digital camera | Ask Ludwig

Cameras typically have a “Mode” dial, although on some the modes are selected on the display. The mode control selects how exposure is set and looks similar to the illustration here.

The mode control has two areas, except for the professional models that have only one, more on that momentarily. One area, shown here with the icons on the dark background, has the automatic exposure modes. From AUTO to the tulip. In any of these modes the camera makes all the decisions. Pretty much all of the time that results in a fine photo. The icons let the photographer tell the camera what the subject is so the camera can make even better choices.

The other area on the exposure mode dial, here with the white background, shows the modes that put the photographer in control – with the camera helping. These modes are typically labelled M, A, S, P. On Some cameras the settings are M Av Tv and P. These are the modes found even on professional models.

M is manual mode. The photographers sets everything, aperture, shutter speed, ISO sensitivity. The A (Av) mode is aperture preferred. The photographer set the aperture and the camera picks an appropriate shutter speed that gives “correct” exposure. In S (orTv) mode is shutter preferred. In this mode the photographer picks the shutter speed and the camera chooses the aperture.

That get us to mode P, or “Program” mode. This is a somewhat automatic mode. The camera picks both shutter speed and aperture, but the photographer can vary the settings, usually with the control wheel.

So what does the camera pick? – That depends on the ISO sensitivity setting and the scene brightness as measured by the camera. Most manuals are not very informative and neither are many of the training tutorials. For my old Nikon D800 the manual contains this graph (I have added the blue line and the ISO label.

What this tells you is that the camera works along the heavy black line. This is when the camera is set to ISO 100. It will pick a point along this line as determined by the scene brightness. Scene brightness on this chart is represented by the EV scale, shown with diagonal lines labeled from -5 to 23. Each increment on this chart is an EV step or stop, meaning a difference in brightness by a value of two, just like the steps on the aperture and shutter speed controls. An EV of 15 corresponds to bright sunny daylight. In your living room with the lights on you probable would have an EV of about 5.

So by this chart in daylight, EV 15, the camera picks an aperture setting halfway between f/8 and f/11 and a shutter speed halfway between 1/250 sec and /1500 sec. That is where the blue line crossed the heavy black line. If a cloud comes over and it gets a little darker the camera will pick a spot along the black line toward the left and the top. For half the light, that is one stop less light, the camera will open the aperture one half stop and increase the exposure time by the equivalent of one half stop.

The heavy black line goes horizontal at the maximum aperture that the mounted lens provides. It is drawn here for an lens that is f/1.4 at the widest. It will increase the exposure time by one stop for each stop less available light.

Similarly on the other end, at high levels of light, the camera will not stop down below f/16 and compensate for the change in light with shutter speed alone.

But wait, there is more!

In this P-mode, the camera control wheel will let the photographer vary the aperture and shutter speed, making the compensation in one for a change in the other.

Look at the blue line, the one I added to this graph. This is the EV 15 line, for a typical sunny daylight scene. The camera picked the point where the heavy lines cross. The control wheel allows moving up and down along the blue line. Notice that the aperture and shutter settings along this line all give the same exposure. So the photographer can pick either the aperture or shutter speed needed while the other parameter is set to maintain the same exposure.

There is still more. Many cameras allow “auto-ISO” and can also set the slowest allowed shutter speed. When that limit is reached at low light levels – along the heavy black line – the camera will increase the ISO to maintain the exposure.

P-mode is a semi-automatic mode freeing the photographer from fussing with the camera and concentrating on the subject being photographed. It can thus lead to better results. So P-mode is “pretty professional”. Use it. You will grow to like it.

The full inquiry stated that the dark spots were usually seen in the sky part in photos and sometime they were almost indistinct, other times very noticeable. Spots in photo The camera used was a DSLR and the lens kept clean.

Here are samples of such spots. These five photos (these are enlarged sections) show the same sky and tree at right. They were taken at different aperture settings, as indicated.

You can see that for large apertures (small f/number) the spots are larger and lighter. In the photos at even larger apertures the spots were not noticeable.

This explains the mystery of sometimes getting spots and not at other times – the aperture will, of course, be different for different photos. So sometimes, with a small aperture (large (f/number), the spots are there, and they are not visible in photos taken at large apertures.

These spots are made by very tiny dust flecks on the rear of the lens. Indeed, this dust was almost invisible to the naked eye. When the lens is stopped down the light bundle forming the image at any one spot is very small and even the tiniest dust particle can cast an appreciable shadow onto the sensor.

It is very easy to miss cleaning the rear of a lens. It is not exposed to the elements except when changing lenses, so you might not think about it. Normally we keep the rear caps on, so there are but a few seconds when dust can get on the rear of the lens. Unfortunately, that is time enough. So be sure to not only keep the front lens surface clean, but the rear of of lens as well.

(Oh, yes, I intentionally got dust on the rear element of a lens to take these photos – the things one has to do to investigate readers’ mysteries!)

.:.