Fast lenses, and High ISO - Part III

Started Jul 23, 2014 | Discussions thread
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Chikoo Senior Member • Posts: 1,630
Fast lenses, and High ISO - Part III

Since the part II thread has reached it's limit, please continue the discussion here.

The last post in that thread Great Bustard wrote:

We can shoot whatever shutter speed we like at whatever aperture we like, within the constraints of possible shutter speeds the camera allows and apertures the lens allows (adjusting the ISO setting, of course, to get the desired brightness).

In addition, the same total amount of light falls on the sensor for *all* systems for a given scene luminance, aperture diameter (e.g. the aperture diameter at 50mm f/2 is 50mm / 2 = 25mm), shutter speed, and lens transmission (t-stop vs f-ratio), resulting in the same noise for equally efficient sensors.

For example, if an mFT lens is t/2.5 at f/2 and a FF lens is t/5 at f/4, then photos of the same scene from the same position at 50mm f/2 1/100 on mFT and 100mm f/4 1/100 on FF will result in the same DOF (for a given display size, viewing distance, and visual acuity) and the same total amount of light falling on the sensor, which will result in the same noise *if* the sensors are equally efficient.

Thus, lens speed and exposure are only meaningful within the context of the visual properties of the photo. The reason we choose a faster lens is for a more shallow DOF or for less noise (although the lower noise *necessarily* requires the more shallow DOF). The reason we choose a greater exposure is also for less noise (although a greater exposure *necessarily* comes with either a more shallow DOF and/or a lower shutter speed, increasing the amount or risk of motion blur).

It is really that simple.

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