"Water is not light" contd. :-)

Started Mar 6, 2013 | Discussions thread
GaryW Veteran Member • Posts: 8,549
Re: "Water is not light" contd. :-)

Photozopia wrote:

Bart Hickman wrote:

Photozopia wrote:

...F-stop is not comparable between cameras with different sensor sizes any more than focal length is (because the formula for F-stop includes focal length).

WRONG - an f stop is the ratio between aperture and focal length.

Physical size is irrelevant - the ratio is identical. Thus f stop values, exposure time and even exposure by sensor area - even differing sensor format sizes - is equal.

RATIO in all exposure factors is correct - not physical dimensions ....

That is precisely why it isn't equivalent (between different sensor sizes).

What you need to be comparing when comparing different sensor sizes is how each camera system creates the same photo (same shutter speed, same FOV, same DOF, and same SNR). Keeping exposure constant is an arbitrary gain adjustment at the end.

When you shrink a sensor, you must also shrink the focal length to keep the FOV constant.

If I'm switching between formats, and I'm used to using a 40mm on a FF camera, I don't want to keep using a 40mm on an APS-C camera -- I really want something more like a 28mm.  So as a practical matter, you're right, that you'd want to switch to a different focal length.  But ....

But in order to keep DOF and SNR constant, you must keep the aperture size constant which means F-number increases. So neither focal length nor F-number have the same meaning for different sensor sizes.

You mean, the f number decreases.  This will help with the DOF and SNR, but it does change the exposure -- you may now using a lower ISO setting, and the end SNR will be the same between formats.  But I don't think  you can say that the focal length or f-number have different meanings -- you just changed them, and for perfectly good reasons, and in the process, increased the exposure.

Suppose, for example, I shoot an image at 50mm on a FF sensor and I want a DOF corresponding to, say, F/2. Now I want to compare this "system" with another system using a m43 sensor. First of all I need a 25mm lens, but I need to keep aperture constant, so F-number will change to F/1 even though nothing physically has changed about aperture opening.

Even though the physical aperture is "the same" the 25mm, f/1 lens captures twice as much light per unit area.

I have an iPhone app which allows one to choose two of the three parameters, and it'll show you what to set for the third.  (Shutter, Aperture, and ISO.)  This works with film, and it doesn't matter what format.  If it tells me to use f/3.5, that's what I set the camera to -- the app doesn't know what the focal length is, and it doesn't even know I'm  using an old medium format camera!  (So much for "FF" being really Full... )

You keep shutter speed the same of course. In order to equalize SNR (aka exposure), you must lower the ISO of the m43 camera.

You're done. You'll notice that even ISO is not comparable between systems with different sensor sizes. In this case, the larger sensor has more highlight headroom.


An exposure value (EV) is the same on any format ... What DOF or S/N ratio you get from differing formats is immaterial.

An EV of f2.8 @ 1/125th is the same on any format regardless of focal length/FOV/sensor size or any other physical factor you care to introduce - as any non-coupled hand-held meter will confirm.

I agree, and the talk of equivalence is just adding confusion perhaps.  But I think it's helpful to explain what happens when you switch between formats.  I'm not going to use 40mm on APS-C.  So what happens?  It's good to know that the f-number works the same, and that a wider lens has less DOF.

I do not discuss the facile notion of 'equivalency' you raise elsewhere in this post - do not bother me with your poor equivalent results from smaller formats - I'm only discussing EV - nothing else.

The bottom line of digital cameras is how much SNR you have at a given shutter speed.  (Yeah, DR is important, but I think SNR really cuts to the core of it.)   If I don't care about SNR, I can just use any ol' P&S or camera phone for that matter and enjoy the noise.

Bart has this much right -- the shutter speed is the most important factor when making comparisons.  I need a certain shutter speed in order to stop motion; now, how good are the results?  As a practical matter, if camera A has less noise and/or more detail then camera B, that's what matters -- I can conclude that I'll get better IQ from A.  It doesn't matter how it gets there.

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Gary W.

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