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09/20/2005 02:15:35 AM · #1 |
| Hi, from the hardware specs, it states "f/ 2.8 - f/ 3.7; 10 elements in 9 groups" meaning the aperture max out at 3.7. But from a photo's EXIF, I got a reading of F/8 (FNumber: 8), what does this mean? Please enlighten me, I'm really dumb at these specifications. Thanks in advance. |
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09/20/2005 02:22:47 AM · #2 |
| I'm guessing it's a zoom lens and the two f stops you asked about are at the fastest ones at the short and long end of the zoom range. |
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09/20/2005 02:25:56 AM · #3 |
| so its possible to have F8 after all with this little thing? Its not an EIF error, right? I dont wanna get into DQ trouble here, lol |
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09/20/2005 02:27:23 AM · #4 |
| Nope, it's not an error at all. |
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09/20/2005 02:30:29 AM · #5 |
f/ 2.8 is the fastest at the widest angle
f/ 3.7 is the fastest at the longest zoom
most P/S type cameras can go up to f8. You may not adjust manually , but the camera may automatically adjust. |
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09/20/2005 02:32:08 AM · #6 |
| Thanks TommyMoe21 and faidoi, those are clear answers! you guys are great. |
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09/20/2005 02:35:31 AM · #7 |
f/2.8 ΓΆ€” 3.7 is the range of your maximum aperture at different zoom lengths; the f-stop number is a ratio between the physical size of the aperture and the focal length of the lens. A 25mm aperture on a 50mm lens would be f/2.0, while a 25mm aperture on a 100mm lens would be f/4.0... since the physical size of the aperture on this lens does not change as the lems zooms, the f/stop itself is smaller at full zoom than it is at minimum zoom.
Some zooms (generally more expensive ones) physically adjust the size of the perture throughout the zoom range, so the f/stop wide open remains the same regardless of the degree of zoom.
You can be assured that your lens stops down to at least f/11, I'd guess, probably f/16. Also, there's no DQ involved in mis-stating the shooting parameters in the information field.
Robt.
Message edited by author 2005-09-20 02:37:27.
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09/20/2005 03:16:08 AM · #8 |
The f-stop of a lens is considered the "speed" of the lens. The lower the number, the faster the lens. The tradeoff, of course, is depth of field. Certain zoom lenses have a different effective f-stop depending on the zoom setting. I believe a zoom lens is generally faster (lower f-stop) when it is wide, and slower (higher f-stop) when it is zoomed in.
As the f-stop is set to higher and higher values, less and less light is admitted through the lens. However, the depth of field for the lens increases as the f-stop increases.
The maximum f-stop value for a lens is not usually of any interest. It's easy to stop a lens down, but hard to minimize a lens's f-stop value. The lower the f-stop, the larger the lens must be. So in general, small f-stop ratings for a lens makes that lens expensive because the optics must be just that much more precise.
For SLRs, a maximum f-stop is usually around 22. For most digital cameras (but probably not for DSLRs, it appears that a maximum f-stop value will be around 7.0 to 8.0 or thereabouts. I haven't really looked into this, and am not particularly knowledgeable about the numbers. This is just an educated guess.
The f-stop number is a figure of merit that tells you how much light is being passed through the lens. It's based on the inverse square of the light being passed. An f-stop of 1.0 would mean all the light is being passed. an f-stop of 1.4 means half the light is being passed. An f-stop of 2.0 means one fourth of the light is being passed, and so on.
In the old SLRs, the f-stops were standard values that typically went something like f1.4, f2.0, f2.8, f4.0, f5.6, f8.0, f11, f16, and f22. Additionally, there were half-steps between these numbers which were known as half-stops. The full steps are basically exponential in nature. 1/2 the light (f1.4), 1/4 the light (f2.0), 1/8 the light (f2.8) 1/16 the light (f4.0) and so on. Nature works exponentially, we think linearly. The f-stop numbers are more compatible to the way we think. Note that every other f-stop number is roughly doubled from its two-back predecessor.
Each successive f-stop admits half the light that the previous f-stop did.
Now this all made sense when cameras were purely mechanical, but with the digital cameras, the concept of an f-stop isn't so elegant. There's no need to be constrained to a few light admittance steps with a digital camera. In fact, digital cameras use the f-stop terminology, because it's valid, but they don't need to use the limited number of steps the old cameras were stuck with. So they come up with goofy numbers like 6.3 or 7.5 or whatever. But it's all good. The older numbers were somewhat arbitrary in nature, albeit they made the photographer's job easier. Computers don't need easy numbers to look at...they're happy with just about any number, and don't get befuddled with arbitrary values.
Older non-digital cameras typically had shutter speeds the roughly doubled. So your shutter speed could be changed from 1/60 to 1/125. This would cut the total light that hit the film in half. But if you changed the f-stop from 5.6 to 4.0 at the same time, then you would double the amount of light coming through the lens, and still get the same total amount of light hitting the film. So you can see it was easy to trade shutter speed off against how much light the lens passed. You could easily trade off speed and depth of field, yet still not alter the effective exposure value. This was extemely handy before cameras came with built-in exposure meters.
Alas, the time for this cleverness has come to pass, and we simply don't have to think in those terms any more. It was elegant in its time, but it just isn't all that relevant with the new breed of cameras. |
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09/20/2005 04:10:42 AM · #9 |
Originally posted by hblake:
Alot of stuff... |
Wow. I say a round of applause for one of the most helpful and complete replys I've seen in a long time.
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09/20/2005 04:18:37 AM · #10 |
| this thread also shows that you guys are bloddy good at what you do |
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09/20/2005 08:30:05 PM · #11 |
Well, to follow up on myself...I have run across a web page that goes to great lengths to explain what an f-stop is. So for those of you who want even more information on the mysterious f-stop, here it is: All About the F-Stop
The only thing the article doesn't really cover is exactly where the f-stop value comes from. Generally, my last post covers that, but let me tie it together here...it's not quite obvious at this point.
As bear_music states, the f-stop derives from the ratio of the focal length of the lens versus the diameter of the aperture. Aperture is just a fancy word for 'hole'. This ratio is used to determine how much light the lens passes. If the two numbers are the same, i.e. 50 mm each, then the ratio is one to one. This would correspond to an f-stop of 1.0. What if the ratio were 50/25? Remember I said it's an inverse square thing? Indeed, the inverse square here is 1/4, and the ratio is 2...that is to say f/2.0. Egad!
So you're probably saying, "Well yes, Hank, but why are we doing that 'square' thing here?" In one word, the answer is -- area. Area measurements are always the square of linear measurements. If you have a square that is 5 inches on a side, the area is 25 square inches, the square of 5. An aperture is an area, so we are talking (more or less) the squares of sides here. Film (or CCD sensors, or film, or whatever) have area. They are not one-dimensional, they are two dimensional. So the one-dimensional numbers we like to use as humans really have to be translated to the two-dimensional numbers that represent reality.
But all I'm trying to say here is that there is a precise correlation between f-stop numbers and what they represent. It's not arbitrary. You can rest assured that f-stop numbers didn't just "magically" appear. They have a definite and causal relation to reality.
So if you are interested in the minutia of photography, follow that link above and increase your understanding of the fabulous art (nay, science) of photography. |
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