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F number question
HI!.

I have just started to get into photography and have been doing alot of reading about the different terminology of photography. I understand that an F number is the the way of describing the the dia. of the entrance pupil. SO...... what does this mean in F numbers?.

F2.8 - F3.7 (minimum aperture F11).

It is the range of a dmc-fz50;.

Im not even sure if this is a valid question or if I'm confusing one thing with another. Please long detailed explanations are loved +SIGHT+.

Http://picasaweb.google.com/cattjake..

Comments (6)

SIGHT wrote:.

HI!.

I have just started to get into photography and have been doing alotof reading about the different terminology of photography. Iunderstand that an F number is the the way of describing the the dia.of the entrance pupil. SO...... what does this mean in F numbers?.

F2.8 - F3.7 (minimum aperture F11).

It is the range of a dmc-fz50;.

Im not even sure if this is a valid question or if I'm confusing onething with another. Please long detailed explanations are loved +SIGHT+.

Http://picasaweb.google.com/cattjake.

Common question as well. It means that at the minimum focal length the maximum aperture is f/2.8 and at the maximum focal length the maximum aperture is f/3.7. It also means that at any given focal length, the minimum aperture is f/11..

Note that f/2.8 is a physically larger (wider) aperture than f/3.7, but numerically smaller!.

Tim'Be the change you wish to see in the world.' -Mahatma GandhiE3/E-1/E410/7-14/12-60/50-200/EC-14/C8080http://www.flickr.com/photos/timskis6/..

Comment #1

Hi,.

The f number is the ratio of the diameter of the entrance pupil to compared to the focal length of the lens. The smaller the f number, the larger the opening at a given focal length..

The number range you gave for the FZ-50 tells you what the largest aperture setting you can use on it's lens over the zoom range - at 36mm (35mm equivalent) the FZ-50 has a maximum aperture of 2.8..

As you zoom towards the telephoto end of the range, the f number increases because the lens is not large enough to maintain a constant f/2.8 over the full range of 36-432mm equivalent. At full zoom, the largest opening possible is at f/3.7..

I don't know the exact points at which the maximum aperture changes on the FZ-50, but I'm guessing it's something like this:.

36-49mm equiv. f/2.850-84mm equiv. f/3.085-134mm equiv. f/3.2135-199mm equiv. f/3.4200mm-432mm equiv. f/3.7.

If you want to check exactly what the maximum aperture setting is at each focal length, set the camera to aperture priority and zoom all the way out to 36mm/1x and set the aperture to f/2.8. Zoom in with the lens - as you zoom in, the camera will automatically indicate the new maximum aperture size in f number on the lcd or in the viewfinder..

Using a variable maximum aperture in a lens allows a manufacturer to produce a much smaller and lighter lens..

The f/11 number represents the smallest possible opening the lens can have at any of the zoom settings..

Summary: the numbers tell you what f numbers you can set for the camera in aperture or manual mode, or that the camera can choose in a program or scene mode. For the lens on the FZ-50 you can choose an aperture as large as f/2.8 to f/3.7 depending on the zoom setting and as small as f/11..

If you get the actual focal length of the lens (should be stamped/printed on the front of the lens, something like 6.0 to 72.0 or so), you can calculate the entrance pupil size using the f number should you so desire...

Comment #2

F number = focal length divided by aperture (equation 1). Focal length is, confusingly, abbreviated f (lower case). Aperture is the diameter (NB) of the hole in the diaphragm..

F is dimensionless (ie, just a number) because the units of focal length and aperture are the same - usually millimetres but it would not matter..

If you re-arrange equation 1 to have aperture alone on the left you will see that.

Aperture = f/F.

This is why fussy people write that a lens has a maximum aperture of "f/2.8". Note that in a zoom lens the aperture in mm represented by f/2.8 will change with the focal length..

Remember that the amount of light that a hole lets in is related to the area of the hole, not the diameter, and the area is proportional to the radius squared. So you halve the light when you double the square of the F number. Now, 1 squared is 1, 1.4 squared is 2, 2 squared is 4, 2.8 squared is 8, 4 squared is 16 and so on, so f/1.4 is twice as much light as f/2 is twice as much light as f/2.8 and so on (this is why F is only occasionally a round number)..

The most intractable problem in photography is shortage of light, so you can't use a shutter speed fast enough to avoid blurring the image. So, the largest aperture a lens can give you is usually the limiting factor. There needs to be an awful lot of light before a modern camera can't make the shutter speed fast enough to enable you to use a moderate or even large aperture if you want to, so the smallest aperture a lens can give you is rarely an issue. Plus, the larger the aperture the shallower the depth of field (focal length being constant), and so a bigger maximum aperture gives you the option to have a shallower depth of field - good for portraits..

Primes (single focal length lenses) always have only one F number. If for a zoom lens there is one number (eg, "17-55mm f/2.8") that means the maximum aperture is the same at all focal lengths. Less expensive zoom lenses often do not have as large a maximum aperture at the long end, hence "f/3.5-5.6"). Only the maximum aperture at the extremes is given, and the rest are in between...

Comment #3

And one more explanation!.

On a zoom lens, when you see the F number described as a range it means that the maximum F number changes as you zoom the lens. In this example it starts out at F2.8. As you zoom in it gradually changes to F3.7. Minimum aperture F11 refers to the smallest aperture setting that you can set on the camera..

This is in contrast to a zoom lens that maintains a constant max aperture setting through it's zoom range. In this case the camera will simply have a single max aperture rating. Such lenses are very expensive. Heres one....

Http://www.dpreview.com/lensreviews/canon_70-200_2p8_is_usm_c16/.

Here's a very long and detailed explanation of F-stop.

Http://www.uscoles.com/fstop.htm..

Comment #4

Thanks so much for the explanation!everything makes more sense now.+SIGHT+.

Http://picasaweb.google.com/cattjake..

Comment #5

Thanks for the reference website, I will make sure to check it out!+SIGHT+.

Http://picasaweb.google.com/cattjake..

Comment #6

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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