Probably a silly question

[marius]

I'm new in photography and have two question:

What is the difference between a a small F (let's say F4.0) and and fast speed VS a big F (let's say F12.0) and a slow speed? I took today 2 photos and they seems identical.

What is usually the F for landscapes when the sun is UP? I read about f11 or f15...is there a rule?

Thanks.

[MJB DIGITAL]

It's not a stupid question.

The only stupid questions are just silly (clowning) or questions you already know the answer to.

But....
You might want to grab a book or two about photography.

and to answer your question:

the f/stop number is a value related to how wide your aperture is.
a low f/stop number is a wide aperture
a high f/stop number is a narrow aperture.

consider your eyeball.
when there is a lot of light, your pupil closes down. that is like being at f/22
and in the dark, your pupils open up. that is like f/2.8 (i would say 1.4 but i dont own a lens that fast. ha)

so now consider what you know:

f/2.8 is a wide aperture. therefore much more light will enter through it. with so much light, you will need a faster shutter eh?

now go and study 'depth of field'

8)

have fun

[flyer]

There is no "set" rule as far a f stop is concerned. It all depends upon what you are looking for. A large f number (small opening) will have a larger zone of apparent sharpness than a small f number (large opening). So, if you want to exclude foreground, you'll focus on the background and select a large opening (small f number). Samething to exclude the background in a portrait. But if you want as much as possible sharp, you hold your lens off "infinity" ( the sideway 8) a little bit and select a small opening (large f number), so most of your picture will "look" sharp". Try to experiment a little bit with a subject that is at a fixed distance from the camera and has some background separation. Focus on the subject and change only the f number (camera set on Av and auto ISO), and examine the background closely to see the difference.

[rfortson]

For any picture to be properly exposed, it needs a certain amount of light (photons) to hit the sensor or film. Your camera is the bucket that holds the light, and the lens catches the light.

Now, the amount of light stays pretty much the same for any given scene, so if you have a small aperture (opening) in the lens, designated by a large f-stop, then you'll need to leave the shutter open for a longer period of time to reach the required amount of light. Therefore the shutter speed is longer (for example, 1/15 sec, instead of 1/60 sec or faster). If you open up the lens aperture (smaller f number), then the shutter doesn't need to stay open as long (now it's 1/120 second or something faster). If the lens opening is larger, then the shutter speed is faster. If the lens opening is smaller, then the shutter speed is slower.

I'd suggest "Understanding Exposure" by Bryan Peterson. Great explanation for how aperture, shutter speed, and ISO (I didn't even mention that) interact to create a proper exposure (and there are many ways to get a proper exposure, all providing different looks).

[noblepa]

First of all, I second the recommendation to read Bryan Peterson's book, Understanding Exposure. He does a marvelous job of explaining this. I've been into photography for 40 years and I thought I understood exposure, but I learned a lot from his book.

Marius, you are absolutely correct. Both your shots are probably correctly exposed. This ability to use different combinations of f/stops and shutters speeds to obtain equivalent exposures is called reciprocity, and is one of the fundamentals of understanding exposure.

However, shutter speed and aperture control more than just the amount of light that reaches the sensor (or film). For example, shutter speed controls the camera's ability to stop motion in the subject. Obviously, a shutter speed of 1/1000 second will freeze most motion, while at 1/10 second, the motion may be nothing but a blur. For creative purposes, you may want to have some degree of subject motion in the shot, or you may want to freeze the splash caused by a water droplet falling into water.

Likewise, the aperture you choose affects more than the exposure. No lens is equally sharp at all the apertures it is capable of. Most lenses are sharpest at their middle apertures, such as f/5.6 to f/11. Every lens has a "sweet spot", or aperture at which it is generally sharpest. Unless you know someone who has the same lens and can tell you, the only way to find out is trial and error and closely examining a lot of photographs.

Aperture also controls Depth Of Field. When a lens focuses on a certain spot, that is called the critical focus distance. Only objects at that distances are truly in focus. However, objects slightly in front of and behind the critical focus distance will also be in acceptable focus. This is depth of field (DOF). For a given lens, the smaller the aperture (higher f-number), the larger this range of acceptable focus will be. If you are shooting a landscape and you want as much of the scene as possible to be as sharp as possible, you choose a small aperture. If you are shooting a portrait and you want your subject's face to be in focus, but you want the background to be a blur, so as to direct the viewer's eye to the face, you choose a large aperture.

This is obviously a greatly oversimplified explanation. Read the book. There are many photography books that will explain this more completely than I ever could.

Try an experiment that Peterson did in his book. Have a friend stand outside, with a scenic landscape in the background. Focus on your friend's nose. Try to have some objects in the foreground as well. That is, in front of your friend. Now, preferably with your camera on a tripod, shoot the scene with as many combinations of aperture and shutter speed as you can. Start with a slow shutter speed/small aperture and work your way to a fast shutter speed/larger aperture. Now, examine the results closely.

The difference in DOF should be obvious. All the pix will be properly exposed. All the pix will be focused on his/her nose. The pictures will all be different. Comparing the first to the last picture should make the differences perfectly clear. In a case like this, a shallow depth of field may be best, as it will tend to draw the viewer's attention to the subject, rather than being distracted by the foreground or background.

You could do a similar experiment to demonstrate the motion-stopping ability of different shutter speeds. This time, though, have your friend run through the picture from side to side. Try to trip the shutter when he/she is in the center of the frame. At slow shutter speeds, they will be a blur. At high shutter speeds, they will appear motionless, perhaps in mid-stride with neither foot touching the ground.

[MJB DIGITAL]

Hell yeah baby!

*applause for nobelpa*