[Photography] Testing the chromatic aberration in Sigma 10-20mm F4.0-5.6

In reviews, it is said that Sigma 10-20 F4.0-5.6 (UWA - Ultra Wide Angle lens) has a low chromatic aberration (CA) (which is a good thing for people like me as I mainly do landscape photography), but I noticed that the aberration varies by quite a margin with different F (DoF - Depth of Field) values.

First some background info:
Chromatic aberration is a kind of distortion where the lens fails to focus all component waves of the white light to a single focal point. The white light consists of component waves of color red, green, yellow, blue and violate (in their increasing order of frequencies). The refractive index of lens glass varies with the wavelength; shorter waves bends more than the longer ones. See the following diagram:

When this happens, the sensor averages them and the edges in the image becomes distorted. See the Wikipedia page for an example. I was told that this happens mainly from the light entering closer to the edge. So, in theory, if you have a smaller aperture (i.e. later DoF value), aberration should be small. A simple technique to correct this distortion is to have a concave lens right next to the convex lens so that the effect of irregular bending of wavelength is corrected by dispersing them. But with mm focal lengths, it seems that such a simple technique does not work - it's much more difficult to correct this effect with UWA lens as they support very small focal length.

Test setting:
I used a Canon EOS T1i body and took multiple pictures at the same focal length (10 mm) but varying the depth of field (F values at 4/4.5/5, 5.6, 8, 11, 22, 29) at the same light condition. Here are some sample photos.

Unprocessed JPEG image for focal length = 10 mm, F = 4.0:



Unprocessed JPEG image for focal length = 10 mm, F = 11.0:



Unprocessed JPEG image for focal length = 10 mm, F = 22.0:

Note that due to the crop factor (1.6 in EOS T1i), the actual focal length is close to 16 mm.

I was expecting the CA to monotonically decrease with increasing DoF value (think in the line that with larger F values, we have smaller aperture and light will pass through mostly middle of the lens). However it was not the case. Maybe the complex inter-leaved lenses have other effects. The CA visible to the naked eye decreased 4 (highest CA), 5.6 8, 11 (lowest CA) and 11 and 16 had similar quality. At 22, CA was quite high and image quality was less than that for at 4. I am not in a position to explain this bell-curve shape like behavior. Would be very much interested to know the technical details behind the scene.

I repeated the above experiment for focal lengths 12, 14 and 16 mm. I found a similar pattern. The visible CA decreased as the F value is increased closer to the current or little above the current focal length but beyond that visible CA increased with increased F values.

So, with Sigma 10-20mm F4.0-5.6 on a Canon EOS T1i camera, if you want to take landscape photos with minimal CA, set the F value closer to the current focal length.

What's your identity (and religion)? [not a security post]

Yes, what is your identity? We immediately think of the country we were born (the nationality), the religion (or a sub division of it) we follow, the ethnicity, the cast to which we belong, our parents, our siblings, the native languages we speak, our skin color, our height, etc. don't we? But wait...if you come to think about it, we pretty much don't have control over any of the above attributes; we don't have control over who our parents are, where we are born and so on; YET we not only label ourselves using those attributes, but sometimes, go so far as to start an arms struggle based on the differences in these attributes. Look at the current news -- most of the conflicts are due to these attributes -- the attributes we did not earn ourselves, but GIVEN to us (different religions interpret how this inheritance works differently -- but the underlying core is the same -- there should be some source of energy which does everything in such an orderly manner -- some of which are beyond human imagination) It is also sad to see that we discriminate people based on these labels that they don't have control over; high cast or low cast, black or white, short or tall, and so on. Your nationality is not your identity, and so are the skin color, the religion (literally), mother tongue, etc.

So, what exactly is your identity? Identity is something that you build yourself with a good intention and over which you have control. And that serves the greater good. Most religions I know of teach us to build this identity. However, looking at the current affairs, the religious identity is gravely misunderstood. This religious label is not your identity. Your religion becomes your identity only when you are truly honest to yourself (for example, treat everyone with the same spoon irrespective of the uncontrollable attributes) and truly care to make the world a better place to everyone (for example, by sharing your knowledge, wealth, etc., by raising your voice in a peaceful manner for the oppressed, the weak, etc.). In short, don't be evil. I hope this post gave you some food for thought.

Twitter

After a long wait, debuted in Twitter :) My twitter id is nabeel_yoosuf. Hoping to share interesting/useful links and events there.

Tracking patients remotely

When we talk about GPS, we immediately think about going from point A to B. Technologies similar to GPS have been used to track patient remotely. The basic idea is that these devices, which are, most of the time, attached to the patient, report the location information to a central location and if the movement patterns deviate from the normal patterns, they detect an anomaly. That anomaly could be something good (for example, a patient who is recovering making some movement could be a positive sign, no movement at all could be a negative sign).

There have been commercial as well as research projects in this regard. For example,

Remotely monitory elderly location: here here
A research project to track the recovery from a surgery: here
A device to track dangerous psychiatric patients: here
And many more

Even though these devices/techniques are designed/deployed with good intension, one concern here is people who are being monitored have no control over their own data, i.e. their movement information. And they don't have control over who can view their data. Hence, it could lead to serious privacy breaches. I'd like to see a system where it gives more control to the target (to someone on behalf of the target) over their information.

Security :-)

Hi & Lois (via Schneier on security)

Security by deterrence

When no one can watch (or trace one to what they do), the possibility of doing something bad (steal, break in, erase/modify/add data, etc.) is quite high. I was thinking about security cameras in supermarkets and shops. Is it more effective to have those cameras well visible to everyone or have them hidden? In my opinion they should be installed in visible locations; if they are not visible, there will be more bad people attempting to do bad things; it's true that you can track them down going through the surveillance videos and prosecute them - but think about the cost you have to incur; it is far more economical to indicate some sort of a warning signal. This will reduce the number such incidents and, yes, you can take necessary actions against those few bad incidents where bad guys dare to ignore the warning. Thinking in this line, you actually don't need real cameras installed all the places - you can safely have a few fake ones installed along with the real ones - they will act as a deterrence factor. (If you cannot afford to have a video surveillance system, it is at least good to have some fake cameras installed.)

What about firewalls, ID (Intrusion Detection) systems? I think we can make a similar argument about them.

Another side note, have you ever come across a situation where you cannot keep the lunch packets or any other food items from your co-workers in an office or classmates in a school? One crude way to do that is to take a bite and keep it :-) it'll surely act as a deterrence. A good way to have it packaged as if it's not a lunch packet - most hungry people won't bother to open that. This sandwich bag seems to be a good idea as well (though won't work after others figure out your trick)

[Math] Transitivity of relationships

Most of the relationships in the real world that we know are transitive. For example, if Bob is taller than Tom and Tom is taller than Alice, it naturally implies that Bob is taller than Alice. At the same time, there are many other relationships where the transitivity is not clear. Take for example, a triangular series among Sri Lanka, India and England. Let's say that India beat England and England beat Sri Lanka. Does that mean India will beat Sri Lanka (comprehensively)? Not necessarily - in fact, it has proven numerous times in the past that the transitive relationship does not hold (a tournament would be boring if it were the case). In other words, the relationship is probabilistic in nature.

Some more not necessarily transitive examples in the technology/science field:
(Social networks) Bob is a friend of Sam. Sam is a friend of Tom. It does not necessarily imply that "Bob is a friend of Tom".
(Trust relationships in security) Alice trusts Bob to keep a secret. Bob trusts Mary to keep a secret. It does not necessarily imply that "Alice trusts Mary to keep a secret" since Alice needs to trust on something else to make the transitivity working. That something is Bob's ability to judge Mary's trustworthiness to keep a secret.

I also find that some relationships can never be transitive. For example:
(Family relationships) Mary is mother of Alice; Alice is mother of Eve. It is incorrect to say "Mary is mother of Eve".

In computer science, we mostly deal with deterministic transitivity. Take for example, Lamport's clock; if an event A occurs before an event B and an event C occurs before the event A, we safely conclude that the event C occurs before the event B. And the transitivity always holds. But, what about probabilistic transitivity?