softness <-> aperture


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jonnie84

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#1
i've been seeing that many mention pictures taken at eg. f1.8 is quite soft, so therefore many photographers dun utilise that and use a smaller aperture, like f2.8.

sorry but someone enlighten me on this? has this got to do with the lens? or is it just common among apertures? if so, which aperture is the sharpest? if the lens, so we'd have to play around and figure out the optimum aperture?

how come at f1.8 is softer than at f2.8? ok maybe this is more technical? dispersion of light or something like that?
 

espn

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#2
jonnie84 said:
i've been seeing that many mention pictures taken at eg. f1.8 is quite soft, so therefore many photographers dun utilise that and use a smaller aperture, like f2.8.

sorry but someone enlighten me on this? has this got to do with the lens? or is it just common among apertures? if so, which aperture is the sharpest? if the lens, so we'd have to play around and figure out the optimum aperture?

how come at f1.8 is softer than at f2.8? ok maybe this is more technical? dispersion of light or something like that?
Physics actually.

The center of a glass is always the sweetest at f/8+. So as you step down the aperture, more of the center is used thus your image comes out more so-called sharp as the light is more concentrated to a point. Image takes more time to form when aperture used is smaller too, so the DOF is less.

For larger apertures, a lot of available light, so the intended image forms faster, the background DOF is more too.

Most zoom glasses don't perform well at wide open but of course there ARE exceptions (eg: AF-S 70-200VR, AF-S 17-35, AF-S 17-55). Or rather most zoom glasses with variable aperture values (eg: AF-S 24-120VR f/3.5-5.6) don't perform well wide open as the glass used is not that perfect.
 

Snoweagle

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#3
Cos the wider the aperture (e.g. f/2.0 and below), the depth of field is a lot shallower than compared to f/2.8 and above. Therefore usually pics look soft at wide aperture ends.
 

jonnie84

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ohh i see...so other than the quality of glass affecting, i gather that at bigger apertures, rather than say not sharp, is just that the area that is sharp is reduced due to shallow DOP. right?
 

lsisaxon

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Nov 29, 2004
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#5
espn said:
Physics actually.

The center of a glass is always the sweetest at f/8+. So as you step down the aperture, more of the center is used thus your image comes out more so-called sharp as the light is more concentrated to a point. Image takes more time to form when aperture used is smaller too, so the DOF is less.

For larger apertures, a lot of available light, so the intended image forms faster, the background DOF is more too.

Most zoom glasses don't perform well at wide open but of course there ARE exceptions (eg: AF-S 70-200VR, AF-S 17-35, AF-S 17-55). Or rather most zoom glasses with variable aperture values (eg: AF-S 24-120VR f/3.5-5.6) don't perform well wide open as the glass used is not that perfect.
PRO explanation.. with espn I learn something everyday! :thumbsup:
 

Snoweagle

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#7
jonnie84 said:
ohh i see...so other than the quality of glass affecting, i gather that at bigger apertures, rather than say not sharp, is just that the area that is sharp is reduced due to shallow DOP. right?
In summary, it's the shallow DOF.
 

lsisaxon

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#8
jonnie84 said:
i've been seeing that many mention pictures taken at eg. f1.8 is quite soft, so therefore many photographers dun utilise that and use a smaller aperture, like f2.8.

sorry but someone enlighten me on this? has this got to do with the lens? or is it just common among apertures? if so, which aperture is the sharpest? if the lens, so we'd have to play around and figure out the optimum aperture?

how come at f1.8 is softer than at f2.8? ok maybe this is more technical? dispersion of light or something like that?
It is called spherical aberration or coma.
 

jonnie84

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#9
lsisaxon said:
It is called spherical aberration or coma.
wa cheem. haha... yup think i'm clear bout this liao. thanks to all the PROs who replied!!
 

lsisaxon

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#11
espn said:
Chroma or coma?
Coma. A term used when you have a primary focus and a secondary focus because of the spherical surface and spherical aberration. Aspherical elements used in modern lenses are to avoid these.

http://www.astrosurf.com/lombry/report-aberrations2.htm
See Spherical Aberration.

Another one.
http://www.opticsforteens.org/what/geo-pg7.asp

The spherical aberration correction will also affect the bokeh of the lens. When you cloe down the aperture, the area of the lens used to form the image is reduced so the effect is reduced.

Edit: This is a better read. http://www.vanwalree.com/optics/spherical.html
 

unseen

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Dec 14, 2004
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#12
espn said:
Physics actually.

The center of a glass is always the sweetest at f/8+. So as you step down the aperture, more of the center is used thus your image comes out more so-called sharp as the light is more concentrated to a point. Image takes more time to form when aperture used is smaller too, so the DOF is less.
Can someone explain why is the center of a glass sweetest at F/8+?
Can someone direct me to a scientific reference?

The only physics theory that I've found applicable so far is that for many high megapixel cameras with small sensor size, diffraction occurs above F/8 or so.. That's the theory we use to calculate minimum aperture possible for our experiments in uni for our 1/3 inch ccd cctv cameras.
 

lsisaxon

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#13
unseen said:
Can someone explain why is the center of a glass sweetest at F/8+?
Can someone direct me to a scientific reference?

The only physics theory that I've found applicable so far is that for many high megapixel cameras with small sensor size, diffraction occurs above F/8 or so.. That's the theory we use to calculate minimum aperture possible for our experiments in uni for our 1/3 inch ccd cctv cameras.
Probably that region for spherical surfaces is able to approximate elliptic optics so that the focus for rays passing through the centre of the lens and the rays passing through the edges of the aperture does not differ much.

If you read http://www.vanwalree.com/optics/spherical.html on the paragraph under Focus shift, you will find that there is a possibility that the focus point may shift if you used small aperture. This will definitely affect SLRs which uses the maximum aperture to focus then stopping down during the exposure. In this case, the focal point may have already shifted.

For very small pixel dimensions, yes, the optical system is also diffraction limited.
 

Apr 12, 2005
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#14
unseen said:
Can someone explain why is the center of a glass sweetest at F/8+?
Can someone direct me to a scientific reference?

The only physics theory that I've found applicable so far is that for many high megapixel cameras with small sensor size, diffraction occurs above F/8 or so.. That's the theory we use to calculate minimum aperture possible for our experiments in uni for our 1/3 inch ccd cctv cameras.
As far as I can remember what I've read before, it has something to do with the different refractions of different light colour wave lengths. When light hits the glass lens at a more acute angle (i.e. away from the centre the lens), the differences in refractions between different colour wave lengths are greater and so the image formed on the image sensor or film is less sharp than when light is concentrated around the centre of the lens.

So apparently, from the above, the actual aperture size used relative to the size of the image sensor will affect the sharpness of the image and so a different breed of lenses has been developed specifically for digital cameras which have smaller than full frame sensors. E.g Nikkor DX.
 

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