Having the concept of camera obscura, all that was necessary was to find the formula that would fix the image after taking it out of the “camera”.
The known quality of silver to darken with sunlight made it the best material to test. Various silver haloed solutions were tried, the difficulty was getting it to stop “developing”, fixing the solution so that bringing the material out into the light was possible. This is what the three basic chemical baths are today. 1)developer, 2) stop, 3) hypo or fix.
Now the only skill set to master in order to have a “perfect” photo, is the dance of three different controls. You have to know the basic sensitivity of the medium that you are capturing the light. For our purposes we call it the film or light sensor. A number ranking of ASA or today’s ISO of 125 is considered basic. Film ISO can go from 25 all the way up to 2000. The higher the number the more sensitive to light. There are specialty film that is even lower and higher for special purposes.
In digital cameras the higher ISO settings came with increased “noise” in the image so that there was extreme penalty in setting higher ISO. The newer cameras now have come a long way towards noise suppression, and super high ISO settings of 32000 don’t seem to generate much noise.
Staying with the basic ISO 125 gives us the possibility of using the f16 Daylight Rule.
The rule states that if you use ISO 125 film, and you set the shutter speed at 1/125 of a sec, and the aperture at f16, that your photo will be perfectly exposed if it is a normal sunny day outside. We call this the “the dance”. If we start with this premise we can find out some interesting things.
The aperture of f16 is a pretty small opening. It can get smaller on some lenses, and it can get very much larger on most lenses. Unfortunately,only the most expensive lenses can get as large as a f1.2. f1.4 aperture. Very cheap lenses are typical f2.0 or higher. We tend to say that the f1.2 lens is very fast and gathers a lot of light. When you want to take photos in a low-light condition, you want a fast lens, hopefully a f1.2.
In the f16 Daylight Rule, the assumption is that the shutter speed is 1/125 of a second. If you are using a fast lens of f1.2, but that still doesn’t capture enough light, then one option is the extend the amount of time that the shutter is left open, in others the opening is as large as you can, so you leave the opening open longer. The problem is that the subject may move while it is open longer, and the photo may appear blurred. 1/125 of a sec is pretty good, but you can often slow it to 1/60 of a second, get twice as much light with very little blur. Move the shutter to 1/30 of a second and you will have to tell everybody to keep still. 1/15 of a second and you will have to pay particular attention to keeping yourself still, or everything will be blurry.
The “dance” is that there are dozens of settings that will give you a perfect exposure, but if you change one element, you must change the other. Make the opening larger and you have to speed up the time.
So let’s say that a setting of f1.2 and a 1/1000 shutter speed is a perfect exposure setting. Then perhaps an f8 aperture and a shutter speed of 1/125 is also a perfect exposure, so is there any difference in the photo? The biggest difference is the focus. Using the large aperture opening requires that the focus is extremely accurate.
In fact, the cameras that don’t have a focus ring to control the lens can get away with this by having a very small aperture of f16 or more. Cheap disposables with no focus control have a very small aperture. This is why photos in low light conditions (f1,2 opening) very often are fuzzy, because the focus is so crucial. And you can’t tell if it focused because it is so dark. This phenomenon of focus is called the “depth of field”.
If you want only the subject to be in focus, then focus critically then set the aperture at a very fast setting. After the shutter speed is adjusted for the proper exposure, then the result Is that only the subject is crisp, everything in front and everything in back is a soft blur. Nice effect!