Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
bobn2 Forum Pro • Posts: 51,522
Re: "fast" is relative

EinsteinsGhost wrote:

bobn2 wrote:

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.

Nope. You simply don't get it. Unlike you, I won't run around screaming that my travel zoom is a fast lens when I use the camera at high ISO.

Well you should be, according to your own definition of 'fast'. I don't use your definition, so I don't have that problem. Remember we are here because you claimed that yours was the only definition of fast. Well, it had better not be because it's a pretty poor definition, as we've seen.

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.

Almost there! Where you didn't want to trust my prediction we were going.

So, why do you shoot Auto ISO?

By and large, I don't. I haven't found an Auto ISO which can be configured to do the right thing with respect to the exposure of the shot.

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.

Actually the argument in favor of Exposure Triangle looks like this: "If you increase the ISO it 'allows' you to set a faster shutter speed"

That's not an argument in favour of the 'Exposure Triangle'. Firstly the 'triangle' geometry doesn't work. Secondly, the effect of increasing ISO is very different from the effect of increasing exposure. Increasing ISO resally doesn't 'allow' you to use a faster shutter speed, it just signifies that you've decided to use a lower exposure, so yiou can set a faster shutter speed. So now we come to the next question, why might one not allow oneself to use a smaller exposure?

Do you agree?


I do.

That doesn't surprise me at all.

But, what is happening there?

What has happened is that your lack of knowledge of the basics of photography has allowed you to be deceived by a popular but flawed mnemonic graphic into thinking that the ISO control is 'allowing' you to use a smaller exposure,when all that is happened is that you have decided to use a smaller exposure and used the ISO control to set the camera up for that exposure.

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.

And yet, you couldn't use a faster shutter speed because you claim having reached a limit on your lens being f/4.5 .

Of course I could have set a faster shutter speed, but that would have reduced exposure. In manual, I can set any combination of values that I like. Looks to me like you camera is controlling you, rather than you controlling your camera.

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.

You don't have a choice.

I have a choice whether or not to trust you, though if I trust you to be wrong, I know that my trust will be rewarded amply.

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