guide to effects on f stops

[stigmundfreud]

Quite often its seen that at wide apertures that CA can be a problem as can other issues with softness, same is said that a good lens offers good results at f11 etc

does anyone have a link to a good guide to explain the effects beyond dof ans shutter speed than the aperture can have on the final result/image quality?

Thanks

[paul911]

Does this help? It’s aimed at Cannon cameras but the logic should be the same!

http://web.canon.jp/imaging/enjoydslr/part2/2Ac.ht...

[stigmundfreud]

will look thanks!

[VxDuncan]

I think in its simplest terms, the closer to the edge of the lens the light ray hits, the more it has to be refracted to produce an in focus image. Therefore any abberations (errors) in the lens are magnified, also the greater the effects of differing wavelengths (colours) of light being focused, as different wavelengths will be refracted by different amounts.

This is the cause of CA - it's impossible to create an optic that refracts all wavelengths by the same amount - therefore you have Acrhomatic lenses (that do a fairly good job) and APOchromatic lenses that are classed as focusing all wavelengths to the same point (within a tiny amount). Usually this is done by using exotic glass - ED (extra low disspertion (of different wavelengths)) and SLD (Super Low dispersion).

The coating on the elements also has an effect, as it's thickness is optimised to the wavelength of the light passing through, hence multicoating for different wavelengths. This assumes and angle of incidence of 90deg, but at wide appertures this ain't quite the case, hence you get some fall off.

Then you get spherical abberations, caused by the shape of the glass which can produce "coma", which is similar to a star burst filter at the edges.

Basically by using the middle of the lens you use the bit that curves least and is less comprimised that the outside!

Would be interested in a decent guide though...

[ThatPhilBrettGuy]

Don't forget about diffraction effects though. Stopping down only works to a certain level, then diffraction effects start killing the image. Here's a good read on it.

http://www.bobatkins.com/photography/technical/dif...

[jon-]

VxDuncan said:

I think in its simplest terms, the closer to the edge of the lens the light ray hits, the more it has to be refracted to produce an in focus image. Therefore any abberations (errors) in the lens are magnified, also the greater the effects of differing wavelengths (colours) of light being focused, as different wavelengths will be refracted by different amounts.

This is the cause of CA - it's impossible to create an optic that refracts all wavelengths by the same amount - therefore you have Acrhomatic lenses (that do a fairly good job) and APOchromatic lenses that are classed as focusing all wavelengths to the same point (within a tiny amount). Usually this is done by using exotic glass - ED (extra low disspertion (of different wavelengths)) and SLD (Super Low dispersion).

The coating on the elements also has an effect, as it's thickness is optimised to the wavelength of the light passing through, hence multicoating for different wavelengths. This assumes and angle of incidence of 90deg, but at wide appertures this ain't quite the case, hence you get some fall off.

Then you get spherical abberations, caused by the shape of the glass which can produce "coma", which is similar to a star burst filter at the edges.

Basically by using the middle of the lens you use the bit that curves least and is less comprimised that the outside!

Would be interested in a decent guide though...

That is a decent guide. Good write up!

[stigmundfreud]

ThatPhilBrettGuy said:

Don't forget about diffraction effects though. Stopping down only works to a certain level, then diffraction effects start killing the image. Here's a good read on it.

http://www.bobatkins.com/photography/technical/dif...

excellent guide there! Intersting to see the difference between the L and the IS too!!!

[ThatPhilBrettGuy]

stigmundfreud said:

ThatPhilBrettGuy said:

Don't forget about diffraction effects though. Stopping down only works to a certain level, then diffraction effects start killing the image. Here's a good read on it.

http://www.bobatkins.com/photography/technical/dif...

excellent guide there! Intersting to see the difference between the L and the IS too!!!

Yup. The description of what diffraction actually is isn't great, but he gets the point across really well I think.

It's amazing how few people know about the effect at all. Even some pro's, when you point out to them that shooting at F22 might give them a big DoF but there are diffraction downsides, just give you blank looks.

[stigmundfreud]

Certainly is something we'd never covered so it was good to learn and something that I'll keep into consideration for the future!

Thanks