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04-01-2012, 06:18 PM   #1
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f stop on a zoom?

Hello everyone I am a little confused I have been reading alot and people always say they have to stop down to get better sharpness from a zoom. Does this mean I should lower the F stop number or increase it?

thanks
Corey

04-01-2012, 06:24 PM   #2
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Its not just for a zoom, but for any lens - prime or zoom.

When the aperture is wide open, the f stop has a low number. By making the aperture physically smaller, by increasing the f stop number. Generally, most lenses are sharpest at around f8.

04-01-2012, 06:41 PM   #3
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Bt realistically, we are taling abou tiny dfferences you'd never see unless you are blowing up your images to 100% on your monitor. You should not let this concern outweigh the *much* more important cmsiderations choosing an f-stop: what kind of DOF do you want, and how fast a shutter speed do you need. Making your your f-stop decision based solely on the small differecnes in sharpness and ignoring the real reasons for having different f-stops vailable in the first place is like choosing your driving speed based soloely on what gets you the best gas mileage, with no concern over the speed limit, road conditions, etc.
04-01-2012, 07:29 PM   #4
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Ok thanks for clearing that up for me.

04-01-2012, 07:57 PM   #5
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Intro to apertures (aka F-STOPS FOR DUMMIES):

An aperture is a hole through which light passes. A camera lens aperture is almost always formed by iris blades that open and close. An aperture setting or f-stop is a ratio, a fraction, the ratio of the iris opening to the focal length. So if the focal length is 100mm and the iris opening diameter is 50mm, then the ratio is 50:100 or 1:2 and is called f/2. If the opening on that lens was 25mm across, the aperture would be 25:100 or f/4.

Just like 1/2 is a bigger quantity (fraction) than 1/4, so an f/2 aperture is bigger (and lets in more light) than an f/4 aperture. The smaller the f-stop number, the bigger the hole. We call that WIDER or FASTER. The bigger the f-stop number, the smaller the hole. We call that TIGHTER.

A lens aperture that is set to the largest possible iris opening is said to be WIDE OPEN. If we use a smaller opening, with a larger f-stop number (but smaller fraction), we say that we have STOPPED-DOWN the lens.

A wide-open aperture passes more light, allowing a faster shutter speed. But there's something called DOF (depth of field), which is the range of distances from a lens that are acceptably sharp. When a lens is wide-open, that DOF may be very thin -- only subjects within a very narrow range will seem sharp. Stopping-down the lens makes the DOF thicker, so much more of a scene is sharp.

We stop-down a lens both to control the shutter speed, and to control the DOF. If we want more of a picture to be sharp, we stop-down the lens. But we don't want to stop-down too far. There's something called DIFFRACTION, which is a blurring caused when light hits the edges of the aperture. The tighter (smaller) the aperture, the greater the diffraction.

So if the aperture is too wide or too tight, we get unsharp pictures. What a pickle! How do we balance those? With our dSLRs, the absolute diffraction limit (the point where diffraction starts) is around f/8. But that only matters if the camera is immobilized (like on a tripod) and we're peeping every pixel like anal dorks. In practice, we can stop-down to at least f/16 without diffraction being a bother.

How to use this? A wide-open lens, especially a fast one (with a large maximum aperture like f/1.4) will seem somewhat soft. If we're capturing action or if we want thin DOF, that doesn't matter so much. If we want a sharper picture, we may stop-down the lens to f/5.6 or f/8 or f/11. Adjusting the aperture gives us great control of how a picture looks.

Then there are funny details like the sequence of f-stop numbers: 1--1.4--2--2.8--4--5.6--8--11--16--22--32--etc. And Waterhouse stops. And why they're called f-stops. And why we count the number of iris blades. And the price of faster f-stops. And why we prize zooms with fixed maximum apertures. Worry about that stuff another time. Hope this helps!

Last edited by RioRico; 04-01-2012 at 08:08 PM.
04-01-2012, 10:40 PM   #6
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I have always found the term stopping down a little bit out there. Now if I have a fast lens at 1.7. If I decide to stop down. It could be anything between 2 to 22.
When people use the word stop down and dont give a reference value its difficult to understand what they are talking about.

And it becomes even more confusing when someone says I made three stops down, or two stops down.
04-02-2012, 07:07 AM   #7
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QuoteOriginally posted by Culture Quote
I have always found the term stopping down a little bit out there.
I think the term comes from lens apertures having click-stops or detents at each iris position. So it's pretty straightforward to go from "adjusting aperture down a couple stops" to "stopping-down a couple" to just "stopping-down". Then rather than indicate the other direction by saying "stopping-up a couple", we say "opening-up a couple stops" or just "opening-up".

And since these aperture adjustments affect exposure as well as DOF, we refer to shutter-speed or general exposure adjustments the same way. We tweak the EV or ISO by stops, etc. So a 'stop' is a doubling or halving of exposure. It's just how the jargon evolved in Anglish. I wonder how the jargon goes in other languages?

As for the f-stops themselves: We use 1--1.4--2--2.8--4--5.6--8--11--16--22--etc because iris openings are fairly round. To double the amount of light passed, we don't open the iris by twice its diameter, but by SQR[2] (the square root of two), a factor of 1.4. But this isn't always the case. I have an old Argus Cintar 50/3.5 lens that is marked 3.5--4.5--6--9--12--18, a rather different sequence.

Then we see half-stops and third-stops (1--1.2--1.4--1.7--2--2.2--2.4--2.8--3.5--etc) and a really clever digital iris might break it down even further: f/3.14159 @ 1/234 second. My mind boggles.
04-02-2012, 07:28 AM   #8
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QuoteOriginally posted by Culture Quote
I have always found the term stopping down a little bit out there. Now if I have a fast lens at 1.7. If I decide to stop down. It could be anything between 2 to 22.
When people use the word stop down and dont give a reference value its difficult to understand what they are talking about.

And it becomes even more confusing when someone says I made three stops down, or two stops down.
Here's an example (FA 31mm F1.9 Limited)



Disclaimer Some people have said that the FA 31mm LTD is very sharp wide open, so take this graph with a grain of salt. And do not use this graph to compare with any other graph.

You see how wide open, the measured resolution is low, but stopped down to F2.8, the resolution seems pretty good, and the at F5.6 the resolution is at it's max? So for this lens, stopping down to F5.6 is better than stopping down to F8. But practically anything between F2 and F16 is better than F1.9, so saying "stopped down is sharper" will remain true. The question is, are you asking for maximum sharpness, or acceptable sharpness.

Usually with really good lenses, it doesn't make too much of a difference. Shooting my Tamron 70-200 F2.8 wide open, it's already so sharp that I don't ever go to F4. If I want to really go sharp, then I just stop down to F8 and bang, razer sharp, and no CA.

That's another thing - shooting wider will allow for more chromatic aberrations. Same with vignetting.

And RioRico explained what F-stop means in terms of steps.

04-02-2012, 09:57 PM   #9
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What I meant was that if someone has a 1.7 lens and says I stopped down. Where is the reference point from which he or she stopped down.
You see for me I am learning everytime I read a post. So i want to know if he stopped down from 1.7 or from 2.8. And how many stops did he make? Did he really mean one stop or two stops even though they just say I stopped down.
04-03-2012, 03:12 AM   #10
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It depends on the context. Someone just saying to "stop down a lens" is ambiguous, it could mean a stop, it could mean 5. If that person is talking about avoiding shooting with the aperture wide open, it's safe to assume they meant 1-2 stops from its maximum aperture. That's generally enough to improve the sharpness of a lens that is soft wide open.

If someone said specifically, "stop down by 2 stops" and your maximum aperture was f./2, then you would click the aperture down to f./2.8 (one stop) and then again to f./4 (two stops from where you started).


Last edited by Philoslothical; 04-03-2012 at 03:28 AM.
04-03-2012, 07:08 AM   #11
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QuoteOriginally posted by Culture Quote
What I meant was that if someone has a 1.7 lens and says I stopped down. Where is the reference point from which he or she stopped down.
There is no reference point. It's all arbitrary. A parallel: When you're driving and you slow down or speed up, there is no fixed standard, no reference point, other than your speed before you changed speed.

QuoteQuote:
You see for me I am learning everytime I read a post. So i want to know if he stopped down from 1.7 or from 2.8. And how many stops did he make? Did he really mean one stop or two stops even though they just say I stopped down.
They meant whatever they wanted to mean. It's quite arbitrary. Some might say they stopped-down a little or a lot, just as a driver may speed-up a little or a lot. Some might give the actual numbers: I closed-down two stops to f/5.6, or I sped-up 50 klicks to 90kph. (But not in a school zone.)

As Phil said, we might *assume* that if someone said they stopped-down, they did so within a lens' "sweet zone", which is usually 1-2 stops from either end of the aperture dial. For an f/1.4 lens, that might be between f/4-f/8. If they stopped WAY down, that might be at the far end, maybe f/16. And if they stopped-down just a bit, it may be just one stop or less: for that f/1.4 lens, to f/1.7-f/2 or thereabouts. But it's arbitrary.
04-03-2012, 07:45 AM - 1 Like   #12
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QuoteOriginally posted by coreyhkh Quote
Hello everyone I am a little confused I have been reading alto and people always say they have to stop down to get better sharpness from a zoom. Does this mean I should lower the F stop number or increase it? Corey
Zoom lenses have a different optimum (highest sharpness) aperture at different focal lengths, especially if the wide-open f-stop increases as the focal length gets longer.

A very easy way to ensure you are using the optimum f-stop when shooting with a DA or FA lens is to use the Hyper Program mode (P) on the camera and set the Program Line to MTF priority. By default, the camera will set the lens aperture to its optimum value (lighting conditions permitting) as you zoom in or out and it will behave as in Av mode with the optimum f-stop selected. You can always switch instantly to Av or Tv mode with the front and back dials and return to P with the green button. This is particularly useful with long range zooms like the DA (L) 55-300mm.
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