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Messages - dougkerr

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EOS Bodies / Re: Anyone else want a cropped sensor?
« on: February 14, 2012, 02:42:56 PM »
All my sensors are at the original size as manufactured. Cropping them is really delicate work.

Best regards,


Lenses / Re: Canon 85L vs. 135 L in terms of focal length
« on: February 09, 2012, 09:28:57 AM »

I have a question related to different focal lengths and want to decide which one would work better for me.

I'm planning to go full-frame this year and want to have better understanding of the difference between 85mm and 135mm focal lengths for portraits. I love fixed lenses and looked through several sample shots made using both, but there are things that you can't predict by just looking at those pictures. I'm not able to try both by myself, that's why I'm asking for help here.

The only thing that comes to mind is different "compression" you get using different focal length (the more is your focal length the less is the distance between objects).
I can't really contribute to helping you decide what focal length lens would best suit your planned work, but I do want to clarify the concept of "compression of distance".

This is a "phenomenon" that occurs as the vantage point of the camera is moved farther from the group of objects (or features) that are included in the frame. It actually has nothing to do with focal length (except through another consideration I will get to in a bit).

Consider for convenience a "landscape" shot. There are two identical telephone poles (same height), one of them 100 feet farther away from us than the other along a straight line from any camera position we choose.

Suppose we first place the camera 100 feet back from the nearer pole. Thus the two poles will be at distances of 100 ft and 200 ft. Thus their heights on the image will have ratios of 1:0.5.

Next we place the camera 200 feet back from the nearer pole.Thus the two poles will be at distances of 200 ft and 300 ft. Thus their heights on the image will have ratios of 1:0.67. Their heights in the image are more nearly the same than in the first case.

Because we judge the relative distances of objects known (or believed) to have the same size based on their relative angular size to our eye, or to their relative sizes on a photographic image, it will seem as if the two poles in the second image are "not as far apart" ans the same two poles in the first image.

If we print both images at a scale such that the nearer pole is the same height in both images, then the second pole will be smaller in the first print than in the second. Thus the second pole will seem less far behind the first pole in the second print than in the first - the phenomenon of "distance compression".

And we will actually see this even if we don't print the two shots such that the nearer pole was the same height in both prints.

Note that I have said nothing yet about focal length. I might have used the same focal length for both shots - or maybe not. But the "distance compression" effect comes only from the distance from which we take the shot.

Now why does focal length seem to get into the act? Because if my object in each case was a picture of the two poles, I would probably have used a greater focal length in the second case (so as to best fill the frame with my two subjects, in the interest of best exploitation of the resolution of the camera).

Said another way, suppose I first took a shot with a 50 mm lens, and located the camera so the nearer pole filled some fraction of the frame height. Then I wanted to try a 100 mm lens. To get the nearer pole to fill the same fraction of the lens height, I would have to position the camera twice as far from the nearer pole as in the first shot.

And, as we saw earlier, that would make the two poles seem "closer together" in the second shot than in the first shot.

Now, I will return to the portrait context. First assume that I set up with an 85 mm lens and position the camera for a certain type of framing ("waist up", for example). I take the shot.

Next I want to try a 135 mm lens, and I still have the same framing in mind. So I will have to shoot from farther back.

The different point of perspective for the second shot causes the face proportions to be different - we have encountered a case of "distance compression" (perhaps desirably). But this is wholly a creature of the difference in camera location - not of the focal length itself.

Best regards,


PowerShot Cameras / Re: *UPDATE* Canon PowerShot G1X Revealed
« on: January 05, 2012, 10:52:40 PM »
Hi, Freak,

Yerry - just checked that link. Wow - 1/1.5" is pretty close to APS-C size (at 1.5 crop)!
This absurd system of specifying sensor sizes goes back to the "2/3 inch" Vidicon tube (so-called because of the diameter of its "bottle"), which had a target (sensor) 8.8 x 6.6 mm in size. That  relationship was continued, proportionally, for small-sensor digital cameras (since stating the actual size would have worried the consumer by seeming "terribly small").

But because even those numbers sounded so small (still worrying the marketing people), the further convention was adopted of specifying the "size" as the reciprocal of a number larger than 1 (to make the stated number bigger).

Thus the "2/3 inch" size became the "1/1.5 inch" size.

And that is the size presumably being discussed here - implying a sensor size of about 8.8 x 6.6 mm.

Best regards,


PowerShot Cameras / Re: *UPDATE 3* Canon PowerShot G1X Revealed
« on: January 05, 2012, 10:36:44 PM »

This will be a 2/3" sized sensor (close to the 8.8 x 6.6 mm Fuji X10)...
Yes, I assume that this is the case.

This is the size that started this whole absurd sensor size designation scheme in  the days of the "2/3 inch" Vidicon!

Best regards,


United States / Re: Why not have the aperture in the lens body
« on: December 17, 2011, 06:41:35 PM »
hI, M,
As I was sitting here debating on taking an old Tamron lens apart, I had an idea about camera/lens design.  The idea is to put the aperture diaphragm inside the camera body near the lens mount, instead of the rear of the lens.
The physical aperture needs to be at a place in the optical train substantially away from where the rays from a point on the object converge.

For example, to take it to an extreme, suppose the aperture were put just in front of the sensor. Then it would not affect the illuminance across the image (the exposure), but would merely adjustably mask the image.

Best regards,


Hi, s,
When Canon describes the 1Dx as having "100 to 51200 native ISO (expandable to ISO 50-204800)" what does that really mean?  Does "native" mean setting the sensitivity of the sensor itself and "expandable" mean setting it after the signal comes off of the sensor?  I've googled around and haven't seen a great explanation.

In the sensor systems we are most familiar with, the electrical outputs of the individual photodetectors are amplified (on an analog basis) and then given to an analog-to-digital converter (ADC) to be digitized. Both amplifiers and ADCs are "multiplexed"; that is, there is not normally one per photodetector. There are many schemes for this multiplexing.

The basic mechanism for changing the ISO sensitivity of the system is to change the gain of the analog amplifiers, normally among a fixed set of predetermined values (controlled by a digital command to all the amplifiers).

The resulting repertoire of sensitivities are often described as the "native" sensitivities of the system.

However, ISO sensitivities greater than that given for by the greatest provided amplifier gain - as well as sensitivities between those given by established amplifier gain values, or lower than those, - are provided by digital scaling of the digital values out of the ADC.

Sometimes the former of these - or all of these - are described as "expanded" sensitivities.

This technical article from 2006 gives some insight into this concept of sensor operation:


Best regards,


United States / Re: Broken tripod collar
« on: November 23, 2011, 07:51:37 PM »
Hi, s,

Here's a non-Canon replacement for $8.99 with free shipping:


I bought their one for the 70-200 f/4L IS USM, and it is lovely!

Best regards,


Lenses / Re: Battle of the 50mm's - 1.8, 1.4 and 1.2L
« on: November 23, 2011, 02:20:13 PM »
Hi, s,

I have one comment about the EF 50 mm f/1.4 USM. The manual focus arrangement on it is dreadful.

It is the only Canon EF lens with non-ring USM that has full-time manual focusing. The unique mechanism used to do that is full of "play", so manual focus is very difficult to do.

Other than that, it is a nice bottle.

Best regards,


Lenses / Re: Battle of the 50mm's - 1.8, 1.4 and 1.2L
« on: November 23, 2011, 02:16:32 PM »
Hi, n,

The 50mm f/1.2L is something of a special case.  That lens suffers from a particularly bad focus shift (many lenses have some focus shift, especially, fast primes, but usually not enough to notice).  Focus shift is when the focal plane of the lens changes when you change the aperture of the lens.
As I'm sure you know, this is often largely a result of uncorrected spherical aberration.

In fact, the Canon AF system has specific provisions (sometimes rather primitive) to (somewhat) compensate for this.

Best regards,


Lenses / Re: Owners of 70-200f2.8L IS mk.II & 70-300 L IS
« on: November 22, 2011, 03:38:32 PM »
Hi, K,

Would like to know if you guys have trouble keeping up with this solid lens and did you get used to it.

I had the EX 70-200 mm f/2.8L IS USM (not the "II"). It was too big and heavy for me. I sold it and bought the f/4. It is lovely.

I also have the Extender EF 1.4x (not the "II"). The combination performs excellently.

The AF performance is much better than on the f/2.8 (not II), although I would expect even more improvement on the f/2.8 II.

Best regards,


A colleague was kind enough to arrange for a machine translation of the Japanese patent publication.

I have not yet perused it at any length. The translation is a bit clumsy but is still very welcome.

The patent indeed refers to a flash unit using (for example) three sets of LED's so that the chromaticity of the flash burst can be set, presumably (as mentioned by one of the correspondents here) so that the flash illumination can match the chromaticity of the ambient illumination. [Note that this involves more than the correlated color temperature, although much of the discussion proceeds as if that is the only parameter of interest.].

Presumably this concept itself has been covered in earlier patents.

The thrust of this patent (as it seemed from the earlier characterization) is to provide for automatic flash exposure control to perform properly in this situation.

The basic idea ("matchable" flash burst chromaticity) sounds very worthwhile.

Best regards,



imho what that means is that the guide number (power output) will be different for different color temperatures the flash is set to.
But what does it mean "the color temperature the flash is set to"?

Perhaps it mean that the needed output of the flash for "proper" exposure in a particular case depends on the white balance setting of the camera.

I'll try and get an English translation of the patent publication and see what it actually talks about.

Best regards,


Hi, Dilbert,

The basic discussion in the cr report makes no sense to me. I fail to see the connection between color temperature and "exposure".

Imagine, if you will, a flash that has the same temperature as either an incandescent light, the sun or fluorescent lighting, as well as the normal "tungsten" value.
Yes, a flash unit with controllable chromaticity - controllable "color temperature" (CCT, actually), if you will - would be nice.

I'm not sure I get that from the description of this new machine, though - but who knows.

Best regards,


The basic discussion in the cr report makes no sense to me. I fail to see the connection between color temperature and "exposure".

We may be suffering here from some "loss in translation".

In any case, Canon Speedlite flash units have for some time (I believe from the 580EX onward) reported to the body an estimate of the color temperature of the specific burst used for a shot so that the body can take this into account when doing white balance color correction.

Best regards,


EOS Bodies / Re: Canon Japan EOS-1D X The Birth of Entirely new Flagship
« on: October 21, 2011, 05:52:18 PM »
Hi, N,

AFAIK, the Tamron and Sigma lenses send false information (read: that the aperture is at f/5.6 when it's actually f/6.3)

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.

The fact that the camera will not allow the AF system to attempt to work below a certain aperture is a firmware limitation.

The fact that it will not work (or work well)  below a certain aperture is a hardware (optical) limitation.

As others have pointed out, the latter is not "knife-edge". If we can force the camera to "let the AF work" at an aperture of, for example  f/8 (for an "f/5.6" AF detector). it may work with some difficulty. At f/6.3 it will likely work "pretty well".

Best regards,


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