# Fast lenses, and High ISO

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

EinsteinsGhost wrote:

Great Bustard 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).

What shutter speed can you use on mFT at f/2 that you cannot use on FF at f/4?

Obviously, you didn't try to understand the point made. Let me make it simpler:

Scene brightness: 9 EV

ISO: 100

With these conditions, f/2 will have a faster shutter speed of about 1/4000s. An aperture of f/4 will give you 1/2000s. A larger or smaller sensor will not change that.

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.

What shutter speed can you use on mFT at f/2 that you cannot use on FF at f/4?

Obviously, you didn't try to understand the point made. Let me make it simpler:

Scene brightness: 9 EV

ISO: 100

With these conditions, f/2 will have a faster shutter speed of about 1/4000s. An aperture of f/4 will give you 1/2000s. A larger or smaller sensor will not change that.

So, set ISO 200 and your lens is magically twice as fast. Perhaps it should be called the speed control.

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Bob

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