# Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
Re: "fast" is relative

Chikoo 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.

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.

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.

And that makes you just as much wrong as tko.

Or just as right, more likely in fact.

Photographic exposure is independent of media size.

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

Bob, fast is because it allows you faster shutter speeds.

Fine. That's what I said too. What determines how fast a shutter speed?

That was my original post. With high ISO do we need lenses with larger apertures just to allow more light when high ISO can achieve it for you with lesser light.

The only thing 'ISO' achieves is applying a given exposure to brightness function so that the image taken with your chosen exposure looks right at the end.

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Bob

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