been reading and reading abt lenses being soft when wide open and sharper when stopped down? why is this so? is there an easy explaination or its just the rule of most lenses?
why are lenses sharper when stopped down?
- Thread starter yeppie99
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ALL lenses behave this way, even the Canon L lenses and Nikon's professional lenses, and Minolta's G lenses. It's a characteristic. Even your eyes behave this way. That's why you see some people squint when they are trying to see clearly - squinting is like stopping-down your eyes.
I can't explain the "why" part (maybe experts like Ian/Jed can) though.
Regards
CK
Gee THANKS.
I must point out that the issue is horribly complex and there are all sorts of views, even the major lens manufacturers aren't in agreeance over many aspects, so do you really want an answer?
Sounds like a technical discussion comming up. I will move this thread to "Technical Discussions".
Originally posted by ckiang
How about a short one, if you can?
Regards
CK
Here goes, the simplified version ....
Firstly there's a lot of confusion about terminology where this issue is concerned and to be honest there really aren't any guidelines. Terms like 'sharpness' and 'soft' aren't exactly scientific in nature and different people have different interpretations of what constitues the exact meaning of both 'sharpness' and soft along with numerous other terms. However, it's fair to say that what most people refer to as 'sharpness' has little to do with the resolving power of a lens but rather is concerned with contrast and it's perception by the viewer.
The resolving power of a lens is something that is NOT related to sharpness as perceived by the human eye. Resolving power or in other words a lens's resolution is expressed in lines per mm and refers purely to how many lines per mm a lens can seperate at a given distance, magnification and contrast level. In the case of typical test targets the contrast level is often in the range of 1000:1 between the black lines and white background. This level of contrast is highly un-natural when compared to the real world where the average scene contrast level is around 6:1.
Most people consider a lens sharp or unsharp based solely on how 'edges' appear in a photograph, for example the edge surrounding a feather on a bird, or it's beak, or the edge of a building against the sky. What you are actually looking at in these cases is a contrast difference between one object and the next, with the accuracy of the zone of change being mistakenly called sharpness.
Take for example a birds feather against a blue sky. The lens that has the best transition zone between the feather and sky will be perceived as being the 'sharper' lens, as the transition is more defined (crisp) thus producing the psychological effect of the lens being sharper than one that has a less well defined zone between the two contrast levels.
Another problem is the perception of edge contrast at the periphery of the film image. The contrast at the edge in many lenses is not at its peak with the lens fully open, but it does increse as the lens is stopped down in most cases thus giving the impression that the lens is sharper at a smaller aperture. (eg: 1-2 stops down from wide open)
When a lens is subjected to a comprehensive resolution test it's always the case that maximum resolution is reached with the lens wide open and by the time a lens is stopped down to f16-f22 the resolution is limted by Rayleighs law and resolution is effectively limited by the equation. In fact a lenses resolving power decreases as the aperture is made smaller, however other optical abberations often mask the decrease in resolution at larger apertures.
NOTE: This is a greatly simplified version .. I don't have a couple of hours it would take to write up a really comprehensive answer (with the necessary mathematics and explainations etc) at present due to time constraints.
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