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Clarification on f/stop, vs. Shutter speed vs. ISO


amol

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Hey,

 

I need a little help, making sure I understand exposure ratios between aperture,

Shutter speed and ISO.

 

I'm trying to decide between the Tamron 17-50/Tokina 16-50 f/2.8 and the Sigma

17-70 2.8-4.5. I don't own either lens.

 

One of the major differences, other than focal lengths, and "lens quality" are

the aperture ranges. 2.8 vs. 4.5

 

 

So, I really like the f/2.8 for low light work. Aside from DOF stuff, I'm

trying to figure out how much I can change the ISO, so that the 4.5 will be give

me the same shutter speed, as a f/2.8... For example:

 

Let's say the meter tells me, at ISO 200, the correct exposure is 1/60 @ f/2.8.

Now, if I have a the Sigma set a 70mm(or some other variable aperture lens), and

it only gives me a max aperture of f/4.5.

 

So, it is one-and-half stop difference, from 2.8 to 4.5? If I understand

exposure correctly, the equivalent of ISO 200, 1/60 @ f/2.8 would be:

 

ISO 400, 1/60 @ 4.5. Is this correct? Or maybe ISO 400, 1/40 @ f/4.5?

 

 

Basically, I still want to "hand-hold" and still get the shot, by bumping up the

ISO, but not too much. On my Rebel XT ISO 400, is still decent, of course, ISO

100/200 is better. I like the range of the Sigma 17-70, but still want to be

able to get a sharp/non-blurry shot.

 

 

Thanks,

Amol

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If my maths is right, f/4.5 is 1.27 stops slower than f/2.8.

 

The calculation is : sqrt(4.5/2.8) = sqrt(1.6) = 1.27

 

So call it 1.33 for convenience, if your camera has 1/3 stop increments. If not then either 1.0 stop or 1.5 stops would be pretty close.

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f/4.5 is about 2,53x slower than f/2.8<br><br>

root of 2 = 1.41<br>

root of 4 = 2<br>

root of 8 = 2.82<br>

 

root of 16 = 4

<br>

<br>

 

aperture f/1.41 gives 2x more light than f/2.0

<br>

<br>

 

Hmmmm... I don't know how to explain it in English, but you'll probably find many web sites about EV, aperture values etc. using google.

<br>

<br>

 

Maybe this will help: <br>

<br>

 

<table border="1" cellpadding="2" cellspacing="0">

<tr>

<td valign="top"><p align="right"><strong> 0EV</strong></p></td>

<td valign="top"><p align="right"><strong> -1EV</strong></p></td>

<td valign="top"><p align="right"><strong> -2EV</strong></p></td>

<td valign="top"><p align="right"><strong> -3EV</strong></p></td>

<td valign="top"><p align="right"><strong> -4EV</strong></p></td>

<td valign="top"><p align="right"><strong> -5EV</strong></p></td>

<td valign="top"><p align="right"><strong> -6EV</strong></p></td>

<td valign="top"><p align="right"><strong> -7EV</strong></p></td>

<td valign="top"><p align="right"><strong> -8EV</strong></p></td>

<td valign="top"><p align="right"><strong> -9EV</strong></p></td>

</tr>

<tr>

<td valign="top"><p align="right"> 1</p></td>

<td valign="top"><p align="right"> 1*1,4142</p></td>

<td valign="top"><p align="right"> 1,4*1,4142</p></td>

<td valign="top"><p align="right"> 2*1,4142</p></td>

<td valign="top"><p align="right"> 2,8*1,4142</p></td>

<td valign="top"><p align="right"> 4*1,4142</p></td>

<td valign="top"><p align="right"> 5,6*1,4142</p></td>

<td valign="top"><p align="right"> 8*1,4142</p></td>

<td valign="top"><p align="right"> 11*1,4142</p></td>

<td valign="top"><p align="right"> 16*1,4142</p></td>

</tr>

<tr>

<td valign="top"><p align="right"><strong> f/1</strong></p></td>

<td valign="top"><p align="right"><strong> f/1,4</strong></p></td>

<td valign="top"><p align="right"><strong> f/2</strong></p></td>

<td valign="top"><p align="right"><strong> f/2,8</strong></p></td>

<td valign="top"><p align="right"><strong> f/4</strong></p></td>

<td valign="top"><p align="right"><strong> f/5,6</strong></p></td>

<td valign="top"><p align="right"><strong> f/8</strong></p></td>

<td valign="top"><p align="right"><strong> f/11</strong></p></td>

<td valign="top"><p align="right"><strong> f/16</strong></p></td>

<td valign="top"><p align="right"><strong> f/22</strong></p></td>

</tr>

</table>

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<b>Shutter speed:</b>

<br><br>

 

<table border="1" cellpadding="2" cellspacing="0">

<tr>

<td valign="top"><p align="right"><strong> 0EV</strong></p></td>

<td valign="top"><p align="right"><strong> -1EV</strong></p></td>

<td valign="top"><p align="right"><strong> -2EV</strong></p></td>

<td valign="top"><p align="right"><strong> -3EV</strong></p></td>

<td valign="top"><p align="right"><strong> -4EV</strong></p></td>

<td valign="top"><p align="right"><strong> -5EV</strong></p></td>

<td valign="top"><p align="right"><strong> -6EV</strong></p></td>

<td valign="top"><p align="right"><strong> -7EV</strong></p></td>

<td valign="top"><p align="right"><strong> -8EV</strong></p></td>

<td valign="top"><p align="right"><strong> -9EV</strong></p></td>

</tr>

<tr>

<td valign="top"><p align="right"><strong> 1</strong></p></td>

<td valign="top"><p align="right"><strong> 1/2</strong></p></td>

<td valign="top"><p align="right"><strong> 1/4</strong></p></td>

<td valign="top"><p align="right"><strong> 1/8</strong></p></td>

<td valign="top"><p align="right"><strong> 1/15</strong></p></td>

<td valign="top"><p align="right"><strong> 1/30</strong></p></td>

<td valign="top"><p align="right"><strong> 1/60</strong></p></td>

<td valign="top"><p align="right"><strong> 1/125</strong></p></td>

<td valign="top"><p align="right"><strong> 1/250</strong></p></td>

<td valign="top"><p align="right"><strong> 1/500</strong></p></td>

</tr>

</table>

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BTW - 0EV is the quantity of light exposed with aperture f/1 during 1 second with ISO 100.

 

This font is weird - all the EV numbers in shutter speed table above are minus values.

 

Difference between ISO 100 and ISO 200 = 1 stop = 1.41x

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<i>BTW - 0EV is the quantity of light exposed with aperture f/1 during 1 second with ISO 100.</i>

<p>

Actually, it's not. EV's just indicate a combination of shutter speed and aperture. ISO doesn't have anything to do with it, and it's not a measure of light levels. People commonly combine it with ISO 100 (sometimes implied) as a measure of light, but on its own it is technically incorrect to do so.

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0 EV (zero EV) = quantity of <b>registered</b> light on ISO 100 film while=((f/1), t=1 second)

<br>

or

<br>

0 EV = quantity of light coming through the lens with aperture (f/1) during 1 second.

 

<br><br>

After all, you're right - ISO information is unnecessary :)

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If your question: >>>exposure ratios between aperture, shutter speed and ISO<<<, is asking for the relationship between aperture / shutter speed / ISO then (forgetting issues such as: exact maths, precise apertures, some [older] f-stop/shutter speed intermediates and reciprocity etc) simply put:

 

 

1. ISO, Shutter Speed and Aperture can be thought of, and are often referred to in `whole stop equivalents`.

 

 

2. There are intermediate values for all three parameters.

 

 

Usually the intermediates are referred to as `half stops` or `third stops`: but quarter stops are used, sometimes in the darkroom and with cine equipment.

 

 

ISO Film Speed, is a standard, in one third stop increments.

 

 

Shutter speeds, once in whole stops only, are now available on some cameras in third and half stop values. (see below).

 

 

3. There is general acceptance for the progressions listed below.

 

 

Their can be slight numerical differences, between lens / camera manufactures in representing half and one third stop increments especially at large apertures (F1.2 is a common example) and representing the longer shutter speeds (1/24 sec and 1/25 sec a common example).

 

 

The progressions below ignore disciplined and exact maths:

 

 

3a ISO one third stop progression:

 

 

[25], 32, 40, [50], 64, 80, [100], 125, 160, [200], 250, 320, [400], 500, 640, [800], 1000, 1250, [1600], 2000, 2500, [3200], 4000, 5000, [6400].

 

 

3b ISO half stop progression do not really exist, because ISO, the term applied now to represent `film` speed, and thus applied to DSLR to represent `sensitivity`, has its roots way back. The roots of film speed (ASA, DIN, GOST etc) were mostly always termed in one third stop progressions.

 

 

However flim users (still) will refer to `pushing` or `pulling` a film in half stop increments, which if required a numerical representation, would be like this:

 

 

[25], 37, [50], 75, [100], 150, [200], 300, [400], 600, [800], 1200, [1600], 2400, [3200], 4800, [6400].

 

 

4a Aperture (f-stops) in one third stop progression:

 

 

[1.0], 1.1, 1.3, [1.4], 1.6, 1.8, [2.0], 2.2, 2.5, [2.8], 3.2, 3.6, [4.0], 4.5, 5.0 [5.6], 6, 7, [8], 9, 10, [11], 12, 14, [16], 18, 20, [22].

 

 

4b Aperture (f-stops) in one half stop progression:

 

 

[1.0], 1.2, [1.4], 1.7, [2.0], 2.4, [2.8], 3.5, [4.0], 4.8, [5.6], 6.7, [8], 9.5, [11], 13, [16], 19, [22]

 

 

5a Shutter Speed one third stop progression (seconds):

 

 

Like ISO, shutter speeds have their roots in film days (and lens caps and later clockworks), and were predicated on `one second` as their base. Thus standard `one stop` increments are now:

 

 

[1], [1/2], [1/4], [1/8], [1/15], [1/30], [1/60], [1/125], [1/ 250], [1/500], [1/1000], [1/2000], [1/4000].

 

 

Technology has allowed the following third stop increments:

 

 

[1/8],1/10, 1/13 [1/15], 1/20, 1/25, [1/30], 1/40, 1/50, [1/60], 1/80, 1/100, [1/125], 1/160, 1/200, [1/ 250], 1/320, 1/400 [1/500], 1/640, 1/800 [1/1000], 1/1250, 1/1600 [1/2000], 1/2500, 1/3200 [1/4000]

 

 

5b And technology has allowed the following one half stop increments:

 

 

[1/8], 1/12, [1/15], 1/24, [1/30], 1/45, [1/60], 1/90, [1/125], 1/180, [1/ 250], 1/380 [1/500], 1/750 [1/1000], 1/1500 [1/2000], 1/3000 [1/4000]

 

 

6. For any correct exposure COMBINATION of all three parameters, we can manipulate any TWO parameters at a time, by moving STOP EQUIVALENT in one parameter and moving the OPPOSITE STOP EQUIVALENT in another parameter.

 

 

Example 1:`Let's say the meter tells me, at ISO 200, the correct exposure is 1/60 @ f/2.8.`

 

 

Moving only Shutter Speed and Aperture, keeping the ISO constant at ISO 200, moving to longer shutter speeds, other correct exposures: 1/30 @ F4; 1/15 @ F 5.6; and 1/8 @ F 8.

 

 

Moving to shorter shutter speeds: 1/125 @ F2; and 1/250 @ F1.4; and 1/500 @ F1 would be correct also

 

 

Example 2.

 

 

 

Also, moving the ISO and aperture but keeping the shutter speed constant at 1/60 the following would be correct:

 

 

ISO 100 1/60 @ F 2; ISO 50 1/60 @ F1.4 and ISO 25 1/60 @ F 1.0

 

 

Moving the other way: ISO 400 1/60 @ F4; ISO 800 1/60 @ F5.6 and ISO 1600 1/60 @ F8.

 

 

Example 3.

 

 

Keeping the aperture constant and moving the ISO and shutter speed:

 

 

At F2.8 the following would be correct: ISO 100 and 1/15s; ISO 50 and 1/8s; ISO 25 and 1/4s and also, ISO 400 and 1/125s; ISO 800 and 1/250s; ISO 1600 and 1/500s; ISO 3200 and 1/1000s

 

 

Example 4 (your question). The meter tells me, at ISO 200, the correct exposure is 1/60 @ f/2.8. I have a lens which only gives me a max aperture of f/4.5.

 

 

Answer: from the tables above, F4.5 is one and one third stops slower than F2.8, so therefore keeping ISO200, referring the third stop shutter speeds above, you will need to let in one and one third stops more light, i.e. have a shutter speed of 1/25 s (if you camera allows), if not 1/30 is not that far off the mark.

 

 

But if that is too slow to hand hold: then you could make your `film` one and one third stops more sensitive. And you would get ISO 500 1/60 @ F4.5.

 

 

But your camera might not have ISO500 capacity, so you could correct to ISO400 for the one stop you need and open the shutter for the extra third you still require: Hence ISO400 1/50 @F4.5.

 

 

Obviously ISO800 1/100 @ F4.5 will be correct as would be ISO800 1/80 @ F5.0 and also ISO800 1/60 @ F5.6.

 

 

There are many more combinations, the procedure is the same.

 

 

The question also requires deciding what is best to sacrifice and what is necessary to prioritize: in the example above which is better (or worse) to shoot wide open at F4.5 or to increase the ISO and shoot at F5.6 at ISO800, and both keeping in mind what is the limit of the photographer`s capacity to hand hold at slow shutter speeds and still shoot a good keep record.

 

 

Whilst I have had to teach EV in theory classes, I personally believe EV is of little practical value in the field.

 

 

Photographers speak, in common parlance here, mostly in `stops`, as outlined above.

 

 

An example is I would say it this way: `two point eight at a sixtieth at two hundred ISO` or `two point eight at sixty at two hundred ISO`. Note that normally the ISO usually is referred to last.

 

 

Or, when reading a light meter I would say to you: `I have two point eight at sixty, 200ISO`.

 

 

With your new lens I would say to you: `You are now using a four point five lens. You need to bump the ISO to 800 and shoot five point six at sixty, I think it best to stop the lens down a third, and not shoot wider open, but are you steady at sixty without a pod? Or do you need a faster shutter, can it (the camera) handle 1600ISO if you need to shoot at one twenty five?`

 

(I have only proof read the above numbers once, I believe they are all correct!)

 

WW

 

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Wow, William, that is the most detailed answer I have ever seen. I need to re-read it again, just to make sure, I got it all. The whole EV mentioned above, didn't seem to make sense, in relation to my question.

I know "basic" exposure values, and roughly understand the concepts of whole stops on apertures and shutter speeds. I appreciate the details on 1/3 and 1/2 stops.

 

"You need to bump the ISO to 800 and shoot five point six at sixty". So, in theory, if this is gives the correct exposure, (ISO 800, 5.6 @1/60). Then, I could also bump the ISO to 400 5.6 @ 1/30; or ISO 400 4.5 @ 1/60; or ISO 1600 5.6 @ 1/125. If I understood correctly, all these are equivalent exposures, right?

 

The Canon Rebel XT, manages noise relatively well @ ISO 800, though, I would probably prefer to use ISO 400. I can handhold at 1/60, probably 1/30, but 1/30 may blur if the subject is moving around. Perhaps, the Tamron 17-50 2.8 would be more suitable for me, even though it is lacking in the zoom range (50-70mm, compared to the Sigma 17-70).

 

I just need to figure out which is more important for me, the 2.8 or extra 50-70mm.

 

Thanks,

 

Amol

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>>> ISO 800, 5.6 @1/60 If I understood correctly, all these are equivalent exposures, right? <<<

 

Almost:

 

 

ISO 400 5.6 @ 1/30; YES

 

 

ISO 400 4.5 @ 1/60; WRONG

 

(you have not moved the aperture a full stop)

 

ISO 400 F4 @ 1/60 is correct, but you do not have F4 on that lens, remember that is the problem. And also how sharp is the lens wide open: hence my first post I said ` I think it best to stop the lens down a third, and not shoot wider open,?`

 

 

ISO 1600 5.6 @ 1/125; YES

 

 

Re read it again, but you I am sure you have the guts of it.

 

RE this new question:

 

>>> I just need to figure out which is more important for me, the 2.8 or extra 50-70mm. <<<

 

 

Well we all have our own opinions here, but I figure it like this.

 

Leaving aside `lens quality` and the technicalities like the compression, which a portrait image will have when shot with a 70mm lens compared to a 50mm lens.

 

Ask yourself: In most circumstances, how easy will it be to walk a short distance closer to the subject to better frame it with a 50mm lens, than if I had used 70mm lens.

 

My answer: I almost always could get closer.

 

Then ask yourself: If I want a shutter speed fast enough to freeze action. Or if I need to use a fast enough shutter speed to ensure I do not get camera shake. . .

 

And the sun is setting, quickly, and I am inside, and it is a once in a lifetime portrait of a most beautiful person lit with available light streaming through the window diffused by shear white curtains.

 

I do not want to spoil the scene with flash.

 

I have run out of suitable ISO, I want no more noise or grain.

 

Will I be able to ask the sun to hang around for a few more minutes so I can just shoot a few more frames (pardon, passionately I regress to film talk) shoot off a few more `images`?

 

My answer: I have never been able to manufacture more natural light than what is available to me.

 

All else equal, I have always bought the fastest lenses I could afford, and at times waited until I could afford them.

 

There are many other reasons why one would prefer a faster lens, but the above, in my opinion is the cornerstone on which all the other reasons are built, and it is the crux of lens designers` reason for being, since lenses were invented; the desire to manufacture faster lenses.

 

In the field one just CANNOT harness more available / ambient light than what there is to harness.

 

Have a great life.

 

WW

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William, you are definitely a great teacher. And very eloquent with your words. So, I take it, from your response, you would get the Tamron 17-50 f/2.8 over the Sigma 17-70 f/2.8-4.5?

 

In the $300-450, are there any other lenses I should be looking at?

 

The Canon 17-55 IS f/2.8 looks very nice, but alas, cost about $900-1000. (Even if I could save money, I doubt I could justify such a purchase. I only do photography as a hobby, therefore, I do not make money with it. If I was doing it for income, then I would probably get the "best" (Canon 17-55 2.8)

 

I'm also wondering what the reviews of the Tokina 16-50 f/2.8 will be like.

 

 

Thanks again,

 

Amol Kumar

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Thank you for the compliment.

 

 

Now recommending is a different kettle of fish: the decision is yours.

 

 

All else being equal, I would always opt for a faster lens. And in a zoom I would always opt for a fixed aperture zoom (i.e. the same aperture throughout the zoom range).

 

 

But I have NOT used any of the lenses you mention. It would be both morally and professional wrong for me to comment on them as a recommendation to buy.

 

 

However I will comment on the point you make about `justifying the purpose`: it is very important.

 

 

By way of an example when I changed my 35mm gear to Digital I bought a Canon 20D.

 

 

I knew I wanted to develop a digital camera SYSTEM.

 

 

As part of that system, I knew, from experience, I wanted a fast portrait lens.

 

 

I knew I did NOT want to use the lenses I owned (a different manufacturer) with EOS adaptors.

 

 

I was fairly confident I did not want to step out of the EOS range of lenses, but I remain open to something, value for money, if it comes along.

 

 

The first lens I collected was the `kit lens` 18 to 55mm: it cost me US$45 more than the body alone. IMO, good value for money and a usable lens if one understands its limitations.

 

 

Getting back to the portrait lens, I had a few choices which I narrowed to primes only: 50mm, 85mm 100mm and 135mm. I deleted the 100mm, and concentrated on 50 and 85mm only (the 135mm I have decided upon, I am saving for it.).

 

 

At 50mm I had three choices 50mmF1.0 (used), 50mmF1.4, 50mmF1.8MkII.

 

 

I used the 50mmF1.0 and it was heavy and slow to AF and it was large and it was wonderful and it was like a beautiful woman seducing me: and I wanted it dearly. But I bought the 50mmF1.4 and kept the spare AUS$1500 in my wallet. The bottom line was I could not reconcile the extra money.

 

 

A similar story with the 85mm: I bought the 85mmF1.8. I could not reconcile the difference in money for the F1.4L version.

 

 

Note in the above examples we are talking about very little aperture differences.

 

 

However, when it came to choosing a telephoto zoom, there was no comparison in my mind the 70 to 200 F2.8 is worth the extra compared to the 70 to 200 F4.

 

 

One whole stop and the flexibility it gives me is worth the extra cash.

 

 

And further thinking about all the `arrangements` of my `system`; and I would rather use the 70 to 200 with an x1.4 teleconverter (making it a fixed aperture F4 zoom) than buy the 100 to 400mmL zoom, with variable aperture. So I got the teleconverter too. (but I am saving for the 300mm F2.8).

 

 

Do not be overawed with the money outlay: be it large or small the logic, IMO, is the same. We each want to spend what we want to spend, and we each must develop our own rules for our system.

 

 

For example, it doesn`t matter to me how good the EF-S 17 to 55ISF2.8 lens is I would never buy one, because as part of my system I want two bodies, one APS-C sensor and one Full Frame 35mm (like a 5D) and I want full lens compatibility between the two. (I know the kit lens does not conform, but hey I bent the rules for a few dollars!)

 

 

I suggest you read other threads here at Photonet, there are many about lens choice and many about these lenses you mention.

 

 

You will get a feel for who`s opinions you appreciate: you will find I push a research, camera SYSTEM and THINKING AHEAD attitude: others, whose opinion I respect say: research a bit, buy it, try it, if you do not like it sell it on Ebay, because you will not loose all that much. And there are others who only buy second hand at a bargain, but to be successful, they need to know what a bargain is.

 

 

But, I have never really come up against any convincing argument to NOT consider the system one wants to end with: and how all the parts of that system relate and work for what one wants to achieve.

 

 

Hope that helps your decision.

 

 

WW

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