Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
OP Chikoo Senior Member • Posts: 1,630
Re: Fast lenses, and High ISO & Transmission losses

EinsteinsGhost wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses?

The issue is primarily due to f-stop performing double duty in photography (video appropriately uses t-stop for exposure).

F-stop is primarily about DOF, and t-stop is about speed. But since most photography lenses are not specified in t-stop (in fact, the only lens I know, and have it, is Sony 135mm f/2.8[T4.5] STF where the t-stop is also specified).

Transmission losses. That is a wonderful aspect that I did not give due consideration in this rambling of mine, and opens up the door to a whole another discussion regarding flash & studio lighting. If you have your camera + flash program setting based upon a certain transmission loss, then you will most likely see over exposure or underexposure if using the camera's built in program features when you switch lenses for the same aperture and shutter speed. Which also brings into the picture, the number of elements. The more the elements, the higher the transmission losses for the same optical material in use.

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