Do yourself a HUGE favor and buy a copy of Bryan Peterson's "Understanding Exposure"... read it, study it, learn it. That might be the best $18 you ever spend on your photography.

That said, you first need to understand the basics of exposure (which we can only cover in a limited way in forum posts).

There are three factors (variables) controlling exposure: lens aperture size, camera shutter speed and the sensitivity of the recording medium (film or digital image sensor). In modern DSLRs, all three are easily adjusted using controls on the camera itself. Of course, the image sensor and shutter are within the camera body itself. But the aperture is in the lens and in most modern camera systems is controlled by electronic communication between the camera and lens (along with other lens functions such as autofocus, and in some cases image stabilization).

The lens aperture is actually a ratio between the lens focal length and the size of the opening allowing light to pass through the lens. For example, a lens aperture of f/2 with a 50mm lens means the aperture is 25mm in diameter. The same f/2 on a 100mm lens must be 50mm in diameter. Or an f/4 aperture on a 100mm lens is 25mm in diameter. Divide the focal length by the f-stop with any combination, and you'll be able to determine the actual physical diameter of the lens aperture opening. (This math explains why low numbers like f/1.4 mean big apertures, while small numbers like f/16 mean small ones.)

Now a teleconverter changes the effective focal length of a lens, while the physical size of the aperture within the lens remains unchanged. With the teleconverter installed, the ratio or relationship of the focal length to the aperture size obviously changes, too. It just so happens that a 1.4X teleconverter causes one stop of "light loss" or reduction in effective aperture... while a 2X teleconverter causes a two stop reduction.... Less common 1.7XC and 3X respectively casuse 1.5 stop or three stop effective reduction of aperture.

Most cameras take this into account and display the effective aperture that the teleconverter causes. Every DSLR I've ever used has done so. If I put a 1.4X teleconverter on my 500mm f/4 lens, the largest aperture I'm able to set is f/5.6. Or, if I put a 2X on that same lens, the largest aperture available will be f/8. Or, if I had stopped down the lens set to f/8 without a teleconverter, then add a 1.4X, it the camera will then display the new effective aperture of f/11. (Of course, the effective focal length of the lens changes too... 700mm with 1.4X or 1000mm with 2X).

My range of effective apertures changes when the teleconverter is added. But since the camera is taking the teleconverter into account, any available aperture I set will give the same exposure. In other words, while I can no longer set f/4 on an f/4 lens once a 1.4X teleconverter is added.... if I instead were to stop down and set f/8 in both cases.... with and without the 1.4X... it will give the same exposure. (The difference is that f/8 is one stop down from wide open when the 1.4X is installed, but it's two stops down from wide open without it.)

Each full stop smaller reduces the light allowed to pass through the lens by 1/2.... In other words, f/2 allows in half the light that f/1.4 allows. f/2.8 cuts in half again... and f/4 cuts it in half yet again. (My cameras and many others are actually adjustable in 1/3 stop increments... some cameras only adjust or can be set to adjust in 1/2 stop increments... some lighting and light meters work in 1/10 stop increments.)

Full f-stops are: f/1.0, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc.

To help remember those, notice that every other stop's denominator doubles.... 1, 2, 4, 8, etc. and 1.4, 2.8, 5.6, 11, etc.... the higher numbers/smaller aperture designations are "rounded off".

If you want to work in 1/3 stop increments you might want to look up and print out a chart showing 1/2 stop increments (I sure can't remember them all!) When I don't have time to consult a chart and am not working in full apertures, I just "count clicks" when I make adjustments... since my cameras are set to 1/3 stop increments, every three clicks of the dial is a full stop change.

Similar is true of shutter speeds and ISO... "Full stops" of shutter speeds are 1 second, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1000, 1/2000, etc. Full stops of ISO are 100, 200, 400, 800, 1600, 3200, 6400, etc. Cameras might present these in full, 1/2 or 1/3 stop increments, too (I often leave my ISO in full stop increments, to be able to change it more quickly... I still have plenty of flexibility to precisely adjust my exposure with 1/3 stop increments of shutter speed and lens aperture.)

Hope this makes sense! I'll leave it to Bryan Peterson to explain further and tell you why you might select different combinations of aperture, shutter speed and ISO depending upon what you're trying to do with a photograph.

Music theory is excellent example of the descriptive
vs the prescriptive. MT is descriptive. Treating it as
prescriptive helps composers to stay the course of
tradition and history, which conditions the ears, the
mental expectations, of an audience ... assuming an
audience has rather consistently remained with the
geo-cultural purview of the MT in use. IOW stuff will
sound right, instead of "out". But go to another geo-
cultural situation and what sounds right to people in
that place may sound "out" to you.

Similar concepts apply in the visual realm.


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