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04-27-2010, 08:56 AM   #1
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Understand apeture

Ok, I understand the basics of apeture. Wide open, low numbers give you depth of field and more light. Big numbers give you focus throughout the scene and increasingly less light....

In my experience once you get past, oh I don't know, fstop 8 or so you're not really getting any of the depth of field effect. And people talk about 8 being the 'middle' of the aperture range.

So what I'm wondering is how do you distinguish when to use the larger fstops. If I'm not going for a depth of field effect do I use 8, 14, 16, 22???

In general I go for the lowest number - widest fstop when shoot DOF. But when shooting say a landscape do I just reverse that and go for the biggest fstop? I know this isn't the way to go as I hear the largest fstops give some bad lens effects or something so generally I just go with 16 without really understanding why.

Thanks for any clarification of this...

Ken

04-27-2010, 09:07 AM - 1 Like   #2
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Let's pretend for this that we are talking about a film camera where ASA (ISO) cannot easily be changed with the turn of a dial and that it is fixed at ISO 100.

If you are actually thinking about it (I suspect most people don't), Change your aperture for 2 basic reasons. First, you want to use a different shutter speed. Second, you want to control what in the photo appears focused. DOF is a term that states how much in front of and in back of the focus point is in reasonable focus. The higher the f number (smaller aperture), the greater the DOF. The lower the f number (larger aperture), the LESS the DOF will be.

You can actually measure, quantitatively, How much distance will be in focus at a given aperture and focus distance. The scale is on most lenses. It gives you both distance as well as the bracket for the aperture stops.

If you're looking at taking a shot where you want a given shutter speed (to stop or blur action for instance), you would adjust your Aperture for that.

Pretty simple really, just decide what you want your photo to look like and then adjust your aperture and shutter speed to make it happen.

04-27-2010, 09:32 AM   #3
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Learning to control your depth of field is a part of the creative process. If you have a scene where you want to have a LIMITED depth of field, and isolate an item from the back ground by blurring the back ground, you might want to use a low number F stop, like 2.8 or 4 where you will have a shallow depth of field, or narrow range of things in focus. If you want to include many things in your composition, like in a landscape, and want a greater depth of field you might choose a higher number F stop like F 11, F16, or F22.

That is as basic as it gets. Other things come into play, hyper-focal distance for one, focal length of the lens, distance from the subject, these too are factors in choosing your aperture or F stop to create the image you want to make, even the type of lens or the number of aperture blades.....or the natural bokeh of the lens.

Two things that will perhaps give you a better understanding of aperture, and how depth of field works, are a good on line Depth of Field Calculator and a Hyper Focal Distance chart.

Here is an article you might find interesting

Understanding the Hyperfocal Distance

and here is an on line depth of field calculator

Online Depth of Field Calculator

While all of this might sound daunting at first, it really is the basis for creative photography. It gets easier with time, and use. But with that being said, it is just the beginning as well. You will soon find that there are other factors that you might want to take into consideration to create the images that you "see in your vision of the world" that you want to portray in your photographs.

Play with the on line depth of field calculator by plugging in different focal lengths and distances, different F stops. You will soon see how the changes affect what your photograph will look like once taken.
04-27-2010, 09:39 AM   #4
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I guess where I'm missing is out is on knowing when to use f11, f16, or f22? How do you make that choice?

04-27-2010, 10:24 AM   #5
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QuoteOriginally posted by Javaslinger Quote
I guess where I'm missing is out is on knowing when to use f11, f16, or f22? How do you make that choice?
Again, we are fixing ISO at 100 here...

You use f11, f16, or f22 when A: You don't care (or do) about how Shallow the DOF is, and B: When your shutter speed dictates it for proper exposure.

04-27-2010, 10:25 AM   #6
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By understanding what those aperture settings will do based on the lens you are using, the subject you are shooting, and the effect you want to portray in your photograph.

There is no hard and fast rule that says use this F stop if you want to do this. It's all variable.

Look at that depth of field calculator I posted the link to.

Say I am using my K 20 and my sigma 105 macro to shoot something that is 1 foot away from me.

At F 16, the Depth of Field is .01 foot

Now lets say I am using my sigma 10-20 and shooting a landscape with F 16 and I focus on something that is 20 feet away, the depth of field - with the same aperture of F16 is now from .98 feet to infinity.

There is NO hard and fast rule to tell you when to use a certain aperture. You have to understand what effect choosing an aperture will have on the image you want to make based on the focal length of the lens and the distance to the subject.

Now there are "rules" like the sunny f 16 rule, or the old adage F8 and be there, but in general, the aperture you choose, along with the shutter speed, and now with digital the ISO, will determine the final look of the image, based on your lens choice, subject and distance to the subject.
04-27-2010, 04:21 PM   #7
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QuoteOriginally posted by Javaslinger Quote
I guess where I'm missing is out is on knowing when to use f11, f16, or f22? How do you make that choice?
You use those when you need more DOF than f/8 can give. While experience will allow you to anticipate you when this will be the case, chimping can show you directly just as well. That is, take the shot at f/8, then zoom in on the LCD. If not enough of the scene looks in focus for your purposes (and only you know what your purposes are), then stop down more and try again - knowing that while DOF will increaee, overall sharpness will *decrease* as you pass f/11, due to diffraction (an optical phenomenon Google can presumably tell you all about if you're curious).

Instead of chimping, you can use the (optical) DOF preview so you see in the viewfinder what's going to be in focus - although since the viewfinder darkens, and doesn't allow you to zoom in, it's really only useful to get a rough idea.
04-27-2010, 04:51 PM   #8
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QuoteOriginally posted by Javaslinger Quote
I guess where I'm missing is out is on knowing when to use f11, f16, or f22? How do you make that choice?

Ken,

Generally speaking, I am working to get LESS depth of field, not more. So if I had to give up an f-stop or two on my camera, I would much rather give up f/22 or f/16 than give up f/2 or f/2.8. When I buy lenses, I almost never worry about the minimum aperture available; I'm always primarily interested in the max aperture.

In addition, you should be aware that, with most lenses, as you stop down to these very small apertures, at some point you start to encounter a technical problem known as diffraction. When sharpness matters most to us, we often want to shoot at the lens's sweet spot. This is never at the extreme ends of its aperture spectrum. A lens might not be at its best at f/2.8, so you might stop it down to f/5.6 or f/8 or smaller (depending on the lens). But by f/16, most lenses do start to be affected by diffraction.

That said, I have to admit that I use f/11 fairly regularly, when I have enough light to do so. I use f/16 seldom, and f/22 very rarely.

*

Why would you use a very small aperture like that? There are two basic reasons, well, two basic situations in which going to f/11 or f/16 makes good sense.

First, you might go there deliberately because you have some flexibility in your settings and really want more depth of field. Ansel Adams belonged to a famous group of large-format landscape (well, mostly landscape) photographers named Group F/64. Apparently f/64 was the smallest aperture available for their cameras, and they used it frequently because those big cameras gave them very little depth of field otherwise. Say you're using a 300mm lens from 5ft away to photograph a flower or insect: even at f/16, you'll have less than half an inch of depth of field! Or say you were trying to shoot a line of Marines standing at attention in a parade. You are looking at the line from the side. Say you've got a 40mm lens on your camera and you're about 20 ft from the first Marine in the line. If you want to narrow your depth of field as much as possible, then by all means, open up the aperture wide. But if you would like to see a line of nicely in-focus chins and nicely in-focus eyes straight forward, well, you would need to stop down to f/11 or f/16.

Second, if there's a ton of light and you can't control it, well, you may need to stop down the aperture simply in order to get a correct exposure. It doesn't happen very often. To understand why, consider the "sunny f/16" rule. This old rule of thumb for guesstimating exposure says that, if you're shooting in full, bright, midday sun—very bright natural conditions, in other words—and if your ISO = 100 and your shutter speed = 1/100th, then the correct aperture will be f/16. Now think about that for a sec. ISO 100 is a minimum on my cameras; I can't slow the camera down any further using ISO. But I don't usually shoot outdoors at 1/100th sec! Say I'm taking a portrait of a child. I'd start at 1/200th sec as a minimum shutter speed—one stop faster than the sunny f/16 rule. That would mean, in bright full sun, my aperture should be adjusted to f/11, one stop wider open, to offset the change in shutter speed. If I increased the shutter to 1/400th or 1/500th sec, I'd quickly have the aperture down into the range of f/8 or f/5.6. Ah, but if I was photographing a building using a tripod, and I was out in very bright light, then f/16 might be just the ticket.


Will

04-27-2010, 05:14 PM   #9
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There is a point when closing down the aperture (higher F-number) does not increase depth-of-field because you reach what is called the diffraction limit. As soon as you pass that limit, everything gets softer which hides the fact that you have more depth-of-field. In fact, it makes things worst.

On the Pentax K-7 for example it is F/13. It is a physical property related to the pixel and aperture size, there is nothing that can be done to bypass it. For a K10D it was F/16. That is why tiny (non-DSLR) cameras often stop at F/8, their pixels are extremely small.

The only time you should go beyond the diffreaction limit is if you know you won't print very big and you want to get a slower shutter-speed for creative effect. In general, it is better to just put in a good quality ND filter in that case.

- Itai
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04-27-2010, 05:37 PM   #10
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QuoteOriginally posted by Javaslinger Quote
Ok, I understand the basics of apeture.
Except, apparently, the spelling.

woof!
04-28-2010, 12:03 PM   #11
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QuoteOriginally posted by Itai Quote
There is a point when closing down the aperture (higher F-number) does not increase depth-of-field because you reach what is called the diffraction limit. As soon as you pass that limit, everything gets softer which hides the fact that you have more depth-of-field. In fact, it makes things worst.
While it's true that once you past the point of diffraction, the overall IQ gets worse, that does *not* completely hide the fact that you *do* in fact have more DOF. The in focus areas at f/22 won't be as sharp as the in focus areas at f/8, but if a given part of the picture is in focus at f/22 and not at f/8, that object *will* be sharper at f/22 than at f/8, diffraction or no diffraction. At least that's been my experience with my lenses (while the pixel size does indeed have implications for diffraction, the lens itself plays a role to in terms of how sharp things appear at different apertures).
04-28-2010, 01:25 PM   #12
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Marc & Will's posts basically sum it up - there's a balance between desirable depth of field and undesirable image softness from diffraction at high apertures that must be considered.

Nevertheless, I have been personally impressed at my f/22-f/40 results (f/40 from the kit lens at 55mm) as the apparent image softness is quite hard to pick out. So IMO, if you need the extra DoF, don't hesitate to stop down, using hyperfocal distance and if necessary DoF calculators to your advantage.
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