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How can I tell how many stops are between each F-number?
For example I meter 5.6 and I see F8 on the dial. How can I tell how many stops less light F8 is or how many stops more light F5.6 is? Also between a larger number for example what is the difference between F16 and F2? Is there a formula or some kinda of scale I can memorize. I want to know for light ratios..

Thanks,.

R..

Comments (13)

There is a formula. Each full stop represents a doubling (or halving) of the amount of light getting through. The amount of light will vary as the square of the aperture diameter. So a doubling of light will mean the aperture increases by SQRT(2) or about 1.4x..

So the sequence of full stops is....

1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32.

Where each number is 1.4x the previous number..

Frank HollisMass Spectroscopist in the UKCan0n 2oD and 4oD..

Comment #1

So then F8 is two stops less light than F4 and F 32 is two stops MORE light than F64? That sounds easy enough thanks, that is very helpful to know..

R..

Comment #2

Ryan21 wrote:.

So then F8 is two stops less light than F4 and F 32 is two stops MORElight than F64?.

Indeed...

Comment #3

There is a definate relationship between f/stops. That is used is the inverse square law of light. The larger the nunber the smaller the f/stop. The smaller the number the larger the f/stop. Each time you open up your lens you are letting in twice as much light. Each time you close down the lens you are letting in 1/2 as much light.

3 stops eight times the amount of light 4 stops 16 times 5 is 32 and 6 is 64 7 times is 128. So a slight miscalculation makes a dreastic difference...

Comment #4

WrightStuff wrote:.

There is a definate relationship between f/stops. That is used is theinverse square law of light. The larger the nunber the smaller thef/stop. The smaller the number the larger the f/stop. Each time youopen up your lens you are letting in twice as much light..

While your math is right, I think your math terminology is wrong..

The "inverse square law" refers to the way energy density diminishes with *distance*. For example, a flat subject gets 1/4 the light energy if it is moved 2x the distance from the light source. If you're not referring to distance, the phrase "inverse square law" shouldn't apply, as the full phrase is "inverse square of the distance.".

An aperture is a hole in the light path (from subject to sensor). The cross section of the light path grows as the square of the radius. As the radius doubles, you are correct, the cross section area will quadruple. If you want to just double the cross section area, you need to increase the radius by a smaller amount, i.e., sqrt(2) ~= 1.414..

They're really two sides of the same coin, but unlike flexible physics equations like PV=nRT or V=RI, I would hesitate to refer to one relationship (distance to target) and another relationship (cross section of aperture) by the same "inverse square law" name..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #5

Ryan21 wrote:.

For example I meter 5.6 and I see F8 on the dial. How can I tell howmany stops less light F8 is or how many stops more light F5.6 is?Also between a larger number for example what is the differencebetween F16 and F2? Is there a formula or some kinda of scale I canmemorize. I want to know for light ratios..

Thanks,.

R.

They way I learnt to remember it, from reading a book is this..

The F-Stops go something like this.

1.0 1.4 2.0 2.8 4.0 5.6 8.0 11.0 16.0 22.0.

Now, you will see that every alternate number is half or double the next/previous one. Thus you have:.

1.0 2.0 4.0 8.0 16.0.

And.

1.4 2.8 5.6 11.0 22.0.

So now it's pretty easy to remember..

Hope that helps...

Comment #6

Hi,.

The ratio refers to area of lens etc so square the numbers and they make sense..

1 is 1.

1.4 is 2.

2 is 4.

28 is 8.

4 is 16.

56 is 32 and so on..

The numbers are rounded a little(!) to make them easy to engrave on lens barrels, or were engraved on lens barrels..

Regards, David..

Comment #7

There's no "inverse" law here. It's simply that the "/" in.

F/1.4, f/2, f/2.8, f/4 ....

Denotes division. If you leave out the "/" in writing the number, someone may think that the 1.4, 2, 2.8, or 4 is a multiplier ... but of course, nobody's bothered to tell the lens that!..

Comment #8

If you have a lens with limits of 3.5 to 5.6, why can you, or why does the camera allow you (or prohibit you in some cases) of using a higher f value?.

For example...I am new to dSLR's, and am just learning all of this, but... I was shooting pictures in our kitchen, trying to see the differences of shooting the same image with different settings...I understand WB now much better, but when I choose the A program setting...aperature priority...it only allows me to go so far (the camera chooses shutter appropriately). Most of the time I dont know how to set the aperature lower...and have never seen this value go down below 4..

At the other end of the spectrum... I think in my head that my limit is 5.6, but many of my images have a much higher f value, some even to f/22..

Can someone explain this? preferably in laymans terms?.

Thanks in advance....

Jeff..

Comment #9

Huskypup wrote:.

If you have a lens with limits of 3.5 to 5.6, why can you, or whydoes the camera allow you (or prohibit you in some cases) of using ahigher f value?.

Those are the lower limits of the lens, not the upper limit..

What you have is a zoom lens that has a lower limit of f3.5 at the wide end of the zoom and f5.6 at the long end of the zoom..

The upper limit will typically be f22 and that applies for either the wide or long end of the zoom. So at the wide end of the zoom you have a range of f3.5-f22 and at the long end of the zoom f5.6-f22 range..

For example...I am new to dSLR's, and am just learning all of this,but... I was shooting pictures in our kitchen, trying to see thedifferences of shooting the same image with different settings...Iunderstand WB now much better, but when I choose the A programsetting...aperature priority...it only allows me to go so far (thecamera chooses shutter appropriately). Most of the time I dont knowhow to set the aperature lower...and have never seen this value godown below 4..

You can only go as low as the lower end of the lens's f range. Your lens apparently stops at about f4. I have a zoom that has a lower limit of f2 so that is the lower limit that my camera allows with that lens attached. If I then attach my f4 zoom then f4 is the lower limit that the camera allows..

At the other end of the spectrum... I think in my head that my limitis 5.6, but many of my images have a much higher f value, some evento f/22..

F5.6 is the lower limit that you can set not the upper. See above..

Can someone explain this? preferably in laymans terms?.

Thanks in advance....

Jeff.

A member of the rabble in good standing..

Comment #10

What you are referring to is the lowest F stop that the lens will go to at each end of the zoom ie. at 55mm it's f/3.6 at 200mm it's f/5.6 (assuming a 55~200mm) and the highest f stop is around f/22..

EDIT, must learn to type faster  .

Cheers DavidThis space NOT for rent..

Comment #11

I am just using the kit 40-150mm lens. And thank you to all who have replied, I didnt know those were lower limits..

Makes much more sense now..

Jeff..

Comment #12

Hi,.

The trouble is, there's more to the zoom lens than just the zoom range or power but the people in PR like to simplify things and - no surprise here - they go for vague figures. So every one is told x3 or x10 or x10 power zoom but everything else is hidden away in the small print..

When they quote something like f/35 - f/56 they mean that it's f/35 at the wide end of the zoom and f/56 at the tele end of the zoom but both figures are for the wisest aperture available. The smallest aperture available is probably around f/11, f/16 or f/22 depending on the focal lengths involved. It's not unknow for some lens to have just two apertures available (like f/28 and f/56) and no way of knowing where they are available..

Then you need to know the actual focal lengths available and the equivalent in 35 mm terms which can be quite usefull. You might find you can zoom in steps of 01 mm all the way through the zoom or you may not. I've had lenses that only had a handfull of zoom "steps" and others that went smoothly all the way and could be stopped anywhere on the scale. (only you don't find this out until you look at the EXIF from all the pictures over a long time)..

The real focal lengths are needed for DoF and hyperfocal length calculations and the 35 mm version is usefull for _rough_ comparisons as the aspect ratio then starts confusing the issue further..

Regards, David.

PS Typing that made me realise how simple life was when I just had 35mm film and MF to deal with (sigh)...

Comment #13

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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