why is aperture only adjustable in certain increments?

[spacediver]

Wouldn't a photographer get more control over the quality of an image if she were able to adjust the aperture with as much precision as, for example, shutter speed? Are there any cameras that allow a finer control over the aperture?

 

[Overread]

Most cameras by default now let you adjust aperture in 1/3 and 1/2 increments as well as full stops.

I also suspect that in the real world shooting, outside of auto modes where the camera controls a/all settings, the benefit of finer and finer aperture (and other settings) control becomes a case of diminishing returns. That is to say its got a peak point and then will trail off since the number of choices gets greater and great and thus the complication greater still. You also have to mate that with the accuracy of reflective meters and complex scenes in-camera. Of course as meters get more and more accurate its potentially possible that the point of diminishing return shifts.

 

[spacediver]

thanks, that makes a lot of sense! I find it interesting that in discussions of aperture, in particular articles that explain it, the issue is rarely discussed. I wonder if some students are left with the erroneous impression that a full aperture stop is somehow of fundamental importance in optics. My understanding is that it's more historical and arbitrary (but I'm still quite new to all this).

 

[480sparky]

Technically, you can adjust a lens to any aperture within it's range. However, doing so with any precision may be difficult to do, at best. At least by hand. Maybe electronically a greater degree of precision can be had.

But I'd venture to say that 1/3 of a stop is, for all intents and purposes, 'close enough'. If f/9 (1/3 stop away from f/8) is 'too much' exposure, would the ability to shoot at f/8.7 (1/4 stop from f/8) be that huge of a difference?

And there's actually DSLR lenses manufactured without the standard 'stops' we associate with lenses. They're called 'de-clicked' or 'cine' lenses.

 

[Overread]

Full stop increments are often how things are taught because its easier. Also learning things in full stop ranges means that you get an idea of where a stop starts and finishes. This makes it then easier to think of stops in 1/2 and 1/3 increments whilst balancing them to other settings (esp if you might, say, have your aperture on 1/3rd and your ISO on full).

From what I recall the only setting that has some bonus to full stop increments is the ISO; since in-between ISO stops the settings are made up by going up or down and then software boosting or lowering the value. Some software packages for noise control also use their default settings based upon noise levels at full stop intervals.

 

[Derrel]

In the digital era, minute exposure corrections are often done in the post processing software stage. Now that sensors have tremendous dynamic range, and low noise in the shadow areas, it is possible to adjust exposures significantly, to such an extent that 1/3 EV precision has proven quite adequate in almost all normal use cases.

Shutter speeds on the other hand CAN be adjusted in very small increments, by electronic shutters controlled by automated light metering systems.

 

[spacediver]

In terms of exposure, yes. But there's also depth of field, though I imagine that 1/3 EV is still a good amount of precision here.

 

[table1349]

Keep in mind you really can't adjust your shutter speed with any more precission than your apeture. If you think I am wrong, try adjusting your shutter speed to say 1/364th&1/2 or 1/523rd. That is why exposure depends on the exposure TRIANGLE so you can adjust each of the three components as necessary to get the exposure you want.

 

[spacediver]

Keep in mind you really can't adjust your shutter speed with any more precission than your apeture. If you think I am wrong, try adjusting your shutter speed to say 1/364th&1/2 or 1/523rd. That is why exposure depends on the exposure TRIANGLE so you can adjust each of the three components as necessary to get the exposure you want.

good point, although there is bulb mode. As for the triangle, for my purposes, I shoot everything at the lowest ISO and at a constant aperture, and only vary shutter speed (I'm doing more technical imaging).

 

[Overread]

In terms of exposure, yes. But there's also depth of field, though I imagine that 1/3 EV is still a good amount of precision here.

.

Honestly in real world situations 1/3 is more than enough aperture control. Indeed I honestly think that depth of field wise you'd be very hard to see 1/3rd stop differences in most situations. Any finer really would only be viable for very niche uses most likely in some form of scientific setup which is way out of the norm for DSLRs

 

[spacediver]

color me niche. I'm using a reverse lens macro setup with a precision linear stage to measure the modulation transfer function (MTF) of various displays. Currently I'm taking some baseline measurements to quantify the camera's MTF, and am testing different apertures to find the sweet spot between lens aberration and diffraction. That's what prompted this question.

 

[Ysarex]

The original lens apertures of the 19th century were holes drilled in metal plates that you dropped into a slit in the lens barrel -- they were called lens stops. Back in those days there was little (basically none) choice of films and they all had the same ISO of about 6 so shutter speed wasn't a big deal you just hung your hat over the lens and when you removed your hat you counted 1 mississippi, 2 mississippi, 3 mississippi, etc. and put the hat back. You had a sunny day stop and a cloudy day stop.

As the 19th century neared an end a number of innovations were showing up. These included an adjustable iris aperture, faster films with a couple to chose from, and shutters with as many as 3 to 5 speeds. Efforts at standardization then began to show up early in the 20th century and the convention of a "stop" as a unit of measure equal to a factor of 2 was adopted. That measurement unit was then applied to all three exposure variables -- shutter, lens aperture, and film speed. This was a critical advance in achieving precise exposure control. It permitted the shutter speed and f/stop values to be reduced to a single number (EV exposure value) which could then be associated with relatively standard outdoor lighting conditions and film speeds. For example Kodachrome (ISO 25) was properly exposed outside on a sunny afternoon at EV 13.
EV 13 is:
1/1000 sec at f/2.8
1/500 sec at f/4
1/250 sec at f/5.6
1/125 sec at f/8
1/60 sec at f/11
etc.
(And yes I remember Kodachrome 25). This method of exposure control in the early/mid 20th century was physically moved to the cameras so that you could set exposure on the camera by setting the EV value. Many camera designs in fact locked the shutter speed and f/stop to the EV setting so that adjusting one adjusted the other and the EV value remained constant. For example if you set the camera to EV 14 and the shutter was set to 1/250 sec then the f/stop would be f/8. If you wanted a different f/stop and set f/4 then the shutter would follow and set to 1/1000 sec.

Using this method it was fairly easy to learn to associate an EV value to relatively constant lighting conditions: Tri-X outside in the sun is EV 17. A cloud covers the sun is - 2 EV. Full overcast is - 4 EV, etc. When I got started light meters in cameras were options.

So the standard f/stop scale developed out of a logical design progression to make exposure control manageable. I still have that camera in the photo above (gettin' buried with it). It's fully mechanical and works without batteries. It has no exposure meter -- just load a roll of film and go.

Joe

 

[KmH]

As mentioned virtually all DSLRs are set by default to adjust aperture, shutter speed, ISO, exposure and flash compensation in 1/3 stop increments. The DSLR owner is given the option to change that to 1/2 stop or full stop steps.

At any rate a 1/3 stop change in exposure is so small an amount most people would not notice the difference between 2 photos made one right after the other of the same scene with 1 of the exposure triad of settings changed by 1/3 stop in between photos.
To this day I am unable to think of/visualize the 3 exposure settings in any working relationship that can be compared to a triangle.

A 'full stop' is a convenient unit that denotes a doubling or a halving.
Lens aperture f/stops, regardless the stop increment, refer to the lens aperture area relative to the focal length of the lens.
Consequently f/2 is a 50 mm wide aperture if the lens focal length is 100 mm, while f/2 is a 100mm wide aperture if the lens focal length is 200 mm.
Since the amount of light any lens aperture can let in depends on the AREA of the lens aperture, and not the lens aperture diameter, a full stop change of lens aperture is + or - the square root of 2 the lens area.

 

[spacediver]

Some great replies, and the historical insight is very valuable Ysarex.