# Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
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 Re: You forgot something. In reply to Great Bustard, Jul 20, 2014

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

Great Bustard wrote:

EinsteinsGhost wrote:

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.

Obviously, you do not understand ISO. Why would a FF photographer feel compelled to shoot the same ISO as the mFT photographer?

I don't see a reason for a FF photographer feeling compelled to shoot an ISO 100 shot at ISO 3200.

Neither do I:

I think this image from your gallery agrees:

Its not ISO 100 image. It is an ISO 400 image.

I see both photographers to shoot at the lowest possible ISO and in this case, it will be the base ISO.

It's fair to say that if f/2 meters at 1/4000, and thus f/4 would meter for 1/1000, then, sure, 1/1000 is more than likely to be "fast enough" in almost all circumstances. What happens with f/2 meters at 1/400? Will f/4 at 1/100 be "fast enough"?

Not necessarily (depends on subject). Which is when you might increase the ISO (as you or your camera did, in the image above).

Please tell us about the exposure differences here as they relate to the visual properties of the photo:

• 50mm f/2 1/100 ISO 400 on mFT
• 100mm f/2 1/100 ISO 400 on FF
• 100mm f/4 1/100 ISO 1600 on FF

Same exposure on first and second (f/2, 1/100s, ISO 400). Lower exposure in third by two stops.

That is correct! However, you forgot to tell us how this relates to the visual properties of the photo.

The visual property that an exposure is all about is brightness of the scene. For same brightness, same exposure is expected. With higher ISO (third bullet above), you're doing just that, increasing brightness by two stop to compensate for reduced exposure by two stops.

So what we see here, really, is that exposure is merely part of the equation:

• Exposure (photons / mm²) = Sensor Illuminance (photons / mm² / s) · Time (s)
• Brightness (photons / mm²) = Exposure (photons / mm²) · Amplification (unitless)
• Total Light (photons) = Exposure (photons / mm²) · Effective Sensor Area (mm²)
• Total Light Collected (electrons) = Total Light (photons) · QE (electrons / photon)
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