Messages

1

EOS Bodies - For Stills / Re: F/8 Autofocusing impressions

« on: Today at 01:11:30 PM »

... everyone who had Canon tele-converters could not have this option!

Smells like Canon is doing this on purpose ... because what I can't understand is if the 6D can AF down to -3EV, then it should also be able to AF with an f/11 lens (in bright daylight, that is).

The AF aperture limit issue that we normally discuss does not evolve light levels. It is based on optical-geometric considerations.

An AF detector is like a rangefinder, with two sensors looking at the scene  through two "virtual sub apertures" located on opposite edges of the real exit pupil.

To attain a certain accuracy, these sub-apertures must be located a certain distance apart. This distance is like the "baseline length" of a classical rangefinder  (I do not mean "rangefinder camera").

The distance between the two sub apertures is built into the optical system of each AF detector pair. If they are farther apart than the diameter of the exit pupil for the maximum aperture of the lens in place, then no light passes to the detector pairs and AF fails. (So typically the Canon cameras block any attempt when the aperture is not "suitable").

But of course the whole situation is more complex than that, and their are various steps that can be taken to "cheat around" the optical-geometric limitations.

Some of the principles are described here:

http://dougkerr.net/pumpkin/articles/Split_Prism.pdf

Best regards,

Doug

2

Technical Support / Re: Teleconverter and DOF

« on: March 01, 2013, 01:08:21 PM »

When comparing DoF issues, we must be very careful about what the conditions of the two situations are. We may say, "all other things being equal", but what does that mean? Same focal length? Same field of view?

For now. let's compare a situation of the same lens with and without a 2x focal length converter. Let's assume that focus is at the same distance, the focal length setting of the base lens is the same (and thus the focal length of the combination is twice that), and the f-number of the base lens is the same (so the f-number of the combination is twice that). And we will assume a consistent circle of confusion diameter limit (COCDL)

Then, in the configuration with the focal length converter in place, the depth of field will be substantially less than for the configuration with no focal length convertor.

If the setting of the base lens is f =  100 mm and f/3.5, with focus at a distance of 10 m, then, using a COCDL of 0.031 mm, the full depth of field is:

No focal length converter:  10.62 m

2x focal length converter:  4.50 m

But perhaps for our purposes it would be best to think in terms of a constant field of view (a constant focal length). Thus, with the focal length converter in place, we would set the focal length of the basic lens to (for our example) 25 mm. Then the results would be:

No focal length converter:  10.62 m

2x focal length converter:  67.98 m

Woof!

If we look at out-of-focus blur performance (which is what we are sometimes more interested in than depth of field), we also see interesting results.

Suppose that in both cases we assume focus at 10 m and are interested in the blur on a background object at 50 m. We will always use the base lens at its maximum aperture, f/2.0.

First suppose that with no focal length converter aboard, we set the focal length of the base lens to 50 mm, and we leave it set thus when we add the focal length converter. Then the results are (in terms of the diameter of the circle of confusion (blur figure) produces for a point on the background object:

No focal length converter:  0.10 mm (3.24 times our reference COCDL)

2x focal length converter:  0.20 mm  (6.52 times our reference COCDL)

This time, when we mount the focal length convertor, we reset the focal length of the base lens to 25 mm (to hold the actual focal length and thus the field of view). Now the results are:

No focal length converter:  0.10 mm (3.24 times our reference COCDL)

2x focal length converter:  0.05 mm  (1.62 times our reference COCDL)

Best regards,

Doug

3

PowerShot Cameras / Re: G15 or G1X for my mom???

« on: January 19, 2013, 09:39:48 AM »

My wife has excellent results from her PowerShot SX150 IS, which is very compact. Of course with the very small sensor, the noise performance becomes an issue at higher ISO sensitivities.

A problem for me is that it has no viewfinder of any sort, but the chimp panel is quite nice.

Doug Kerr

4

EOS Bodies - For Stills / Re: Solar eclipse questions...???

« on: May 19, 2012, 12:05:09 PM »

Here is another recent forum discussion about solar photography:
http://www.canonrumors.com/forum/index.php?topic=6385.0

I'm heading to Albuquerque to be in the center of the annular eclipse path...  the forecasted sky cover is higher than I would like (27%) but I'll take my chances.  I'll be using two stacked ND filters with a combined optical density of 3.9

About 13 stops - often recommended for this purpose.

Best regards,

Doug

5

EOS Bodies - For Stills / Re: Solar eclipse questions...???

« on: May 19, 2012, 12:02:58 PM »

Hi, Don,

One source tells me to use the filter for direct shots of the sun but to remove it during the total eclipse...

Good advice for a total eclipse event. Note however that this time there will be no total eclipse.

I don't know what the best advice is during the "complete annular" phase.

Best regards,

Doug (Just barely outside the path of "full annular" eclipse, in Alamogordo, N.M.)

6

Lenses / Re: What can tilt-shift lenses do that post-production cannot?

« on: May 10, 2012, 10:00:20 AM »

Hi, Random,

The more you have to correct for buildings "falling over," the more you source data you lose from having to stretch a portion of your source file.  The shift function can eliminate or minimize that effect and can help maximize the number of pixels you have on your "subject."  With a normal lens, you might have to change the angle to keep the subject vertical lines vertical, which might give you too much background/foreground that has to be cropped out.

Tilting the focal plane is not something that can be done in PP.  You can't make an OOF area more in focus.  Let's say you take a picture of a deck or sidewalk from where you are on the ground to the horizon.  A regular lens will have a portion of it OOF closest to the camera because of its DOF even if shooting at f/11 or smaller.  With a TS lens, you can shift the focal plane so that it is parallel to the deck/sidewalk and still shoot at f/4 (max aperture).  The sidewalk with lay completely within the focal plane even though the DOF is thin, but the DOF effect can't be seen because there isn't anything on top of the deck/sidewalk.

All well said.

Best regards,

Doug

7

EOS Bodies / Re: Canon Mirrorless Information? [CR1]

« on: May 05, 2012, 10:02:46 AM »

Hi, Ontarian,

Come on Canon, go full frame or don't bother.

I'm not sure I understand the premise that 24 mm x 36 mm is automatically the ideal format for a new genre of serious "general purpose" digital cameras.

In the late 1930's, we were introduced to the concept that cameras with a format size smaller than, for example, 2-1/4" x 3-1/4" could do serious photography. Thus we came to embrace (suspiciously at first) the 24 mm x 36 mm format. This created an entire world of "miniature" photography.

Now, 75  years later, with a wholly different paradigm of imaging, and an enormous history of development and a repertoire of powerful technical tools at our disposal, this very same format size is somehow still held to be "the real thing".

I myself would like to see the power of our tools deployed to make the next generation of "general purpose serious digital cameras" (body and lenses) a bit smaller and lighter than dictated by arbitrary adherence to the 24 mm x 36 mm format.

Best regards,

Doug

8

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,

Doug

9

Lenses / Re: Canon 85L vs. 135 L in terms of focal length

« on: February 09, 2012, 09:28:57 AM »

Hi,

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,

Doug

10

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,

Doug

11

PowerShot Cameras / Re: *UPDATE 3* Canon PowerShot G1X Revealed

« on: January 05, 2012, 10:36:44 PM »

Hi,

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,

Doug

12

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,

Doug

13

EOS Bodies / Re: Anyone have a good explanation of "native" vs "expanded" iso?

« on: December 03, 2011, 07:36:47 AM »

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:

http://dougkerr.net/Pumpkin/#CMOS-APS

Best regards,

Doug

14

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:

http://www.gadgettown.com/Tripod-Mount-Ring-Canon-EF-70-200-2-8L-D00465.html

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

Best regards,

Doug

15

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,

Doug