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Messages - dougkerr

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EOS Bodies / Re: Canon Japan EOS-1D X The Birth of Entirely new Flagship
« on: October 21, 2011, 05:48:10 PM »
Hi, CT,
Yes, I know...but Doug stated, in effect, that at lens apertures narrower than specificed for the AF system, i.e. f/5.6, the AF detetors cannot function at all, i.e. it's an optical limitation, as opposed to a firmware limitation.  The fact that an f/6.3 lens can AF, as can an f/8 lens under certain conditions, with an f/5.6-sensitive sensor, suggests that it's a firmware limitation, and not an optical limitation.

Well, the pupils have a certain size, and at f/5.6, they're not shaded at all and the focus sensors get the full amount of light. But when you attach a slightly slower lens as f/6.3 they start to get shaded, but there's still light coming thru to the sensors. So it works worse in low-light conditions. As people report here, the pupils are nearly fully covered at f/8 depending on the lens. This is why it may still work with a lot of light.

And the body may in fact let the AF system (try to) work at a reported aperture of f/6.3. But not f/8

Best regards,


EOS Bodies / Re: Canon Japan EOS-1D X The Birth of Entirely new Flagship
« on: October 21, 2011, 08:49:25 AM »
Hi, CR,

There does [do] seem to be some things left out of the EOS 1D X that photographers are worried about. The biggest one I have seen is the omission of f/8 AF sensors. This means that a 500 f/4L IS with a 2X teleconverter wouldn’t have AF at the center point, like the current 1D and 1Ds cameras. I am still waiting for official confirmation from Canon about this issue. Is it a hardware thing? or could it be addressed with software?

The minimum aperture required for a particular AF sensor pair to function depends on the spacing of the "AF sub-exit pupils" for the two detectors.

At the smallest workable aperture for a particular detector pair, these are located (by design)  against the edges of the actual lens exit pupil (as it is during AF, at the lens' maximum available aperture). Their separation is in effect the "baseline" of a rangefinder, which is what a phase detection AF detector system actually is. Each detector of a pair works from light coming only through its own sub-exit pupil.

Thus, the spacing is different for the AF detector pairs with different "minimum aperture" requirements.

This siting is determined by the little optical system that feeds each AF detector pair, which among other things includes a pair of physical apertures. The sub-exit pupils are images of these.

The greater the spacing of the sub-exit pupils, the greater the "precision" of focus detection, which is why the detector pairs that require an f/2.8 minimum aperture are more precise, and are desirable to use (when they will work).

But if the sub-exit pupils are located outside the boundaries of the exit pupil of the lens proper (as when the lens aperture during AF is smaller than that which supports the sub-exit pupil spacing of the particular detector pair of interest), then the AF detectors get no light at all, and thus cannot function.

And, prudently, the camera body declines to include various classes of AF detectors from the AF scenario based on knowledge, reported by the lens, of its maximum aperture.

This is thus a hardware consideration, and cannot be overcome by any firmware fiddling.

Note finally that this is in no way a photometric issue; having "enough light" (in the exposure sense) is not the critical issue here. This is why AF detectors requiring a minimum aperture of, say, f/5.6 cannot be put into play merely by limiting ourselves to "brighter" scenes.

When a 2X extender is placed on a lens which, by itself, has a maximum aperture of f/4, the result is a new lens, with more elements, longer and weighing more, having a maximum aperture of f/8. It is popular to say that the "effective maximum aperture" is now f/8. But that is the real aperture of our "new lens". Thus it is the exit pupil produced by that actual new aperture which is the limiting factor on which AF detector pairs will work with that augmented lens in place.

Best regards,


Hi, Rad,

Mathematically speaking the noise in an image actually contributes to a reduction of resolition of an image. You can't make out individual specs of sand on a picture of a beach when there are grains of noise for example.
An important notion, of fundamental importance in the telecommunications world (as revealed in the work of Shannon and Hartley, for example).

But in the digital camera case we must note that the signal to noise ratio varies with the luminance of the specific pixel (duh!) .

Thus, except in the "shadow" areas of the scene, a decrease in noise will not lead to a "measurable" improvement in resolution.

So the point you make is important with regard to "resolution in the shadow areas", but not much elsewhere.

Best regards,


Lenses / Re: Nodal Point on non extending Zoom Lenses
« on: September 30, 2011, 09:24:58 PM »
Hi, Edwin,

. . .the nodal point shifts when changing aperture, too!

I have never heard of either of the two nodal points of the lens, nor the center of the entrance pupil (which corresponds to the "no-parallax point"), shifting with change in aperture.

Any of those three can, of course, change with change in focal length (for a zoom lens), or maybe even with focus position (for some focusing systems).

Best regards,


Lenses / Re: Nodal Point on non extending Zoom Lenses
« on: September 30, 2011, 08:29:50 PM »
Well the reason why I refer to this as the Nodal Point is actually cause I own this Nodal Point Rail, which was just made for this purpose to adjust the lens for Panoramic stitching:

See here: http://acratech.net/product.php?productid=62

I understand.

Best regards,


Lenses / Re: Nodal Point on non extending Zoom Lenses
« on: September 30, 2011, 03:38:01 PM »

A short question about the Nodal Point of non extending zooms:

For (non-extending) zoom lenses like the Canon 17-40mm f4.0 L or 70-200mm f2.8 IS II L do you know if the nodal point shifts when zooming, or is that dependable from each lens's construction and cannot be generalized.
If your interest is in multi-frame panoramic photography, you have no reason to be concerned with either of the two nodal points of a lens. They are not relevant to that issue.

You do need to be concerned with the often-called "no-parallax point", which falls at the center of the entrance pupil of the lens.

Note that you can look into the lens from the front and locate this. It is where the aperture diaphragm looks to you as if it is looking from outside the lens in the front.

I just took a quick peek into my Canon EF 70-200mm f/2.8L IS. It does not look as if the entrance pupil moves much (if at all) when changing the focal length.

Best regards,


EOS Bodies / Re: Ball pitching speed / Shutter speed accuracy experiment
« on: September 21, 2011, 04:17:46 PM »
Hi, K,

I am trying to estimate the velocity of a pitcher (the radar gun I have does not seem to be not accurate) using a framed capture at 1/60th of a second.

Does the "scan" aspect of the FP shutter introduce any complication into your process? Perhaps with a shutter speed of 1/60 s it would not be a factor.

Is the travel of the ball parallel to the focal plane (or at a consistent angle to it)?

Interesting project.

Best regards,


EOS Bodies / Re: Nikon unveils V1 and J1 Mirrorless Cameras
« on: September 21, 2011, 04:13:37 PM »
Hi, Peter,

About the only thing I saw of interest was the phase detect autofocus. I'd previously thought that was only possible with a mirror system but see in the Wikipedia article on autofocus it can be incorporated into the sensor.
There has been much patent activity by Canon over the last some while on doing that.

Best regards,


Canon General / Re: "Old Lenses" and "Digital Camera Compatibility"?
« on: September 15, 2011, 12:25:37 AM »
Hi, b,

These include the so-called quasi-telecentric designs, in which the exit pupil of the lens is quite far forward.

I love it when you stop by Doug. Not only do you always drop some serious knowledge; you're the only one that consistently breaks out terms I have to google.  ;D
Well, thanks.

Best regards,


Canon General / Re: "Old Lenses" and "Digital Camera Compatibility"?
« on: September 14, 2011, 08:19:18 AM »
Hi, d,

Are "old lenses" that predate digital cameras different in some way?
One issue relates to angle of incidence - the angle at which rays strike the sensor.

Film responds to oblique rays (those that do not strike at right angles) proportionately to the cosine of the angle of incidence (which is measured with respect to arrival at right angles). This is one ingredient in the fall-off of effective exposure for off-axis parts of the scene (natural vignetting), as well as a loss of effective exposure even for on-axis scene points in the case of a large-aperture lens..

With a typical digital sensor, the fall-off in response may be faster than as the cosine of the angle, thus exacerbating the fall-off in effective exposure. The problem is most pronounced in wide-angle lenses.

To mitigate this, in lenses intended to be usable in digital cameras, design schemes are often used than decrease the maximum obliquity of the rays. These include the so-called quasi-telecentric designs, in which the exit pupil of the lens is quite far forward.

The bottom line is that wide-angle lenses (in particular) not specially designed for use with digital cameras may exhibit more off-axis fall-off, and more "shortfall" of effective exposure at large apertures, than we might wish.

Another issue is that digital cameras may be more susceptible to internal reflections caused by rays bouncing between the sensor and various lens surfaces. For that reason, in lenses specifically intended to be usable with digital cameras, special attention may be given to anti-reflective coating and the curvature of critical lens surfaces.

For that reason, lenses not having that design attention, when used on a digital camera, may exhibit more loss of contrast (through "flare") or even overt "sun dogs" than when used on a film camera.

Best regards,


EOS Bodies / Re: Art Wolfe Confirms the 1Ds Mark IV?
« on: September 13, 2011, 02:02:41 PM »
Hi, L,

A typo vs. what?

A mental slip;an accidental jump between two similar thoughts.

The other thought being what?

Best regards,


EOS Bodies / Re: Art Wolfe Confirms the 1Ds Mark IV?
« on: September 13, 2011, 11:02:11 AM »

Art would be a guy that would have an advanced look, I’m just not sure it wasn’t a typo.
A typo vs. what?

Best regards,


Hi, H,

I have never owned a camera like this in my life. I have a Eos 1 D mk III and not a PowerShot SX30 IS.  ;)
That's a membership "rank", like "Chevy", "Buick", "BMW", and "Ferrari".

I know, it's just too cutesy!

Best regards,


United Kingdom & Ireland / Re: Compatibility question
« on: August 16, 2011, 08:48:42 AM »
Hi, Casper,

I own a Canon 450D at present but am waiting for the next release of the 5D.
In the meantime I'd like to buy a lens which would hopefully fit both. Is the Canon EF 85mm f/1.2L II USM compatible with both cameras?

Note that when the lens is used on an EOS 5D, because of its larger sensor size than that of the 450D, the field of view will be greater.

Best regards,


Lenses / Re: Which is the best "normal" prime for a Crop Camera?
« on: August 13, 2011, 11:31:49 PM »
Hi, p,

I have a 7D and want to get a fast prime lens that would be close to the equivalent of a 50mm on a FF camera.
Well, the lens that, on an EOS 7D, would give the same field of view as would a 50 mm lens on a full-frame 35-mm camera would have a focal length of about 31 mm.

But you might mean something else by "would be close to equivalent".

Best regards,


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