Effect of Aperture on contrast

[Justin]

especially on strongly backilit subjects.

I discovered this when trying to take a shot of the colosseum with the sun setting behind it. On full auto (out of frustration with manual exposure, to see what the camera would do), I would get better foreground exposure than the best I could set manually. Main difference between my manual setting and camera auto being the aperture.

My after-the-fact reasoning is, could this be that, the more light let in (e.g. f3.5), the more the light will burn and eventually blow the highlights, therefore a smaller aperture (e.g. f8/f11+) will let less of the stonger backlight in whilst allowing a longer exposure for the foreground...

...However a smaller aperture affects the entire picture, so there must be something else going on here optically for this to work.

So... what does anyone know about the effect of aperture on contrast and exposure characteristics and how far off is my reasoning?

[sheepie]

Good question - I don't have the answer, but I bet someone here will. *** Bumping *** this one so that it gets a second chance of being seen!

[gstark]

Justin,

I think that the contrast changes you're seeing are a red herring.

The issue is that you have a scene with a vast contrast range that needs to be covered - far greater than what your sensor is capable of recording.

As you bring the exposure into the range of what the correct exposure needs to be, you will, by definition, start to see more detail in those areas that are within shade, whereas those areas that are outside of the shade are .... blown.

What metering modes were you using, and why is it that you think that the camera was metering better than you were capable of doing?

[Steffen]

Justin,
the aperture doesn't affect the dynamic range of the sensor (the darkest and brightest light values it can distinguish) as long as the exposure remains the same.

Stopping down a lens can help image contrast, though, since it cuts down on internal reflections. Images marred by internal reflections in the lens look washed out. That's probably what you are seeing. The stray light hits areas of the frame that would otherwise remain darker...

Stopping down generally also improves resolution (until the aperture gets so small that diffraction takes over as resolution limiter), which is sometimes perceived as sharpness, sometimes as contrast.

Cheers
Steffen.

[Justin]

well, going through my photos last night it appears I deleted the manual exposure, so I can't even prove any of what I said above, pretty hopeless. Let's hope it wasn't just my memory addled by Tuscan sun and wine....

However, 'contrast' is probably the wrong description, which I think you incisively understood. This is about exposure - I will try to take some test shots this weekend to demonstrate what I think I mean.

Cheers,
J