Fstops

Some shots in dpreview are taken with a 50mmf1.4 lens. what is the first stop down from that? Is it f1.8, or f2 or is it f2.8. Being old school I'm pretty clear on the rest,ie f4,.56,8,11 etc. Taa

In terms of full stops, f/2.0, then f/2.8, f/4.0, etc.

Each f numbers is 2 to the power of ( n/2 ), so that each stop is a multiple of the square root of two apart, or about 1.4.

The easiest way to remember is that it's two interleaved sequences :

1.4, 2.8, 5.6, 11.2 and so on ( each 2x apart )

and

1, 2, 4, 8, 16 ... ( each 2x apart ).

--
StephenG

Thanks for the response's. So going by your example, f4 is not a full stop value?

And to continue, f1.4 is the widest full stop possible. F1.2 or f1 are only fractions of a stop. Is this true? Perhaps in the future when there is a f1.o lens that manufacturers could re align the who;e f-stop increments as follows:

f1, f2, f4, f8, f16, f22, f44, f88 . Some how i think somethings not quite right with this equation. Food for thought

Thanks for reply. So f4 and f8 are not true full f-stops? Is this right?

If I have a f2.8 lens and attach a 2 x tele-convertor does that reduce it to maximum aperture of f11 and not f 8 which i previously believed.? Like to hear your thoughts.

cootaman said:

Thanks for the response's. So going by your example, f4 is not a full stop value?

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It is still a full stop, but it is the stop between f/2.8 and f/5.6.

cootaman said:

And to continue, f1.4 is the widest full stop possible. F1.2 or f1 are only fractions of a stop. Is this true?

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No, the widest is indeed f/1.0, which is as sjgcit pointed out. f/1.2 is a half-stop between f/1.0 and f/1.4.

cootaman said:

Perhaps in the future when there is a f1.o lens

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Such wide aperture lenses already exist, but they are not common or necessarily currently produced.

cootaman said:

that manufacturers could re align the who;e f-stop increments as follows:
f1, f2, f4, f8, f16, f22, f44, f88

Click to expand...

Very small apertures introduce distortion; I'm not sure any lens aperture gets smaller than f/32.

cootaman said:

Thanks for reply. So f4 and f8 are not true full f-stops? Is this right?

If I have a f2.8 lens and attach a 2 x tele-convertor does that reduce it to maximum aperture of f11 and not f 8 which i previously believed.? Like to hear your thoughts.

Click to expand...

You have not been reading the responses carefully. Everyone who has responded to your questions has provided useful information but it went right by you. Please generate some effort on your part to achieve an understanding of your questions.

First, do you know what an f/number is? Let me explain. The f/number is the ratio of the diameter of the entrance pupil to the focal length of the lens. So if a lens has a focal length of 50mm and an entrance pupil of 25mm, it's an f/2 lens. If the entrance pupil is 5mm, it's an f/10 lens.

This ratio controls the amount of light reaching the sensor (or the film, if you're an old-style guy). But the amount of light is a function of the area, so the illumination goes as the square of the f/stop. In the example above, we have a 25x change in area, so there's a 25X change in illumination.

Over the years, a standard progression of f/numbers has developed, which has been explained to you. It typically starts at f/1 (although that's not the limit; you can make lenses faster than f/1) and goes as the square root of 2 as far as anyone's interested.

But those are not the only "true" f/stops. You can have f/6.3 or f/24 or whatever. Anything you set is a true f/stop. The ones you mentioned, f/4 and f/8 are, if you looked at the responses, full f/stops in the standard sequence:

1,1.4,2,2,8,4,5.6,8,11,16,22,32,45,64

You have also added a teleconverter question. The effect of a teleconverter on f/stops can't be simpler. You multiply the f/stop by the converter factor. So for an f/2.8 lens with a 2X converter, the maximum aperture is f/5.6.
--
Leonard Migliore

trekkeruss said:

No, the widest is indeed f/1.0

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Correcting myself: there can be apertures wider than f/1.0, but it is definitely not common.

trekkeruss said:

No, the widest is indeed f/1.0, which is as sjgcit pointed out.

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But f/1.0 is not anything like the widest and sjgcit did not say it was.

An "f/stop" is the ratio of the focal length to the aperture stop. If the f/stop is f/2 the aperture diameter is half the focal length. If the f/stop is f/1 they are the same size. If the aperture diameter is twice the focal length the f/stop is then f/0.5.

Generally speaking we don't see lenses, particularly for DSLR's that have an f/stop of even f/1, much less smaller. That is because the front element is necessarily huge and very difficult to grind to close enough tolerances to maintain sharpness.

It probably should be pointed out to the OP that a "full f/stop" does not depend on any given number as such. It just means twice as much light or half as much light. So if we started with f/4.34 a "full f/stop" less light would be at sqrt(2) * 4.34, or 1.4 * 4.34, which is a aperture of f/6.076! Of course lenses are not marked for that, and cameras can't be adjusted to it. But that relationship is what has to be kept in mind. And when it is the set of "full stops" starting at f/1 is pretty easy to remember.

Of course these days 1/2 stops and 1/3 stop increments are pretty common too! Not that you necessarily need to have those number at the tip of your tongue...

Knowing how to get them with a calculator is probably enough. Light reduction by 1/2 a stop means the ratio is increased by sqrt(1.5) times the existing f/stop, as an example. Use 1.333 for 1/3 stop and 1.667 for 2/3 stop.

apaflo said:

trekkeruss said:

No, the widest is indeed f/1.0, which is as sjgcit pointed out.

Click to expand...

But f/1.0 is not anything like the widest and sjgcit did not say it was.

Click to expand...

But it is the widest "full stop" when using the agreed upon scale used by all manufacturers.

apaflo said:

It probably should be pointed out to the OP that a "full f/stop" does not depend on any given number as such. It just means twice as much light or half as much light.

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Yes, of course. But let's not confuse the OP further with too much information!

Got it guys and thanks for the responses. I'm a photographer, not a mathematician and failed badly in maths. It was the response from Srjit ? that threw me off for a while

I missed the relation of 'interweave' in the context to the bottom set of figures that he supplied.

1.4, 2.8, 5.6, 11.2 and so on ( each 2x apart )
and
1, 2, 4, 8, 16 ... ( each 2x apart ).

Thanks again to replying to my first dp posting.