Most lenses have a "sweet spot" (a range in their aperture) where they produce the sharpest images; this may be what you're thinking of..
See this webpage... http://digital-photography-school.com/blog/find-your-lens-sweet-spot/.
Or, you may be thinking of the depth of field. The DOF has to do with what part of the image will be the sharpest and it is controlled by the aperture setting.See this webpage... http://www.dumetier.com/photography/depth.html.
Hope this helps,.
In reading many reviews on lenses, people talk about how "at wideopen" the sharpness is not as good as when stopped down some...Thisdoes not make sense to me and I would love an explanation..
This is a general truth, although not necessarily true for very specific cases. The reasons are actually twofold. A lens takes light from a point on a subject and focuses it onto an image plane. The way this works is that light emerges from the point on the subject in different directions. A larger lens will capture not only light emerging directly toward the lens, which doesn't need to be bent at all to reach the correct spot on the focal plane but also light leaving the subject at slight angles and hitting the middle or edge of the lens. This light needs to be bent more to land on the same spot on the focal plane..
For all the subject point light intercepted by the lens to land on the same exact spot on the focal plane, the lens needs to be perfect over it's entire surface. Since light hitting the edge of the lens needs to bend more, things become very sensitive to the exact curvature of the lens at the edge..
Now, creating a perfectly-shaped surface over a large-diameter lens is challenging, from a manufacturing point of view. This is true enough at one wavelength. When you need to ensure that all wavelengths focus to the same point, the challenge is even greater. This is why a large number of lens elements is required in addition to the main, front element..
When you stop down aperture, you're effectively blocking off the outer edges of the lens in increasing amounts, moving closer and closer to a pinhole ... This means that you increasingly get rid of light from the more problematic regions of the lens near the edge and image quality improves. Note that this isn't always the case. You can take great care in designing and manufacturing a lens to ensure a perfect shape to the edge. However, cost, size and weight constraints generally impose compromises..
The second reason for softness at wide-open apertures is that depth of field is shallowest in this circumstance. This means that only a short distance in front of and in back of the focal plane appears in sufficient focus to seem sharp. It also means that you need to be very precise in your focus lest the details you care about be outside the depth of field. When you stop down the aperture, depth of field increases and tolerance to misfocus increases..
Hope this answers your question..
-Removed content, previous poster answered very well while I was typing my postJust trying to learn.
Blog: http://novicephotog.blogspot.com/Flickr: http://www.flickr.com/photos/9778447@N07/..
But the OP has the other arm of the problem in mind as well.....shutter speed.A picture can be out of focus soft (depth of field issues)A picture can be lens distortion soft (cost of lens issues)And a picture can be motion blurred soft (shutter speed issues).
So it is true that while your stopping the apature down to reduce the lens distortion softness you are slowing the shutter speed and risking motion induced softness..
And while a tripod will help sometimes, if the subject is in motion then the only recourse is a faster shutter speed and a more open apature (or higher ISO and noise issues).
Much of photography is a compromise with many techniques that can optimise the result within the limits of the compromise.A member of the rabble in good standing..
...In reading many reviews on lenses, people talk about how "at wideopen" the sharpness is not as good as when stopped down some...Thisdoes not make sense to me and I would love an explanation..
A wide open aperture at for example F1.8 or F2.8 means a lot of light which means a faster shutter speed but it also means a narrow depth of field..
*** Open wide aperture means narrow depth of field. ***.
A narrow depth of field means only a small amount in front of and behind the focus point which is in sharp focus, the rest will be blurry. You might be taking a photo of a persons face and only the eyes be in focus, the tip of the nose and the hair all blurry..
Stopping down the aperture perhaps to f8 or f11 increases the depth of field and now perhaps the whole face of the person is in focus but the background 10m behind them may still be blurry..
Sometime shooting wide open is a good thing when it comes to reducing your DOF to blur the background. Of course in low light and when DOF is not critical, you can trade f stop for shutter speed.J. English..
In reading many reviews on lenses, people talk about how "at wideopen" the sharpness is not as good as when stopped down some....
Although the point made about depth of field in some of the earlier responses is correct (wider aperture = less DoF), the issue being referred to in the reviews is not depth of field, it is the sharpness at the point of focus. DRG's post gives a good basic explanation - there's a lot more that could be said but in the spirit of keeping it simple I won't!.
I will mention one additional point, though. Lenses also lose sharpness at very small apertures, for a completely different reason. An effect known as 'diffraction' sets in, and becomes the dominant factor above approximately f/16 (it varies a bit), limiting the sharpness that can be achieved even with the best lenses. So you end up with a 'sweet spot' in the range of f-stops - most lenses are sharpest from a stop or so less than their widest, up to about f/11, with the very best performance in the middle of that range...