down to the next "standard" focal length.

There are a few consumer and pro grade cameras and lenses with 'gapless' apertures. Cinematic lenses often have the option to open and close the iris (aperture) in stepless adjustments, much the same way we can manually focus most lenses in steps as small as our hands and fingers can control. This allows "stepless" transitions from one aperture to another while footage is being recorded. There are even a few non-lab grade cameras that can target shutter times in very fine steps. But there is a big difference between targeting specific apertures or shutter times and actually hitting them with a such a high degree of precision.

What would be the point of adding the complexity of the ability to aim for apertures at 1/100 stop intervals if the camera is only accurate to within 1/6 stop? There really isn't one. The same is true of shutter times. What would be the point of adding the complexity of shutter times in 0.00001 second intervals if the mechanical components can't actually be controlled that precisely?

With raw development tools the final image from our creative cameras can be adjusted in very fine steps to compensate for any variation in shutter time or aperture, not to mention the difference between ISO settings and actual ISO used by the camera at that setting. Canon's Digital Photo Professional 4 allows for adjustments to exposure as fine as 0.01 stops, color temperature as fine as 5 Kelvins, color correction as fine as 0.1 mireds along the Blue ←→ Yellow and Magenta ←→ Green axes, and similarly fine adjustments using the HSL tool. Many other raw development tools are similar.

The difference between such cameras used for creative purposes and those used for precise scientific measurements is the degree to which they can consistently hit the targeted aperture or shutter time within a defined very narrow margin of error. This requires the camera to be able to more precisely measure aperture and shutter curtain positions than is required for our creative cameras. Servos for controlling the aperture and shutter movements with such precision are the primary issues from a cost/effectiveness standpoint. That is why many scientific grade optical instruments are based on precise electronic control of the intensity and/or duration of the light used to illuminate what they measure.

For more regarding the technical precision of cameras used for creative photography, please see: Is there a sane reason why ¹⁄₁₂₅ is not, instead, exactly half of ¹⁄₆₀?

Michael C
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    Also, working with fixed intervals makes exposure very easy to keep figured in your head. A stop of aperture can be brought back with one click of ISO and two clicks of shutter, for example. Nevermind carrying figures in your head when it comes to the crunch. – J... Oct 04 '17 at 10:59
  • The motors that control aperture in modern lenses could easily be made to stop anywhere along their range, and timers could be set to any arbitrary value. These things are controlled by software, so I doubt that expense is an important consideration. – Caleb Oct 04 '17 at 11:48
  • @Caleb - If you read the answer carefully that is not what it says. The expense comes from the expectation of precision with less than 0.01% variance from a target value, not from the ability to target an almost infinite number of values. But if your margin for error is around 1/12 stop what good does it do set steps every 1/48 stop? – Michael C Oct 04 '17 at 17:31
  • @MichaelClark Perhaps we each understood the question differently. I think the OP's question is mostly about why exposure settings aren't continuously variable rather than broken into discrete and seemingly large steps. I don't think providing that capability would be expensive. You're right that calibrating each camera against a standard would be expensive, but that seems separate from the number of steps between f/1.4 and f/2. I don't think it's what the OP was asking about. – Caleb Oct 04 '17 at 17:46
  • @Caleb Without the ability to do so precisely, what exactly could be gained by setting apertures in 1/100 stop steps if the camera is only accurate to within 1/6 stop? As already mentioned, there are some benefits for creative cinematography (the ability to do "stepless" transitions from one setting to another while recording), but that doesn't really apply to still photography. – Michael C Oct 04 '17 at 18:05
  • @MichaelClark I doubt that most people ever wonder whether 1/100s f/8 on their camera gives exactly the same exposure as the same setting on any other camera. I can understand the OP's notion that finer control over those parameters might be useful, and that's independent of the camera's fidelity to an objective standard. That said, I think that the reality of the situation is that finer control really wouldn't be useful -- I've never heard anyone wish for an aperture a little smaller than f/1.4 and a little larger than f/1.6. – Caleb Oct 04 '17 at 21:15
  • @Caleb Again, for finer control, one must be able to both aim for and hit the smaller target. It's the second part (actually hitting the target within a very tightly defined tolerance) that gets expensive very quickly. Creating very fine steps is meaningless if the accuracy of the system is several orders of magnitude less accurate. – Michael C Oct 04 '17 at 21:24
  • @MichaelClark Consider the volume knob on a stereo. It might be marked in decibels, but nobody worries much about whether the markings are accurate -- it's enough that you can turn the volume up or down a little or a lot. And if the knob is at least repeatable -- the same setting gives the same volume -- that's as much as anyone other than a sound engineer would care about. I claim that kind of relative precision should be fairly cheap; more expensive absolute accuracy is less important. In any case, I think we agree that either way, that level of fine control wouldn't be helpful. – Caleb Oct 05 '17 at 01:20
  • @Caleb The problem with repeatability is on the mechanical side of the systems, not the control side. For example, the vast majority of Nikon lenses still use mechanical linkages for the camera to control the lens aperture. Even when the camera is set for the exact same Av, the mechanical slack in the system will cause each successive movement to be slightly different. What would be the point of controlling the aperture in 1/100 stop movements if the standard deviation for any particular movement of the aperture at the same setting is +/-1/6 stop? – Michael C Oct 09 '17 at 15:15
  • @MichaelClark The fact that Nikon hasn't needed to do better than that backs up my assertion that finer control just isn't needed. But it surely would be possible to build apertures that are highly repeatable -- companies like Canon and Nikon are masters at building small stepper motors, which are inherently very accurate and repeatable. I don't know if EF lenses use stepper motors to drive the aperture, but either way, doing so wouldn't be "extremely expensive" by any reasonable standard. – Caleb Oct 09 '17 at 16:00
  • @Caleb Again, it all depends upon exactly how precise one desires. For creative photography 1/6 stop is fine. For scientific research, 0.01 stops may not be precise enough. It's well documented that the aperture control in EF or µ4/3 lenses is more precise than mechanically linked lenses used by other systems. That is why Nikon finally broke down and started making "E" lenses with electronically controlled apertures. But even such electronically controlled lenses are not repeatably accurate to the same degree that much more expensive lab grade stuff is. Getting from .01 to .001 costs a lot. – Michael C Oct 10 '17 at 00:06
  • @MichaelClark I think we're on the same page. ;-) – Caleb Oct 10 '17 at 13:19