• Please use real names.

    Greetings to all who have registered to OPF and those guests taking a look around. Please use real names. Registrations with fictitious names will not be processed. REAL NAMES ONLY will be processed

    Firstname Lastname

    Register

    We are a courteous and supportive community. No need to hide behind an alia. If you have a genuine need for privacy/secrecy then let me know!
  • Welcome to the new site. Here's a thread about the update where you can post your feedback, ask questions or spot those nasty bugs!

AF sensor sensitivity

Doug Kerr

Well-known member
Canon often expresses the range of sensitivity of the AF systems in EOS digital cameras as a range of Ev (exposure value). Sometimes "ISO 100" is mentioned. What does this mean?

The intent is to describe a range of scene luminance. Of course, Ev is not a legitimate measure of scene luminance, but rather of photographic exposure. But what is in play here is a convention in which Ev is used to describe scene luminance. In effect, when a scene luminance is described as "EV 8", it means the luminance for which the APEX exposure equation, for an exposure index of ISO 100, would call for an exposure of Ev 10.

I do not endorse this convention - the legitimate way to express, scene luminance in logarithmic terms is with the APEX quantity Bv (brightness value).​

Now, having understood that, it does not seem right that scene luminance should be the critical property. In fact, the critical property is actually illuminance on the AF detector sensor.

We might think that this is a function not only of scene luminance but also of the maximum aperture of the lens in use (as well as of the transmission properties of the AF mirror system, but of course that is essentially constant for any given camera).

But in fact the lens aperture is not in the equation. The images on the AF detector sensors are formed by restricted cones of light, originating from small roughly-circular regions at two opposite edges of the exit pupil of the lens (for the two sensors of a pair). These small circular regions are essentially the exit pupil of the lens so far as the images on the AF detector sensors are concerned. (They are defined by apertures in the AF sensor assembly.)

Of course, the actual exit pupil of the lens must be large enough to span these little circular regions, or there will be no light passing through them and the AF sensors will be "dead". This is why, for example, a certain type of AF detector sensor will only operate with lenses of a certain minimum aperture. For an "f/2.8" sensor, those little circular regions lie wholly outside the exit pupil of an f/8 lens, would receive no light, and the sensor would not work at all with such a lens.

But, assuming that the aperture of the lens in use is large enough to embrace these little regions, the size of those regions, not the size of the actual exit pupil of the particular lens, (along with scene luminance) determines the illuminance on the AF detector sensor.

Thus, the criterion for sufficient illumination on the sensor is stated as a certain scene luminance, independent of lens aperture (again, assuming that to be sufficient for operation of the sensor at all).

Best regards,

Doug
 
Top