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07/16/2005 09:44:22 PM · #1 |
Can someone explain this to me? I've seen comments elsewhere about how setting lenses to certain F-stop's improve the sharpness. I'm just trying to grasp how the aperture has an affect on the sharpness.
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07/16/2005 10:02:07 PM · #2 |
The smaller the aperture the deeper the field...so more things become in focus and less things become "washed out".
however...there is an ongoing argument about the fact that beyond F22 things start to get "dirty again" because of the lens itself...I have yet to be sold on that one. |
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07/16/2005 10:03:18 PM · #3 |
It's called diffraction and is more prominent in cheaper lenses than in the more expensive glass.
edit: spelling
Message edited by author 2005-07-16 22:15:57.
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07/16/2005 10:04:41 PM · #4 |
Originally posted by cpanaioti:
It's called defraction and is more prominent in cheaper lenses than in the more expensive glass. |
Yeah the Canon ones, you gotta watch you don't go to small an aperture. Usually with the Nikon glass, you're fine. ;)
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07/16/2005 10:07:42 PM · #5 |
Originally posted by deapee:
Originally posted by cpanaioti:
It's called defraction and is more prominent in cheaper lenses than in the more expensive glass. |
Yeah the Canon ones, you gotta watch you don't go to small an aperture. Usually with the Nikon glass, you're fine. ;) |
I doubt that that's totally true but good dig. ;o)
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07/16/2005 10:11:05 PM · #6 |
| I've never experienced it myself...and I guess we're splitting hair a bit here BUT I can see the science behind it...and if it's true for Canon it certainly must be true for Mikon....physics does not follow markets! |
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07/16/2005 10:23:41 PM · #7 |
Completely aside fromt he optical characteristics of a given lens, there's something called the "circle of confusion" and it has a definite effect as you stop down. Once and aperture reaches a certan ver small size, things get less sharp because of COC. It's a function of the physical size of the holke, not the relative f/stop. So it comes into play at, say, f/11 on a WA lens and, say, 5/22 on a telephoto, I don't remember the numbers.
Google "circle of confusion" and you'll probably find soemthign about it. it was a signifcant factor shooting architecture with view camera when we wanted to stopw ay down for DOF but then started losing critical sharpness in details and had to balance the two needs.
Robt.
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07/16/2005 10:33:32 PM · #8 |
Isn't it the "Bal of Confusion?"
I've read alot about COC and I really never purpously shot at F-stops beyond my immediate needs but I have done stuff with the 14mm and the 16-35 at f22 (used to be my magic number for products....when I used hotlights) and I can't say that I had diffuculty, must be a very slight issue or more apparent on lesser glass?
Message edited by author 2005-07-16 22:34:45. |
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07/16/2005 10:36:43 PM · #9 |
Originally posted by Gil P:
Isn't it the "Bal of Confusion?"
I've read alot about COC and I really never purpously shot at F-stops beyond my immediate needs but I have done stuff with the 14mm and the 16-35 at f22 (used to be my magic number for products....when I used hotlights) and I can't say that I had diffuculty, must be a very slight issue or more apparent on lesser glass? |
I'm not really an expert on it. I'm really only aware of it in terms of large-format photography. If you're referring to LF product shots, well WA is like 90-70mm and f/22 isn't an especially small physical aperture. We saw the effect when we went down tolike 45 or 64 in an attempt to eke out DOF in architectural interiors where swing and tilt were insufficient.
R.
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07/16/2005 10:40:39 PM · #10 |
| ahhh... well I've never done anything beyond f22 so I guess I've never had to "deal" with that. |
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07/16/2005 10:52:34 PM · #11 |
At very smallapatures the lights defracts of the blades of the iris diaphram, this also contributes to the lowering of sharpness.
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07/16/2005 11:09:37 PM · #12 |
Originally posted by kiwinick:
At very smallapatures the lights defracts of the blades of the iris diaphram, this also contributes to the lowering of sharpness. |
Yes, that's where COC comes from: light actually diffracts off the blades at any aperture, but the diffraction is a greater percentage of the whole, in term sof overlap, at very small apertures. Thickness of the aperture blades is a factor also I believe...
R.
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07/17/2005 01:23:34 AM · #13 |
Here's an excellent link providing information on diffraction, especially with respect to digital cameras.
Note that visible diffraction happens in print at different apertures depending on your print size and print resolution (i.e. enlargement factor). For example, with a 20D a 300ppi 8x10 (viewed at 10 inches) is limited at f/11, whereas the same image printed as a 4x6 is limited just smaller than f/16. Under the same conditions, except with a 20x30 print, between f/3.5 and f/4 is your limit.
That's crazy you say! Well, most people don't consider 10 inches a typical viewing distance (at least for acceptable sharpness) with 20x30 prints; for that size, 3 feet or more is probably acceptable, so you're back down to the f/11 to f/16 range.
Note that visible diffraction can be subtle as it contributes to the image's overall softness. Although you can see it on screen with extreme examples, side by side prints can/will show the effect more readily. Try it for yourself sometime: if you don't see a difference, then don't worry about it. :-) |
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07/17/2005 01:29:27 AM · #14 |
most lenses' sweet spot is around f/8...after that you start to get what's called defraction (i think), it's where light passes through a small opening and get's bent. like if you almost close a door by leaving a small gap for light to come through, if the light projects onto a wall, you wont see a defined line between light and dark.
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07/17/2005 08:32:48 AM · #15 |
Aside from all the diffraction talk;
All lenses get sharper once you stop them down a bit. A 50mm f/1.4 is relatively soft at f/1.4 and gets sharp from f/2 to an optimum at say f/5.6 to f/8.0, after that you start to loose some sharpness again.
Lenses and especially zoom lenses are a compromise. They cannot be good at alle ends. Even when you get Canon's 50mm f/1.0 it will be soft wide open.
The expensive lenses usually have a wider range of good sharpness, good contrast, good color, etc. At certain f-stops (around f/8-f/11) cheap lenses and expensive lenses can come close, altough the better contrast of the expensive lenses still makes a lot of difference.
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07/17/2005 10:15:35 AM · #16 |
| Besides all the optics stuff, sensor dust becomes a lot more prominent at small apertures. |
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07/17/2005 01:11:58 PM · #17 |
| So if diffraction is the cause of softness at smaller apertures, then what is the cause of softness at larger apertures, like f1.8? |
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07/17/2005 01:54:48 PM · #18 |
Originally posted by Olyuzi:
So if diffraction is the cause of softness at smaller apertures, then what is the cause of softness at larger apertures, like f1.8? |
Imagine the lens as throwing a cone of light from its rear element onto the sensor plane. The distance between the rear element and the corner of the sensor is significantly greater than the distance to the center of the sensor. The light has to travel further to reach the corners and it is striking the corners at an oblique angle.
Lens are designed to compensate for this. The amount of compensation needed varies with the aperture. The designers must choose a happy medium. Also, expensive lenses have more elements, and more sophisticated elements, specifically designed for such forms of compensation. Inexpensive lenses have fewer elements, and cannot compensate as well. Aspherical elements (expensive) are a major factor also, as is general precision of build. Obviously, if you have extremely close build tolerances you can narrow down this compensation significantly. In the inexpensive, plastic lenses, the build tolerances cannot be as precise, and so more slop has to be allowed in the compensation to account for this.
This is the major reason why cheap lenses rarely perform as well as expensive ones, and it also explains why some examples of a particular cheap lens may actually perform nearly as well as, or even as well as, professional-grade optics; whoever happens to buy one of these lenses that chances to fall precisely on spec is a lucky fellow indeed, and has a nice piece of glass at a bargain price.
Robt.
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07/17/2005 03:30:11 PM · #19 |
Optics and optical characteristics are very complex. But there are some general guidelines that can help when trying to comprehend the why of things.
Assuming ideal lenses:
1. Most lenses are spherical and produce a spherical (curved) image plane. This effect is more pronounced with short focal length lenses of large diameter. Manufacturers go to great lengths to flatten the image plane using spherical and aspherical elements in combination.
2. Larger lenses are capable of resolving greater detail than small lenses. But large diameters of short focal length have a shallow depth of focus. We have some control over this effect by using an adjustable aperture restriction (iris diaphragm). The smaller the aperture (large f number), the greater the depth of focus and also lower potential resolution (not generally much of a consideration).
3. All lenses (except reflecting lenses) refract and/or diffract light. Much like a prism, this effects light of differing wavelengths in differing amounts causing chromatic abberrations. Manufacturers must take this effect into account when designing lenses to correct spherical abberrations. It ain't easy!
So you see, every photograph you make will be a trade off between all the various optical effects and throw into the mix the noise and sensitivity characteristics of your camera. Learning how to best balance all these phenomena in any given situation is what makes a good technical photographer. But, the hardest part of photography is learning what makes a good, appealing, composition. I can't help you there!
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