I have a canon 135m L lens, I saw in most of the forums that the picture is really sharp at f value of 2.8
Can some one tell if there is any direct relation between F value and sharpness
Printable View
I have a canon 135m L lens, I saw in most of the forums that the picture is really sharp at f value of 2.8
Can some one tell if there is any direct relation between F value and sharpness
The easy answer to your question is that the more you stop the lens down, the deeper the depth of field and your sharpness improves. At a certain point it will start going the other way, there will be a sweet spot that you will get the best IQ out of the lens. At F/11 you may start seeing it decline.
But I am not sure that was the answer you were looking for, were you looking for a relationship formula that would tell you what the optimum F value would be on a given lens? It would be interesting to see if such a calculator or formula existed. Maybe it would be something like (All F values < F/8). Then maybe not.
At the risk of ridicule from other's on the forum, here is a link that might give you some information, and start you down the path of disenlightenment.
http://www.kenrockwell.com/tech/focus.htm
There are several relations between aperture and sharpness.
First one is the Depth of Field (DOF). At wide aperture (small f value), the DOF is smaller/shallower, which means only objects at focus distance will be in focus, everything closer or farther away will be blurred. Often that is a desired effect, that's why people like lenses with a wide max aperture. Keep in mind that there are other factors that affect the DOF, distance and focal length.
Another relation is that it is obviously more difficult to build a lens that at wide apertures produces sharp images (for everything inside the DOF). That is due to the nature of optics. That is why lenses with wide max apertures (small f-values) and great sharpness are typically heavier, bigger and more expensive than lenses with less wide apertures. For example, the 70-200mm f2.8 IS is much heavier, bigger, and more expensive than the 70-200mm f4 IS.
Most if not all lenses will produce sharper images if you use them not completely wide open, but stopped down 1 stop or two. However, if you stop down too much, diffraction kicks in and the image becomes less sharp. Depending on the sensor that will happen at f-values around 8 to 11 or higher.
Not everything there is wrong I guess :-)Quote:
Originally Posted by HDNitehawk
here is another resource:
http://en.wikipedia.org/wiki/Depth_of_field
http://en.wikipedia.org/wiki/F-numbe..._image_quality
Hi, my not too scientific way of understanding it is something like this:
- Imagine a small point on the subject.
- This will reflect light in many directions, think of it as rays of light reflected from this small point.
- A bunch of these rays will travel more or less in parallel towards your lens.
- Now, to get the image sharp, the lens must bend those rays back into a small point on the sensor.
- When you use a small aperture, only the centermost rays are let through the aperture and it is "easy" to collect them in a single point on the sensor. Basically all lenses can do this.
- When you use a large aperture, there are more rays that must be bent correctly into this single point (a larger cross-section area of the lens is used), and to me it seems logical that this is more difficult to achieve.
When it comes to comparing sharpness at different apertures/focal lengths I find the interactive charts at slrgear.com quite useful. At least for the lenses I own, these graphs seem to reflect what I've also seen in real use. The lens you mentioned, 135L, is known for being very sharp even wide open. This is also illustrated here.
There are several relationships. In general, as you stop down:Quote:
Originally Posted by gandhi
- Less defocus blur (if part of the frame is not within the DOF)
- Same or less lens aberration blur
- More diffraction blur
The optimal F-number for sharpness varies with the lens, camera, and specific circumstances. For example, for one subject, f/16 may be sharper than f/45. But for another subject (that requires more extreme DOF), the f/45 is actually sharper (because it gets the *entire* subject into the DOF instead of just a part of it).
Indirect sharpness relationships include things like motion blur/noise related to stopping down when you cannot keep exposure the same (e.g. low light).
The page you linked, like most of the pages on that site, is riddled with gross inaccuracies. I think it's a disservice to point anyone at that site.Quote:
Originally Posted by HDNitehawk
Going to his site is like picking and eating mushrooms, until you can identify the ones that are poison it is probably best to just stay away. The mushrooms are his points of course.Quote:
Originally Posted by Daniel Browning
It could have been worse; I could have linked to CR as well.
That's the perfect way to put it!Quote:
Originally Posted by HDNitehawk
:DQuote:
Originally Posted by HDNitehawk
The best explanation I've read of sharpness as it relates to aperture and depth-of-field is How To Improve Sharpness by Lars Kjellberg at photodo.com. It's an oldie and a goodie. In short:
Large apertures =
- Shallow DOF, which is good if that's what you want
- Less sharp because the large hole allows light from the edges of the lens, which is more prone to distortion
Small apertures =
- Deep DOF, which is good if that's what you want
- Sharpness compromised due to proportionately large percentage of diffracted light caused by such a small aperture hole (see Lars' article if you want a detailed explanation)
Mid-range apertures =
- Mid DOF, which is okay if that's what you want
- Best sharpness because the smaller hole means you are only using the best part of the glass (the center) but the hole isn't too small, which would allowing diffracted light to dominate what hit's the sensor
Of course, the average photographer often judges "sharpness" in terms of "is it blurry or is it clear."
It is harder to get properly focused shots with a wide aperature/shallow depth of field.
On the other hand, in dimmer light, a wide aperature allows you to use a faster shutter which prevents motion blur.
Motion blur and OOF shots can be easily mixed up when one is just getting into photography.
For average shooters, there is a lot to be said for the high-percentage safe shot, in the mid range of aperatures. Not too spectacular or artsy, but not as easy to screw up as long as it is not too dim and the subject is not too fast.