t-stop and constrast

[J Costantini]

Hi, this is more like a concept review than a technical question.

in a lens, i understand the relation between t-stop and resolution of the picture, according to the circle of confusion and optical characteristics (real construction of the elements...)

BUT my question is: how can a change of t-stop T1.3 and T11, for instance, affect the CONTRAST of the image? Does it have anything to do with the interior reflections due to the big or small opening? or maybe the way the light rays touch the emulsion?

 

Thanks

[Chris Keth]

Hi, this is more like a concept review than a technical question.

in a lens, i understand the relation between t-stop and resolution of the picture, according to the circle of confusion and optical characteristics (real construction of the elements...)

BUT my question is: how can a change of t-stop T1.3 and T11, for instance, affect the CONTRAST of the image? Does it have anything to do with the interior reflections due to the big or small opening? or maybe the way the light rays touch the emulsion?

 

Thanks

 

On modern lenses T-stop setting on the lens doesn't affect contrast, only places the scene brightness. On older lenses, shooting wide open or nearly so can result in slightly reduced contrast because of poor optical corrections that scatter light somewhat.

[Satsuki Murashige]

Does it have anything to do with the interior reflections due to the big or small opening?

Yes, you're basically describing lens flare, which as you probably know lowers contrast and becomes increasingly out of focus the wider you open the aperture. This creates a general veiling glare or flashing effect, effectively milking out the blacks in the image. Modern lens designs combat this effect in two ways: first, the lens coatings are extremely efficient and cut down on internal reflections inside the lens, reducing the amount of multiple reflections that create the general soft flare and leaving only the hard "shaped" flares that appear around point sources in or just out of frame. Second, lenses like the Cooke S4s are never fully wide open optically - the iris is still closed down a bit at T2. This accounts partly for the large size of the lens barrel relative to older lens types, since the f/stop is partly determined by the size of the front element. So while they probably could have made the S4s T1.4 lenses, I think they decided to keep them at T2 and increase their performance at that stop you could use them wide open without lowered contrast or sharpness. I suspect the Zeiss Master Primes are designed in a similar way.

[David Mullen ASC]

There is an old rule or concept that stopping down too much increases contrast outdoors.

 

Truth is that a shallower depth of field gives the illusion of less overall contrast, ignoring the extra flare when shooting wide-open that may lower the contrast.

 

If you look at an image on a waveform monitor and then throw the image in and out of focus, you'll see what I mean -- as the image goes completely out of focus, the highlights and shadows blur together. The net result is that the peaks and valleys on the waveform are the nearly same but the majority of the information seems crammed into the middle luminance values. You could see this if you shot a chart of black and white bars - when the chart is in focus, you see the whites and the blacks very clearly on the waveform. When you throw the chart out of focus, it seems to be one big grey field.

 

So I think this is the reason why, when outdoors, if you shoot at a wider aperture for a shallow-focus look, the image seems less contrasty. On the subject itself, the contrast is really about the same, but now the background is softer, blurrier, and thus less harsh in contrast because the values have muddled together somewhat.