Colour Saturation and Aperture

Not posted for a while, but I need an answer to question that has been bugging me!
What is the relationship between colour saturation and aperture ?
This applies to all camers but I use Nikon so posted here.

In theory, in P mode, exposure should be the same even at different shutter speeds and aperture settings.

I always knew this, and obviously aperture has a direct effect on depth of field and shutter speed on "stopping the action" for want of a better term.

However, whilst messing around with my 5omm F1.8 at the weekend, taking the same shot over and over again with the same ISO and but different "P" settings, colour saturation differs from shot to shot.
Same with Aperture priority mode....

My only asumption is that the longer the shutter speeds at higher f numbers (smaller aperure) gives the sensor longer "exposure" for colour saturation, but that is an educated guess and not something I have found in photography books I own.
Maybe I am just being a bit thick, but can anyone explain it any better ?!!
Thanks

JohnnyH said:

Not posted for a while, but I need an answer to question that has been bugging me!
What is the relationship between colour saturation and aperture?

Click to expand...

Changing DOF tends to change the character the out of focus areas.

JohnnyH said:

In theory, in P mode, exposure should be the same even at different shutter speeds and aperture settings.

Click to expand...

If lighting is constant and you are using a tripod. Otherwise you may in fact be comparing different exposure values.

JohnnyH said:

My only asumption is that the longer the shutter speeds at higher f numbers (smaller aperure) gives the sensor longer "exposure" for colour saturation, but that is an educated guess and not something I have found in photography books I own.

Click to expand...

My guess is that it has to do with the lens.
--
Anthony Beach

Contrast trumps saturation as saturation is really a type of contrast, within a colour range. Also, a change of just 1/3 stop in exposure or a slight difference in perspective can dramatically affect both contrast and saturation.
--
Jim

Aperture affects contrast a bit, but in my experience this varies between lenses. On many the variation is only slight. On my (old) Nikon 50mm f1.4, the contrast reduces at wide aperture. This is because the lens quality isn't so good at wide apertures. From f5.6 and up (bigger f-number) there is little change. On most newer lenses of mine there is little variation of contrast with aperture - maybe a little lower fully wide open. Lower contrast tends to appear as lower saturation too, as Jim says. Shutter speed shouldn't affect contrast or saturation.

However, as Jim also says, the appearance of colour saturation changes much more markedly with exposure. A higher brightness picture look less saturated.

Can't think of any other reason for the variation.
--
Simon

Thanks for your thoughts.

I think you are right, it is probably a contrast issue relating to aperure, giving the impression of increased saturation.

I will have another play around making sure the camera is on a tripod, clear skies, same lens, same ISO and just alter the aperture and see what happens.

I have been reading the Digital Photographers Guide to exposure by Peter Cope.

Excellent book, he talks about "EV" values rather than aperture and shutter speeds, which is how I see things, but it didn't explain why different EVz should have different colour saturations.

Apart from some contrast increases as you stop down, I would think that it could be related to the actual exposure.

Sure, the camera will vary the shutter speed to match the aperture, but on many lenses (your 50mm f/1.8 included), aperture is determined by the position of a mechanical lever. There will always be some slight variation between stops, and a small change in exposure will often show up as variation of saturation. Underexpose a bit and you often get more intense colors.

On most lenses, the variation is quite small (maybe + - 1/6th of a stop or so), but it is certainly not uncommon to see variations larger than this. On some lenses (especially 3rd party), + - 1/3 to 2/3 of a stop is not completely unheard of. Most people do not notice, unless you actually test for it -- you may be surprised if you start testing ;-)

If you have really old lenses (I think pre AI-s), the stop down lever is not reacting the same way as the AI-s and later ones, and there you may see even larger variations, unless you use the aperture ring to control it.

It is also possible that you have a sticky diaphragm that does not manage to close down fully in time for the shot. Some old 50mm f/1.8 AI and also some older 35mm f/2 had a habit of developing this. I had one 50mm that developed this problem, but it was correctable with some care. However, if you have a newer lens, this is unlikely to be the case.

-- Per.

JohnnyH said:

Excellent book, he talks about "EV" values rather than aperture and shutter speeds, which is how I see things, but it didn't explain why different EVz should have different colour saturations.

Click to expand...

The reduction in contrast (and saturation) at wide apertures is simply that lenses aren't as good at full aperture. The saturation and contrast don't otherwise vary with aperture or shutter speed per se, BUT do vary with the overall lightness of the image.

Try this: set aperture priority exposure and aperture to f/8. Then take a series of pictures of the same scene with exposure compensation ranging from -2 to +2. You'll see that the under exposed pictures will not only look darker, but will have higher contrast and higher saturation. Similarly, the over exposed ones will have lower contrast and saturation. I'm not sure if it's physics or human perception (the two are not always easy to distinguish) but it's what happens.

--
Simon

Simon Garrett said:

Try this: set aperture priority exposure and aperture to f/8. Then take a series of pictures of the same scene with exposure compensation ranging from -2 to +2. You'll see that the under exposed pictures will not only look darker, but will have higher contrast and higher saturation. Similarly, the over exposed ones will have lower contrast and saturation. I'm not sure if it's physics or human perception (the two are not always easy to distinguish) but it's what happens.

--
Simon

Click to expand...

Hi Simon, I can understand what you said, but in this example you are actually altering "Exposure". An under exposed shot at -2 will be "darker" have different saturation to one at +2.

But, if we assume that a modern DSLR in P mode will correctly meter an image and set the correct "Exposure Value" in terms of f-stop and shutter speed, why is it, that when I rotate the command dial, then the aperture and shutter speed will obviously switch to a different values, BUT the "Exposure Value" should remain the same.

So apart from depth of field and "stopping the action" all colours and everything else should remain the same.

But if you try it, there are always subtle differences in colours and yet the "Exposure Value" should be exactly the same ie. The same amount of light is reaching the sensor.

It's interesting isn't ?
John

JohnnyH said:

So apart from depth of field and "stopping the action" all colours and everything else should remain the same.

But if you try it, there are always subtle differences in colours and yet the "Exposure Value" should be exactly the same ie. The same amount of light is reaching the sensor.

Click to expand...

Did you see my reply above?

In short: Different apertures may result in different exposures, even if the shutter speed is varied to compensate.

The reason is that the effective aperture may not be the exact aperture the camera tries to select. The aperture is controlled via a small mechanical lever and even minute variations in the position can result in some deviation from the selected value.

-- Per.

Per
OK, i think I understand what you are saying.

So in effect, although the "Exposure Value" seems the same with equivalent aperture and shutter speeds, the "actual" exposure between shots will be slightly different.

I guess camera metering has an effect on this as well, because we are limited by the fact that although an "ideal" exposure setting may be recommended by the camera meter, it may for instance fall in between available settings so a "next best" or "nearest" EV will be indicated.

If aperture and shutter speeds were more "linear" (analogue) in how we could change them rather than a "click" (digital) for each setting that would probably help and enable "fine tuning".
John

JohnnyH said:

Per
OK, i think I understand what you are saying.

So in effect, although the "Exposure Value" seems the same with equivalent aperture and shutter speeds, the "actual" exposure between shots will be slightly different.

Click to expand...

Yes, correct.

Obviously if you keep the camera on automatic mode, the actual calculated exposure can also change due to small changes in the scene, etc.

But if it truly has to do with variations in "achieved" aperture vs. "ordered" aperture, just try to put the camera on manual, and do a series of identical EV of, say f/2@1/2000th, f/2.8@1/1000th, f/4@1/500th, ... f/11@1/60th, ... or whatever would be a good exposure of the scene. This will rule out any effects from the metering (but obviously requires a constant scene, more or less).

-- Per.

You might be seeing the effect of vignetting here, JohnnyH. Your f/1.8 lens suffers from heavy light fall-off at its full aperture (f/1.8).

Because you used programme and aperture-priority modes to set the exposure, rather than manual, the camera may have overexposed the centre of the frame with the lens at wider apertures. This is because it tries to average out the scene, and if the corners are very dark it tries to give more exposure to compensate, even if that means overexposing the centre a bit.

As you stop down the lens, the fall-off in the corners diminishes, leading the camera to give less exposure (at centre of the frame). And as has been noted, less exposure may give the impression of higher colour saturation. This would explain why you're seeing higher saturation at higher f-stops (smaller apertures).

To keep the exposure the same between multiple shots at different apertures, you absolutely must use manual exposure. The camera will assuredly not select the same exposure for each frame in aperture-priority or programme mode as you adjust the f-stop, due to the light fall-off issue and slight changes in what the meter sees (e.g. you might move the camera ever-so-slightly between exposures, or a leaf might be blown by the wind and reflect more sunlight, etc.).

What were the shutter speeds for each f-stop at which you took a photo? If the exposure drops at higher f-stops, I think we've discovered why the perceived saturation increased.

Thanks for your thoughts.
I will dig out the files and you know the settings.
John

Philidors shadow said:

You might be seeing the effect of vignetting here, JohnnyH. Your f/1.8 lens suffers from heavy light fall-off at its full aperture (f/1.8).

Click to expand...

Hm... the exposure is always measured before the lens is stopped down.

I don't really understand how the camera could the vary the exposure due to changes in vignetting/fall-off, as this would hence not be visible to the meter in any way?

But it is likely that I simply didn't understand the logic here or completely misunderstood your point, in which case I'd be happy to be corrected

-- Per.