HK Camera Life: Myth Busting: Lens Contrast

[Garfink]

Hiya everyone, I've learnt tons from reading the forum of late and I figure I will contribute something back. So I have started a blog about photography. Its called: HK Camera Life, here is an example article that I have written. Please pay me a visit and tell me what you think! Any feedback would be greatly appreciated.

Myth busting: Lens Contrast

There is often debate on many forums about the topic of lens contrast. In this modern age of lenses, the fidelity of lens contrast is getting higher and higher. The reason for this increase contrast is better control of flare. This flare is caused by stray light that doesn’t come from the reflected light from the objects in the scene and hence doesn’t contribute to the photographic details of the photo. This light from high angle of incidence enter the lens through the front element and bounce around inside the lens barrel and cause reflections on the internal elements and some will reach the sensor or film and exposure the film/sensor in a random but uniform way. This stray light causes the photograph to become lighter in color and tone overall, especially obvious in the dark areas of the photo. Obviously there is a cost to this flare, it veils the midtones and light tones and the dark tones, as a result there is some lost in details or dynamic range all areas. This loss in detail is less apparent in the light areas.

There are two improvements in the modern lens that impact this the most, one is the modern multi-coating on the front elements that disperses the light coming from high angles and the improve absorptive properties of the inner lining of the lens barrel. This results in a high contrast and a greater dynamic range. There is talk that goes around that says that the blacks are blacker, but even though this is true in an empirical sense, a better description and a more accurate description is that the blacks appear blacker because there is less flare. If you consider it in terms of the tonal curve, for any given picture, a high contrast lens with low flare will have a wider dynamic range, which is overall a good thing. In other words: A high contrast lens will provide more tonal information than a low contrast lens.

So why are there so many advocates for low contrast, antique lenses? Why do photos look better sometimes with a low contrast lens?

The answer lies in the limited dynamic range of our sensors and the even more limited range of film. If the curve is wider you will exceed the ability for the sensor/film to capture the dynamic range of the scene. I like to think of it this way: I sometimes use HDR techniques in processing of some photos to improve the dynamic range, the HDR photo that results have too many colors & tones for the monitor to display and you have to remap the colors to shrink the dynamic range to usable levels (within the range of the color space you’re using). There exists very expensive HDR monitors that can exhibit this HDR photos in their original glory, but since these monitors are not in general circulation at the moment we have to limit the dynamic range. The same thing occurs with a low contrast lenses: you essentially are compressing the dynamic range at the level of the lens by truncating the blacks in the photos, this is obviously advantageous with film where the dynamic range of the film is very low and the end result less malleable unlike a RAW file. Traveling with this train of reason, low contrast lenses will benefit positive slide film when compared to negatives, because the dynamic range of negatives are better.

The advantage of low contrast for modern digital sensors is dependent on the scene being photographed. A low contrast scene like my photo of the bas-relief at Angkor Wat is low contrast to begin with, almost the whole scene is uniformly lit & in the shade and the use of low contrast lenses will make this scene look very flat. The bas-relief will look less 3D with less pop and this will result in less impact.

For a very sunny high contrast scenes with details hidden in the shadows and much of the scene are highlights, the scene is beyond the dynamic range of the sensor and as a result of using a low contrast lens, we compress the dynamic range at the level of the lens and when balanced with the degradation of information due to flare, we may still benefit from using a low contrast lens. So my street scene of Lhasa taken with the more nostalgic Summicron would benefit even more from a lower contrast lens.

I think that even for a medium contrast scenes, digital sensors are better off with high contrast lenses.

In the future as the dynamic range of our digital cameras improves, the need for low contrast lenses will decrease. Even in the present, with judicious use of HDR techniques this is already true. Will this make low contrast lenses obsolete one day? Yes and No. Yes for digital and forever no for film users.

 

[Derrel]

You're onto something about the way a low-contrast lens handles wide DR scenes. Another example you could mention would be that relating to the way in-camera use of diffusion filters creates a much different appearance and result than trying to imitate in-camera diffusion in post-processing. Using a diffusion filter over the camera lens can cause light from the highlights to spill over in to the shadow areas, lowering overall scene contrast, and actually altering the image the sensor records. Trying to imitate the effect later by simply re-arranging some pixels does not create the same effect.

 

[Garbz]

Ahh but yes and no.

You see flare does not compress dynamic range. It all comes down to this comment:

There is talk that goes around that says that the blacks are blacker, but even though this is true in an empirical sense, a better description and a more accurate description is that the blacks appear blacker because there is less flare

While what you say here is true, the simple fact is photography is WYSIWYG. What you see is what you get. So yes the black are blacker because they appear blacker. In a true sense you are widening the dynamic range.

However with the onset of flare and lower dynamic range, no compression takes place. The finite darkness of the blacks is determined by the light hitting it. You lose detail and blacks by introducing a different source of light.

This is not compressing dynamic range, this is limiting dynamic range by clipping the lower bounds at a higher value. The kicker here is that one can easily eliminate the detail that is there, but can't create the detail which isn't.

What this means is that if you take two identical images with a "low contrast" lens and a "high contrast" lens but expose the visible highlights to the same level, one will have blacker blacks, the other won't. However the one that has blacker blacks has detail in the shadows that can be either lifted out of the shadows, or just clipped out of the shadows in post processing.

You can make a high contrast lens look like a low contrast lens, but you can't make a low contrast lens look like a high contrast lens.