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f/ stops at different focal lengths.

Adam Mueller , Oct 12, 2010; 10:30 a.m.

Can anyone offer me an introduction or quick tutorial to understanding and using focal lengths with diffe


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Adam Mueller , Oct 12, 2010; 10:33 a.m.

ie - does it matter what length the camera is focused at along with the aperture? Does it matter, does it provide focus problems or anything of this nature? Is there anything in particular I should understand in order to move forward.

Rob Bernhard , Oct 12, 2010; 10:36 a.m.

Your question is sufficently broad and vague enough to not really be answerable. Perhaps if you had a specific example of the types of problems you're running into?

Otherwise, the only thing stopping you from experimenting and practicing with your camera would be you! :) Set yourself an assignment that focuses (no pun intended) on seeing what happens when you change focal length without changing position.

William Clark , Oct 12, 2010; 11:22 a.m.

Here is some information on f stops:


It's expressed in mathematical terms. The way I understand f stops is it's a function of 2 power with the resulting number used as a square root which determines f stop:
2 to the zero power = 1. the square root of 1 = 1
2 to the first power = 2. the square root of 2 =1.4
2 to the second = 4. the square root of 4=2
2 to the third power = 8. the square root of 8 = 2.8
And so on.
F numbers in between, examples are given in the article from wiki.
Differing focal lengths will have an opening based on lens diameter. So f 2.0 on a 50mm lens will have an opening different than an f 2.0 on a 135mm lens.
Quote from wiki, "each aperture has half the light gathering area of the previous one. The actual size of the aperture will depend on the focal length of the lens.."

Hope this helps you.

Charles Beddoe , Oct 12, 2010; 11:28 a.m.

Besides exposure, the main relationship between f-stop, focal length, and focusing distance is depth-of-field. You should find a book on exposure or start Googling.

James Dainis , Oct 12, 2010; 11:29 a.m.

What you are concerned with is depth of field (DOF), the portion of a scene that appears acceptably sharp in the image. With large aperture openings (f/1.4, f/2, f/2.8) the depth of field is small. With small aperture openings (f/11, f/16, f/22) the depth of field is greater.

With short focal lengths (18mm, 24,mm, 50mm) the depth of field is greater. With long focal lengths (100mm, 200mm, 400mm) the depth of field is very narrow.

The closer a subject is the narrower the depth of field, The further away the subject is the greater the depth of field.

Here is a depth of field calculator that you can play with:


Shawn Gibson , Oct 12, 2010; 11:32 a.m.

I always look at it this way: you have 3 main variables: compression, depth of field and distance. The good thing is, all of these things are constant:

The shorter the focal length, the greater the expansion and the greater the depth of field @ all apertures, all else being equal. And conversely, the longer the the focal length, the greater the compression, and the narrower the depth of field @ all apertures.

The other variable is the actual depth of field, which is always affected by distance to subject, and distance from subject to background (the latter of which affects 'background blur'):

The closer you are to a subject, the narrower the depth of field, always, with all lenses and all working apertures.

Not necessarily what you asked, but hopefuly what you're truly asking is implied at least in part of my response?

Adam Mueller , Oct 12, 2010; 11:50 a.m.

What started me off was I saw another post here where someone said f/8 was the sharpest?

Personally, I was under the impression that the higher you go (ie - f/22) the sharper the image gets providing you have enough light to compensate (ie studio flash). I have a small studio set up and I use two flash heads and have my aperture set to f/14. The image comes out very sharp, but whe someone said that it kind of threw a spanner into my thought process works! :-S

Alan Marcus , Oct 12, 2010; 12:02 p.m.

The camera lens performs just like a funnel. Whereas funnels with large mouths allow you pour liquids and fill up a bottle without spilling, into a bottle with a tiny mouth. A camera lens gathers light and directs it inside so it can play on the film or digital chip. More correctly, the camera lens projects an image of the outside world onto a tiny screen inside the camera.

Now the greater the diameter of the lens, the more intense (brighter) the projected image will be. This is because the lens acts like a funnel; the wider it is the more light it can gather. Most times the projected image made by the lens will be too bright. We correct this by reducing the working diameter of the lens as this prevents it from gathering too much light. We call this action stopping down as this restricts some of light rays from passing through the lens. To do this, the camera lens is equipped with a variable size entry hole. This hole is washer shaped and it is adjustable as to its diameter. We call this opening an aperture. When we set our camera to operate in different conditions of brightness, we are generally setting the diameter of this aperture for the brightness of the scene.

Now lenses come in different focal lengths. Most lenses nowadays are zoom lenses meaning the focal length can be changed by the photographer. Some are fixed focus and cannot be changed. The ones that are fixed are often called prime lenses. Nevertheless, when we change focal lengths we change the magnification of the image. If we mount a long lens or zoom to a long focal length the lens, acts much like a telescope and we get a magnified view. If we mount a short lens or zoom to a short focal length the lens gives a wide-angle view.

When we change focal lengths, we are also changing the magnification. Now high magnifications using a long focal length reduce the amount of light that can get inside the camera. Wide-angle lenses have short focal lengths and the image projected inside the camera is quite bright.

Now changes in magnification brought about by changing the focal length would lead to over and under exposure pictures because each time we change focal lengths we change the image brightness. We need a system to prevent all this confusion. The f/number system to the rescue. The f/number takes into account the focal length working diameter of the lens. In other words, the f/number intertwines these two actions and takes the confusion away.

How do f/numbers take the confusion away? The f/number works as a universal brightness setting. If we set a lens to a specific f/number value, say f/8, then set another camera with lens to f/8, both cameras now pass the same amount of light. In other words, any lens set to the same f/number as any other lens passes the same amount of light into the camera. This is true regardless of focal length or the working diameter of the aperture. The f/number system allows one photographer to tell another photographer how to set their camera for best exposure. This is important because there are big lens cameras and and small lens cameras. The important thing is every lens set to the same f/number delivers the same light energy regardless of focal length or aperture diameter.

Kyle Evans , Oct 12, 2010; 12:02 p.m.

The higher your aperture the greater your depth of field. It doesn't inherently make the image sharper.

As a product of the physics of how light works though lenses will have varying sharpness through their range of apertures. At lower apertures for example having a lens wide open the image will soften around the edges as you stop down and get closer to the "sweet spot" of the lens the image will get sharper. Most lenses have an optimal aperture where the image created is at its sharpest and depending on the lens this will be different. Most of my lenses are sharpest around f5.6-f8. As you stop down past a certain point again depending on the focal length of the lens how far the lens is mounted from the image plane and how large the projected image will be (crop sensor digital, medium format, large format) you run into diffraction which is caused when light passes through a very small hole, this causes the image to become fuzzy or soft.

Hope that makes sense. I didn't go into super detail on the physic of it but if you google diffraction it will explain why images become softer at higher apertures.

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