Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 58,507
Re: "fast" is relative

EinsteinsGhost wrote:

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.

So you keep saying, but that is not consistent with your definition of lens 'speed', that it allows you to use a fast shutter speed.

You reduce exposure time value by increasing "sensitivity" of the sensor/film.

You really don't The sensitivity of the sensor never changes. You reduce exposure time value by deciding to use a smaller exposure.

This is why people like to use the term "Exposure Triangle".

Many people get very misled by the 'Exposure Triangle'. Looks like you are one of them.

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.

You need a new definition of 'fast' then, because you can set the same shutter speed with both.

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.

I trust you to be wrong.

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