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12/30/2004 08:14:21 PM · #1 |
Am I right in saying that a shot at 30mm f2.8 won't blur the background as much as say a 500mm f2.8? If so, why does focusing further away make the backgrounds so much blurrier? Or am I completely missing the point of blurred backgrounds?
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12/30/2004 08:20:11 PM · #2 |
True. But the other variable is the distance between the subject and the background. The bigger the distance the blurrier the background.
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12/30/2004 08:40:07 PM · #3 |
If I took a photo of my girlfriend standing 5 feet infront of me at 18mm and f3.5 and there were trees 40 yards behind her, they'd be pretty well in focus. Now if someone shoots a football game and is focused on a player on the other end of the field (I'm just guessing here, but at 400mm) and f3.5 at the 20 yard line with cheerleaders 40 yards behind the player and a wall behind that, the cheerleaders would be way OOF.
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12/30/2004 08:48:08 PM · #4 |
| Hmm... I think at the same aperture the opposite is true. DOF is proportional, so as you increase the distance between you and your subject, you also increase the depth that will be in focus. That's why a focus setting at infinity is just as good at 500 feet as 750. |
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12/30/2004 08:54:24 PM · #5 |
Originally posted by deapee:
Am I right in saying that a shot at 30mm f2.8 won't blur the background as much as say a 500mm f2.8? If so, why does focusing further away make the backgrounds so much blurrier? Or am I completely missing the point of blurred backgrounds? |
WARNING: MATH AHEAD. Laurie, run away!
This has to do with the way f/numbers are calculated for aperture values.
As I'm sure you are aware, smaller apertures correspond to larger f-numbers. The smaller the aperture, the more depth of focus.
To understand why the depth of focus changes as focal length changes, you must first understand that aperture values are represented not as an absolute value, but as a ratio, of the focal length (f) to the diameter of the shutter opening. For example, a ratio of f/4 on a 50mm lens has an opening of 50/4=12.5mm in diameter, while f/4 at 300mm requires an opening of 300/4=75mm in diameter. This also explains why fast lenses are larger than their slower counterparts.
The smaller the opening, the more nearly parallel the light rays coming into the lens, and the more depth of focus is present.
-Terry
Message edited by author 2004-12-30 20:59:38.
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12/30/2004 08:57:45 PM · #6 |
ahh that COMPLETELY makes sense. So basically, 18mm at f3.5 the shutter is not really openeing as wide as a shot at 400mm f3.5...you have put an end to my confusion!
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01/01/2005 07:57:00 PM · #7 |
Originally posted by deapee:
ahh that COMPLETELY makes sense. So basically, 18mm at f3.5 the shutter is not really openeing as wide as a shot at 400mm f3.5...you have put an end to my confusion! |
Uh... confusion. No, that is not how it works. The shutter on a camera will open just as wide no matter (unless you are using a camera that utilizes the aperature as a shutter) what lens you have attached to the camera or what aperture that lens is set at. What it really comes down to is a wide angle lens is capable of greater depth of field then a telephoto lens. That is one reason that a short telephoto - say a 85mm or a 105mm makes a better portrait lens then say a 35mm or a 50mm. The telepoto is much better at seperating the suject from the background then a normal or a wide-angle lens. |
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01/01/2005 08:19:47 PM · #8 |
Originally posted by deapee:
So basically, 18mm at f3.5 the shutter is not really openeing as wide as a shot at 400mm f3.5 |
David,
You just used a term interchangably that isn't interchangable. The SHUTTER isn't the same as the APERTURE or iris.
The SHUTTER is just one size on each camera (like a 10D has one shutter size while a D70 may have a slightly different shutter size while a Minolta has . . . .). The SHUTTER only opens as large as it is allowed to open by design.
The part that makes the APERTURE work is a diaphragm of overlapping blades within each lens. These blades generally are called an iris if you refer to them but most people don't ever talk about them. What they do talk about is the function of these blades. The function of these blades opening and closing is called the APERTURE. As you roll a dial on the lens (in older mechanical lenses) or as you roll a dial on a newer digital/electronic camera you cause these blades to be pulled out away from each other creating a larger opening for the light to pass through the lens. This is a larger APERTURE and, thus, a smaller F-STOP. The larger this opening, the more light that can pass through the lens in a given amount of time and so you get enough light to clearly define a small section of space that is x distance away from the plane of the sensor in your camera. If you dial the F-STOP back the other direction, you make the APERTURE smaller, thus you will have larger F-STOP numbers (say f/8 or f/11 or f/16, etc). This makes a smaller opening between the blades in the lens. Now to get the same exposure that you got before you will have to leave the SHUTTER open longer (slower SHUTTER speed like 1/60th" as opposed to 1/125th"). This allows an equivilent amount of light to fall on the sensor in your camera so you get the same exposure for the same subject X feet/meters distant from your camera. This process of dialing the APERTURE to a higher F-STOP number is called "stopping down" the lens. It has the added effect of giving you a deeper Depth of Field so that more things are in focus when you make the exposure.
Now to get back to Terry's post the F-STOP is a function of the length of the lens. He put it succinctly and his example worked well to answer your original question.
I just wanted to note the difference in the SHUTTER (which you mentioned) and the APERTURE (which is what Terry commented about to explain the difference in f/2.8 at 35mm and f/2.8 at 300mm).
Hope this helps to clarify your comment. |
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