FF Commentary by Thom

Started Nov 4, 2013 | Discussions thread
Ontario Gone
Ontario Gone Senior Member • Posts: 4,183
Re: FF Commentary by Thom

Ian Stuart Forsyth wrote:

Brian is correct Fstop is meaningless between formats what counts is the AP dia . and this is what determines the DOF, SS and total light striking the sensor

Mark Ransom wrote:

Roland Karlsson wrote:

It is not the F-number that determines how fast a lens is, it is the aperture diameter.

Therefore, an F8 lens might be faster than an F2 lens, if the F8 lens camera is more than 4x bigger.

Not sure I understand. Doesn't "faster" in this context mean you can use a faster shutter speed for the same light level and exposure? That's independent of sensor size.

Keep in mind, Mark was asking about SS, light level, and exposure. He mentioned nothing about noise, for good reason, bc even noise varies among different sensors of the same format. You can't place the credit solely on the lens, bc it is the relation between the lens and sensor that is important. Of course FF gathers more light, everybody knows that. But the original question was about a lens being faster just bc it's a FF lens vs a crop lens, and that is misleading. The FF lens may have a larger aperture, but that is meaningless unless the sensor catches all the light.

Don't get me wrong, I understand very well the terms of aperture, I have spent much time studying telescopography and in that world, they don't usually mention F stops. They always refer to aperture. Why? BC there is not standard for sensor size. Many use CCD imagers, web cams, but it's a crap shoot.There is a reason the camera world does things differently. For example, how can you tell, if you were not studied in photography, which lens allowed faster SS:

Example 1: 50mm FF @ 25mm aperture

Example 2: 100mm FF @ 25mm aperture

Both shot on a FF camera fyi.

Now, anybody not knowing what the heck is going on will think you get the same SS from both, if they were to listen to the arguments here, but we know that's not true don't we. Why? BC the intensity drops off with longer FL. This is exactly why F stops are needed. Is it true a FF lens has a larger diameter than a crop, or MFT lens? Yes, but that doesn't matter. I'll prove it one more time.

Take a MF (medium format) lens and slap it on a FF camera. Take a crop lens and slap it on an apsc camera. Is the MF lens several stops faster than the crop lens, according to your standards? Yes. Are we using different sensor formats? yes. Will you get several stops of light more on the FF camera? No, perhaps 1.5. Going by lens diameter is hit or miss, bc its not the lens only, its the lens in relation to the sensor. Keep in mind, this is in line with your statement above, you said "Fstop is meaningless between formats, what counts is the AP dia". Here we have two different formats, yet the lens aperture differences won't reflect what we see in the photos.

F stop was created for a reason.

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