Chromatic aberrations, light wave lengths and the difference between film and digital

[Hamish Gill]

I am lead to believe that digital cameras are more susceptible to chromatic aberrations. i understand that this is because they are flat compared to the multi layered nature and thickness of the emulsion of film. and somthing to do with differing wavelengths of light...
can anyone expand on this and help me get a better understanding of what is happening... or have i just got it all wrong

 

[Vic Shaw]

As I understood it has something to do with the lenses over sensor pixels, but what do I know? I can barely work my camera!

 

[Hamish Gill]

yeah... that sounds familiar

 

[Vic Shaw]

so I guess with the light being at different wavelengths there's already separation going through the lens then this is compounded by the sensor lenses. If only we had knowledgeable people on this forum!

 

[Hamish Gill]

we do... they have gone quite... must be reading up on google

 

[Graeme Harvey]

This should clear it up...

The term "purple fringing" is commonly used in photography, although not all purple fringing can be attributed to chromatic aberration. Similar colored fringing around highlights may also be caused by lens flare. Colored fringing around highlights or dark regions may be due to the receptors[clarification needed] for different colors having differing dynamic range or sensitivity -- therefore preserving detail in one or two colour channels, while "blowing out" or failing to register, in the other channel or channels. On digital cameras, the particular demosaicing algorithm is likely to affect the apparent degree of this problem. Another cause of this fringing is chromatic aberration in the very small microlenses used to collect more light for each CCD pixel; since these lenses are tuned to correctly focus green light, the incorrect focusing of red and blue results in purple fringing around highlights. This is a uniform problem across the frame, and is more of a problem in CCD's with a very small pixel pitch such as those used in compact cameras. Some cameras, such as the Panasonic Lumix series and newer Nikon DSLRs, feature a processing step specifically designed to remove it.
On photographs taken using a digital camera, very small highlights may frequently appear to have chromatic aberration where in fact the effect is because the highlight image is too small to stimulate all three color pixels, and so is recorded with an incorrect color. This may not occur with all types of digital camera sensor. Again, the demosaicing algorithm may affect the apparent degree of the problem.

OK, so I had a bit of help....

http://en.wikipedia.org/wiki/Chromatic_aberration

 

[Vic Shaw]

Bah and you'd think they would be able to ad-lib it.

 

[Vic Shaw]

On photographs taken using a digital camera, very small highlights may frequently appear to have chromatic aberration where in fact the effect is because the highlight image is too small to stimulate all three color pixels, and so is recorded with an incorrect color. This may not occur with all types of digital camera sensor. Again, the demosaicing algorithm may affect the apparent degree of the problem.

I have notice this alot

 

[Pete Askew]

Sorry guys, I was too busy making rude remarks elsewhere!

That Wikipedia entry is pretty comprehensive although it tends to focus (!) on the effects at the interface with the sensor / IR filter / antialiasing layer / microlens. Before the light strikes those layers of course defects / design limitations within the lens come into play and can result in separation of light by wavelength as it passes between boundaries caused by the various elements used in the design of the lens. It is not that a digital camera is more susceptible to the effects of chromatic aberration (they are no more so than film if the source of that aberration is the lens), it is that other effects occur at the sensor (that may exacerbate the issues). Each of these (aberration in the microlens, 'leakage' between pixels, refraction at the interface with the antialiasing filter / IR filter, saturation of the sensor) can add their own colour anomaly to the final image.

In some ways that is why small sensors are more susceptible to such effects than larger and the sensors used in MF capture backs, which lack antialiasing filters and microlenses, are so much less affected and produce such high resolution images with low noise and little chromatic error.

 

[Hamish Gill]

I thought the multi layered nature of colour film helped .. No?

 

[Pete Askew]

Not really no. In some ways it could make matters worse as the focal point of the various wavelengths would differ but not necessarily to the same plane as occupied by the 'colour' layers.

 

[Hamish Gill]

I see... So Is it not the case that film is more forgiving of lenses than digital, or is that for different reasons?

 

[Graeme Harvey]

It's probably more forgiving due to the analogue way the information is collected - you'll get the correct colour if a little unfocused, rather than the correct light falling on the "wrong-coloured" detector and therefore going astray

 

[Hamish Gill]

Sounds reasonable ...