Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
EinsteinsGhost Forum Pro • Posts: 11,977
Re: "fast" is relative

bobn2 wrote:

EinsteinsGhost wrote:

So what? If the speed of `the lens is according to the shutter speed, then I've increased the speed of the lens by increasing the 'sensitivity' (I haven't, by the way) because I can now set a faster shutter speed, have I not?

Yes, you can change ISO and increase or decrease the shutter speed. What does it have to do with whether the lens is fast or slow?

You say that the speed of the lens is determined by the shutter speed it allows you to set. If you increase the ISO it 'allows' you to set a faster shutter speed, therefore under your definition, you have made the lens faster, because now you can set a faster shutter speed.

NO. You don't make a lens faster by increasing ISO. You reduce exposure time value by increasing "sensitivity" of the sensor/film. This is why people like to use the term "Exposure Triangle".

Put it another way an f/2 at 200 ISO is as fast as an f/1.4 at 100 ISO because you can set the same shutter speed.

Shutter speed is as fast. An f/2 lens isn't any faster.

so the shutter speed can be twice as fast to achieve same brightness level in the image. Your change in exposure is only due to change in "sensitivity" of the media (we've discussed that several times before, haven't we? Funny though, you seem to be opposed to the idea of playing with exposure with ISO changes)

I don't know what you mean. I don't remember ever having expressed opposition to 'playing with exposure with ISO changes'.

Trust me, you will be going there sometime very soon.

Why should I trust you when you say I have said things that I haven't?

You don't have to trust me.

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