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Aperture

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Aperture and f stops

I know what aperture is and it took some time to find out what it means. It is easy to find out what it does. So, up front, the definition of Aperture:

A ratio between the distance from the front of the lens to the sensor and the diameter of the opening of the lens.

But guess what? If you hate math and words like ratio and diameter (I know many who do) the above definition is unimportant. You don’t need to know it! Oh, but there are many things you do need to know.

First, when talking to photographers, the smaller the number the bigger the aperture. An f2 is bigger then f8. The biggest aperture is an f1 (okay, there have been bigger but right now no one makes one). With an f1 lens it is bigger across then the length of the lens!

Also, the bigger the aperture the faster and brighter the lens. If someone is talking about a fast lens then they are talking about an: f2.8, f2, f1.8, f1.4, or f1.2. All this means is that you can shoot at faster shutter speeds with these lenses.

Another thing, when someone says to step a lens down a stop or two they are telling you to make the aperture smaller by one or two f-stops. Hey, that is something new. The Aperture number is generally called the f-stop or f “number that you are using”. So, if you have your lens at f5.6 then you would say the f-stop is at 5.6. When it comes to stepping down a stop, you are talking about making your aperture smaller one full stop, say from f4 to f5.6. Okay, we have another new one, Full Stops. Todays lenses can be set to half and one third stops, but most of the time we talk in Full Stops. Here is a list of full  stops.

f1, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22, f32, f45, f64
and it goes on, but more then likely you’ll never see smaller then f32

All the above is for talking about aperture. Lets get on to using it. The bigger the aperture, the more light come to the sensor. f4 lets in twice the light of f5.6, f2.8 is twice as bright as f4.  Using a big aperture allows you to use lower ISOs (less noise) and faster shutter speeds (less blur). So always use the biggest aperture, right? No.

First: Quality

Most lenses are at their best one or two stops down from max. I have a 50mm 1.8 and it is much sharper with better color at f2.8 and f4. This is pretty common. I guess it is hard to make a lens very sharp at it’s max aperture.

Second: DOF

Depth Of Field. This is the amount of space in focus behind and in front of your focal point. It would be easier to show you then describe this, check below

f1.8

f5.6

f11

Third: Shutter Speed

Sometimes you want a slower shutter speed. Why, water falls and maintain streams are a good example. If you want that wispy silky smooth water you need a slow shutter speed, like 1 or 2 seconds! Tripods help too.

Slow Shutter Speed 1/3 sec at f8

Okay, there are three reasons to step down. But don’t go down too far. There is this thing called diffusion. It happens because of the size of the pixels on your sensor. As mega pixels get bigger the pixels become smaller and diffusion becomes more of a problem. What is diffusion? Simply put, blurriness. After a certain point, stepping down the aperture will cause the lens to become less sharp.

With my camera, a 12.2 Mega pixel camera, I start seeing it around f14. I hear the 17+ Mega pixels have issues at f11, some say even at f8. It is easy to find out. Just pick a subject with some detail and good light. Set you ISO high enough so there will be no camera shake at you smallest aperture. Place your camera in Aperture Priority mode (AV on Canon, A on Nikon and Sony) and take some pictures at different apertures. Here are some of my samples

f1.8

f4

f8

f14

f22

Notice that at f1.8 it is not that sharp but f4 and f8 are very sharp. f14 is pretty sharp and f22 is not. In fact I would say that f22 and f1.8 have about the same sharpness even though at f22 everything is in focus and at f1.8 only a small section is.

BTW, if you buy at Full Frame camera like the Canon 5D’s  and 1Ds’s or Nikon d700 and d3’s, then diffraction is less of a problem seeing that they have bigger sensors.

I think that about covers it. If anything is unclear, let me know and I will try and fix it. Ask me questions and I will try and answer.

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Written by photogchris

May 11, 2010 at 4:23 pm

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