DaveC1 Loc: South East US

Yes they do! And most engineers are very proud of that fact.

DaveC1 Loc: South East US

TonyBot

Forget "T-stops". Forget the logarithmic scaling that is what "f-stop" stands for (or that in real numbers, the 1.4 equals 1/2 and 2 equals 1/4). Forget "distance from" and "number of elements" arguments. While all have some merits to them, an extender just doubles the "effective" focal length, optically making it greater than it actually is, and Bebu's explanation cuts directly to the crux of the matter.

DaveC1 Loc: South East US

Respectfully, I don't believe you understand how a teleconverter works based on your explanation. The teleconverter is a single, or multi lens magnifier, 1.4 times in the case that started this thread. This increases the focal length of any attached lens by that factor (300mm X 1.4=420mm) but the teleconverter does nothing to the size of the lens opening and as we all know the f-stop is the ratio of focal length to opening size, as simply as I can put it.

Rich1939 Loc: Pike County Penna.

I was a sales engineer for the first half of my working years. both sides of me hated the other.

DaveC1 Loc: South East US

Yes, and you were probably deeply distrusted by both the sales staff and the engineering staff.

GrandmaG Loc: Flat Rock, MI

Absolutely true! We have two sons that are engineers!

jeep_daddy Loc: Prescott AZ

Yes, it is this simple math that tells all. It is also known as the focal ratio, f-ratio, or f-stop. It is the reciprocal of the relative aperture. The f-number N or f# is given by:

N=f/D
where f is the focal length, and D
is the diameter of the entrance pupil (effective aperture).

For example, if a lens's focal length is 10 mm and its entrance pupil diameter is 5 mm, the f-number is 2, expressed by writing "f/2", and the aperture diameter is equal to f/2 where f is the focal length.

So as to not confuse you, the pupil is the diameter of the actual aperture when wide open. So if you can imagine the focal length of a 50mm lens is 50mm, to be an f/1 lens the actual diameter or pupil of the aperture would have to be also 50mm. The body of the lens would be quite large at 50mm. If you think that is large, imagine a 500mm lens at f/4. The pupil of the lens (aperture wide open) would have to be 1/4 of the focal length or 125mm in diameter. A 100mm cigarette is 100mm in length. So the diameter of the lens housing has to accommodate an aperture that is 125mm when wide open. That's why large lenses are so fat.

CatMarley Loc: North Carolina

here is a graphic to explain. The teleconverter basically works by cropping the circle of light from the lens and projecting the cropped light onto the whole surface of the sensor. The cropped light is less intense (fewer photons), than the image that came directly out of the lens.

TBPJr Loc: South Carolina

Attaching the 1.4 teleconverter doesn't change the aperture the camera sets, it changes your maximum aperture, just as you noted (a 1.4 converter will turn a 2.8 lens into a 4.0)--it reduces the amount of light the lens can deliver. It has no effect on what the metering says is needed or on what the camera wants to set, but it limits the settings available by one stop.

amfoto1 Loc: San Jose, Calif. USA

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.

Photocraig

YUP, but they ALSO sent my kids to college!

BebuLamar

That because the Salesman sells things that the engineer couldn't make and the Engineer made thing the salesman couldn't sell.

ecurb1105

Just remember a teleconverter reduces the light reaching the sensor/film. A 1.4 converter reduces light by one stop, a 2x converter reduces by two stops. You can adjust your exposure y opening your lens or by lowering your shutter speed. You're over thinking things, it's simple in practice. I just don't like teleconverters because of the light loss and image degradation.

drklrd Loc: Cincinnati Ohio