Fast lenses, and High ISO

Started 2 months ago | Discussions thread
EinsteinsGhost
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Re: Fast lenses, and High ISO
In reply to Chikoo, 2 months ago

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).

That being said, while faster lens can make us worry less about high ISO, having high ISO access without worries can allow you even more freedom. In fact, my first digital camera is still with me 10.5 years later by virtue of its faster zoom lens and infra red capability: Sony Cybershot F828. It has a 28-200mm equiv lens with f/2-2.8. This allowed to extend the life of the camera even though I would use ISO 400 as I would ISO 6400 on my APSc cameras today. A vast majority of shots involve ISO 64 or 100.

Now, I took some available light family portraits 750ft below ground at Carlsbad Caverns recently using Sony NEX-6. There isn't much light there (in this case, there was some light from the cafe but still too dark to see in person). I had Minolta 50/1.4 on Speedbooster, which gave me an effective 35mm f/1 lens. I used ISO 3200 and still had only 1/30s for shutter speed (could have improved it a bit with spot metering but some of the ambiance would be lost). The scene brightness value was -5EV. This is an example of when just a superfast lens or high ISO capability worked better when combined.

Would I have loved having Sony a7s instead? You bet! Would have gotten same exposure at ISO 6400 which is impressively clean and composed in that camera.

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