# Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
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 Re: "fast" is relative In reply to bobn2, Jul 19, 2014

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

EinsteinsGhost wrote:

bobn2 wrote:

Albert Silver wrote:

tko wrote:

Remember that F4.0 is considered kind of slow on FF, but is equal to F2.0 on M43rds, which is considered "fast."

That's not entirely accurate. You are describing the depth of field equivalence, from one sensor to the next, not the light. f/2 on a m43rds may have the depth of field of f/4 on a full-frame, but the light will still be f/2.

The 'light' of a FF f/2 and a FT f/4 will be the same, which is the point he is making. In the end, given equally efficient sensors, you can achieve the same result at the same shutter speed using an f/4 on FF as you can on FT. The density of the light of the f/4 is one quarter but there is a sensor four times the area to collect it, so it ends up the same.

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Bob

The point tko is making is wrong. DOF equivalence applies, exposure equivalence does not (for the reason you state above).

Whether the point tko is making is wrong or not depends entirely on what you think is the definition of 'fast'. If you think that 'fast' is to do with exposure when comparing between formats, then he is wrong. However, that's not a very sensible point of view, so if you assume that he thinks that 'fast' means 'puts more light on the sensor' then he is right.

Fast has only one meaning:

Well that is obviously not true.

You're wrong about that, unless you mean that it is also wrongly used for DOF based arguments. Fast is about speed (faster the lens, shorter the exposure time for identical conditions).

There are many possible meanings to the word 'fast' so your statement is wrong. Even if we narrow it down to photographic use as in 'a fast lens' you are still wrong, because quite clearly different people mean different things by it. It is also a colloquialism, so you can't refer to an official definition. So your statement was both untrue and displayed an intolerance for opinions other than your own.

I don't care about different people. I only care about why I would call a lens fast. It only relates to it allowing a lower exposure time. Ideally though, as I mentioned in one of my first posts in this thread, I would rather see T-stop being specified. But, most people probably don't know that either.

Shutter Speed, as in exposure time, the time value part of exposure.

Obviously it doesn't mean 'shutter speed', otherwise it would be 'shutter speed'. And 'fast' as used by people relates to aperture, not 'shutter speed'. So you are very obviously wrong. What it looks like is you have an agenda but haven't thought things through enough to successfully defend yourself, so you end up saying silly things like that. Let me help you out. A 'fast' lens is called a 'fast' lens because it allows you to set a 'fast' shutter speed. That begs the question, what would stop you setting any shutter speed that you want? My answer would be that you might not want to set too fast a shutter speed because it would result in a lower quality image than you would be satisfied with.

Faster aperture relates to faster time value which shows up with faster shutter speed. Sensor/format size has no bearing on this fact.

Try reading this bit of what I wrote:

What would stop you setting any shutter speed that you want?

So, how and why does a large aperture lens 'show up with faster shutter speed'. If you're going to have any chance of winning this argument (don't worry too much, you haven't) you're going to have to display much clearer thinking than you are now.

That you can do anything you want only distracts from the point about what fast lens is, and the role it plays in exposure. And "large aperture" has no meaning. A 200/4 lens has larger aperture than 35/1 but that does not make it a faster lens.

And that makes you just as much wrong as tko.

Or just as right, more likely in fact.

You wish to be right, and that is a fact. That you and tko are, isn't.

You have singularly failed to demonstrate that we are`wrong, you simply assert it. Your argument gives out at 'faster aperture relates to faster time value which shows up with faster shutter speed', that's not taking you very far because you can't explain the why, where as we have. It allows a faster shutter speed for a given noise threshold. Your turn.

Amen to that. It is a shame that a collective that is so vocal in this drama.

Photographic exposure is independent of media size.

Yes, I know that. The real question is how relevant is exposure when making cross format comparisons?

Irrelevant. That is why I argued against exposure equivalence.

Good, and since exposure is irrelevant, you can't say that a f/2 lens on FT is 'faster' than an f/4 lens on FF, since the only way it is 'faster' is if your definition of fast is based solely of exposure, which, as you've just said, is irrelevant when making cross format comparisons.

Eh, no. Stop collective thinking, and try to think for self: Not Exposure Equivalence But DOF Equivalence.

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Bob

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