DOF and Cropping take 2

Started 7 months ago | Discussions thread
moving_comfort
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Awaldram
In reply to Great Bustard, 7 months ago

Great Bustard wrote:

Here we are:

http://www.josephjamesphotography.com/equivalence/#quick

Neither the focal length nor the f-ratio of a lens change as a function of sensor (for example, a 50mm f/1.4 lens is a 50mm f/1.4 lens, regardless of the sensor behind the lens). However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor:

25mm f/1.4 on mFT (4/3) is equivalent to 31mm f/1.8 on 1.6x (Canon APS-C), 33mm f/1.9 on 1.5x (APS-C for everyone else), and 50mm f/2.8 on FF (FX), where "equivalent to" means:

  • The photos all have the same diagonal angle of view (25mm x 2 = 31mm x 1.6 = 33mm x 1.5 = 50mm) and aperture diameter (25mm / 1.4 = 31mm / 1.8 = 33mm / 1.9 = 50mm / 2.8 = 18mm).
  • The photos all have the same perspective when taken from the same position.
  • The photos all have the same DOF (as well as diffraction softening) when they are taken from the same position with the same focal point and have the same display size.
  • The photos all have the same motion blur for the same shutter speed (regardless of pixel count).
  • The same total amount of light falls on the sensor for the same DOF and shutter speed.
  • The same total light falling on the larger sensor will result in a lower exposure than the smaller sensor (the same total light over a larger area results in a lower density of light on the sensor).
  • The larger sensor system will use a concomitantly higher ISO setting for a given brightness of the LCD playback and/or OOC (out-of-the-camera) jpg due to the lower exposure.
  • The same total light will result in the same noise for equally efficient sensors (regardless of pixel count and regardless of the ISO setting).
  • If the 25mm lens at f/1.4 has twice the resolution as the 50mm lens at f/2.8 on FF, 1.33x the resolution of the 33mm lens at f/1.8 on 1.5x, and 1.25x the resolution of the 31mm at f/1.8 on 1.6x, the sensors have the same pixel count and same AA filter strength, then all systems will also resolve the same detail.
  • Other elements of IQ, such as bokeh, color, distortion, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".

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So Awaldram, you've called Falk's paper a 'fallacy', although I doubt you'd really think the whole thing could be a fallacy. I'm left with not knowing what parts of it you agree with or not. So let's move to the above - what do you think is false or 'crackpot' to use your word, in the above?

You seem to not agree with:

  • The larger sensor system will use a concomitantly higher ISO setting for a given brightness of the LCD playback and/or OOC (out-of-the-camera) jpg due to the lower exposure.

Based on what you said here (in reference to Ian stating a correct equivalent relationship he layed out which would have required the FF image to use ISO 225 to match the aps-c ISO 100 with the same total amount of light) :

...Ian/MC said FF required iso225 to match iso100 aps-c - wrong

...On another subject you keep saying iso100 on aps-c = iso 225 on FF

...Again your doing some kind of weird 'equivalence' to push some FF agenda

And you seem to disagree with the following: "...However, the effect of both the focal length and the f-ratio on the visual properties of the photo very much depend on the sensor, and scale in direct proportion to the size of the sensor"

Based on what you said here:

...Ian/MC said DoF is a function of sensor size - wrong

...I said an f2.8 lens is an f2.8 lens on any sensor - Check Proved

...Ian/MC stated a lens alters it behaviour and f2.8 is not the same but depends on sensor size - yet again wrong.

So, anything else you disagree with in the above (or can you be more specific about what you disgaree with in Falk's paper? )

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