Does opening the aperture improve color contrast? (Open aperture producing more punchy colors / closed aperture more dull colors?)

Question

I'm using some SLR legacy lenses on a mirrorless camera that I have, and I am under the (subjective) impression that if the aperture is wide open, the colors are more vibrant, while closing the aperture a few stops down causes them to become dull and less saturated.

This is all anecdotal, since whenever I tried putting it to the test, I haven't been able to verify this. (By varying the shutter speed to retain the same ISO and exposure level)

I've discussed this with a friend, and while he has a similar impression, he can't explain it either; I haven't been able to find anything on the topic of aperture affecting color contrast anywhere.

Anybody that could confirm (or disprove) this theory? Since this is more of an impression, I don't expect it to be necessarily correct; I'm more interested in seeing if this is the common experience and whether there is something in the way lenses are constructed that would explains that, not in my particular case, but in general.

Setup: I'm using a canon fd 50mm 1.8, a Canon FD 135mm F/3.5 SC and a Pentax 50mm f/1.7 SMC-M on a MFT Mirrorless body. Simple adapters with no optics, no light leakage (tested with long exposure times in the dark and flashlight shining at the seams). No visible light source in/around the frame that would cause obvious flare — no filters, even though all these would reduce contrast, not increase constrast. I have a similar impression from using my native 14-42mm ez (3.5-5.6) lens: it feels as if it has much duller colors overall than the bright adapted lenses.

Answer 1

Conventional wisdom says that, if there is any difference at all, the difference would be in the opposite direction from what you are describing.

At first glance the opposite of your impression might be expected when using a 135 film format lens on a smaller sensored digital camera.

If you are shooting in a situation where there is a strong light source (or multiple strong light sources) inside the field of view or just outside the field of view then lens flare, particularly veiling flare, will generally be worse at larger apertures and would reduce overall contrast which might also be perceived as making the colors less saturated.

This would especially be the case when using a lens made for a larger format than the camera with which it is being used. This is because the lens is collecting light over a wide enough field of view to cover a larger sensor than the one in the camera collecting only the center portion of the image circle cast by the lens. All of that light is entering the lens, even if it is not being directly projected onto the smaller sensor. Some of the light from the wider field of view may be bouncing around and getting reflected inside the lens.

Lenses designed for smaller sensors can be made so that they block off-axis light from just outside the field of view that is coming from the same angles that would be included in the field of view for a same focal length lens designed for a larger sensor.

But since you seem to be indicating the opposite, that is not the source of the phenomenon you've described.

Another consideration when using a legacy 35mm film camera lens on a digital mirrorless camera is the adapter required to position the lens at the correct registration distance from the camera's sensor. If the inside of the adapter tube is reflective, it could be allowing light from the larger image circle cast by the lens to bounce around and cause flare which would reduce contrast and possibly cause a color cast that would tend to make the image look less saturated. This would probably be more noticeable at wider apertures because the light projected by the lens is less collimated at wider apertures than with a narrower aperture.

Absent of any details regarding the particular lens, adapter, and camera as well as the lighting conditions and some sample images that's about the best guess I have.

Answer 2

As you mention a subjective impression that you cannot reproduce when trying to test it, I may have an explanation.

Maybe this occurs when you are in a brightly lit environment? (outside by a very sunny day, for example).

Having a smaller aperture makes less light enter the camera, which saturates less and may feel more balanced (a bit like using polarized sunglasses can makes things (the blue sky and little clouds, especially) better "defined", contrasted, vibrant, to your eyes, than without those sunglasses, by both the lowering of the amount of lights on the retina and the polarization as well)

In other words it helps by having a less "oversaturated" image. (I'm not sure I use the right terms)

Answer 3

Optical flare is devastating as it reduces overall image contrast. Let me add, the adverse impact of flare is often most visible in the shadow areas. Optical flare is due to stray light rays that intermingle with the image forming rays. The chief cause is the fact that a modern camera lens is comprised of numerous lens elements. Some are cemented together; others are air-spaced apart. Each element has two polished glass surfaces that reflect away about 6% of the light. A high percentage of these vagrant rays will bath the image sensor (or film).

We employ countermeasures: Lens hoods to block light originating outside the actual subject area and we coat the surfaces of lenses with thin transparent films that mediate internal reflections. We also coat the interior of the camera flat black and install light baffles in an attempt to diminish stray light reflections. Despite all efforts, flare diminishes the contrast of our images.

So the question asked if the aperture diameter is a contributing factor. All lens systems are hurt by aberrations. There are 7 major aberrations. Five are independent of color and two color related aberrations. It is a fact that modern camera lenses are well corrected but aberrations linger. When a modern camera lens is stopped down one f-stop from its maximum working diameter, the impact on the bad effects of these aberrations is cut 50%. By stopping down two f-stops, the typical camera lens is now at optimum as to making a faithful image.

The bottom line: As you stop down moderately image quality increases. As you continue to stop down the image begins to deteriorate. This is due to two phenomenons. Diffraction – light rays that brush the margins of the aperture blades, tend to spread out and become stray, they intermingle the image forming rays. Interference – light rays intermingle and are out of phase. Where wave crests meet they reinforce otherwise they can cancel. As the lens diameter is stopped down, a greater percentage of rays blush the aperture blades.

I think you are on target for a lens one or two stops down from maximum.