Figure 1 – Cross-section view of SLR system: 1: Front-mount lens (four-element Tessar design) 2: Reflex mirror at 45-degree angle 3: Focal plane shutter 4: Film or sensor 5: Focusing screen 6: Condenser lens 7: Optical glass pentaprism (or pentamirror) 8: Eyepiece (can have diopter correction ability). From the Wikipedia and created by CBurnett, in the public domain under creative commons attribution license.

I’d like to return today to our technical discussion of camera optics.  We have looked at what mirrors do to light and at what lens do to light.  Based on what we learned so far take a look at Figure 1 which shows the innards of a single lens reflex or SLR camera.  The first object that we see is the lens.  The lens is actually a composite of multiple lenses.  However, we know that the lens is going to invert the object on the sensor or film.  Up and down is flipped, and so is right and left.  Look at Figure 2.  If the object was the letter F (Today’s blog is brought to you by the letter F) then what appears on the sensor or film is the “lens inverted image.” This is what you see in a large format camera on the ground glass.

Figure 2 – Image inversions in a modern SLR camera. Image may be used under Creative Commons Attribution license.

It is also all that you need to make a camera if it is to be purely digital. This is because in a digital camera you can make all the corrections that you need computationally.  You display and store the file with all the corrections made.

The In the next generation of camera, makers decided that this inversion needed to be fixed.  By putting a mirror into the camera at a right angle they could create a “righted mirror image”  where up and down were fixed but right and left were still flipped.  You can do this with a single lens, in which case the lens needed to be flipped out of the way when the picture was taken.  A second approach was the twin lens reflex, which had two identical lens: one for the picture and one for the viewfinder.  This is, of course, a bit costly and also introduces what are called parallax effects as the object gets closer and closer to the image.

To create the right side up image of the modern SLR viewfinder, makers use a pentaprism, which is actually a set of reflective (like a mirror) surfaces that multiply flip the image until it is corrected and do this in a fairly confined space.  For illustrative purposes you can follow the multiple reflections of the blue and red rays in Figure 3 to convince yourself that geometric points wind up where they need to be.  This is shown in Figure 1 and in more detail in Figure 3.  Again with the modern SLR it is necessary to flip the mirror out of the way during the exposure.  This can be done either automatically or manually before exposure if you are afraid of vibrations affecting your image.

Figure 3 – Schematic showing operation of a pentaprism in a modern SLR camera. Image from the Wikipedia created by Paul1513 and put in the public domain under creative commons attribution license.

Figure 1 – IPhone 4S image taken with the low resolution front-facing camera. (c) DE Wolf 2014.

Why does the Nokia Lumina cellphone offer a honking 41 mega pixel camera on the back and only a 1.2 megapixel front facing camera.  That’s the one you use for all those important selfies.  Remember that the selfie is the new self-expression medium.  So this is important people.  And why is this what all the cell phone companies do?  Well you’re not going to get an answer.  It has become one of those great rhetorical questions like: what is the meaning of life and why is there air?

Fortunately, New York Times reporter Molly Wood has posted a very entertaining and informative video “Your Best Selfie” to answer the next best question: what cellphone gives you the best selfie?  And since she’s done a nice side by side, apples and apples comparison you can weigh in with your own opinion.  Ms. Wood compares the IPhone 5S with its 1.2 megapixel camera, the Nokia Lumina also 1.2 megapixels the Samsung Galaxy S4 at 2 megapixels, and the HTC One with its 2.1 megapixels.

Figure 2 – IPhone image taken with higher resolution rear-facing camera. (c) DE Wolf 2014

Ms. Wood correctly points out that it’s not all about the number of megapixels.  This agrees with all that we have said here about image sharpness.  There’s also optics and sensor quality as well as focusing accuracy.  For my mind there’s also the ability of the camera to accurately judge the white balance.  I mean you can do it yourself, but who wants to do that.  I find that warm orange glow of incandescent light kind of soporific and yucky.

Ms. Wood disses the consistency of the IPhone 5S.  I’m not so hard on it.  But her winner for the best selfie sharpness and color is the HTC One, with the Nokia Lumina being the runner up.  Look at the pictures that she shows and I think that you will agree.

I also decided to do a little testing myself.  Figure 1 was taken with my IPhone 4S’s low resolution front-facing camera – not so great.  Figure 2 was taken with the rear-facing 8 megapixel camera – better but still less than I like.  I decided to leave the glare in the pictures.  It’s a common problem with my IPhone.  Yes, it’s due to the overhead lighting, but my Canon T2i would do a much netter job dealing with it.  And ultimately that’s why we sepend big bagels on cameras.  Both of these selfies could use a lot of improvement.  I have not yet tried out the newer versions of the IPhone or other cellphone cameras myself yet.  But Molly Wood does a pretty nice job in her video.

The thing is that a cellphone is becoming much more than a wireless on the go telephone. People use it to surf the web and take pictures.  A selfie photography with the front facing camera is becoming more and more a popular sport.  So the important question, of course, is when will the industry respond to the user.  I mean cellphones are already growing in size suggesting that two points: first that the initial read about the market that smaller and smaller would always be better and second that people just don’t like squinting at their cellphones.  Makes one wonder if we will retro-evolve (retrovolve?) back to the Maxwell Smart shoe-phone size cellphone all the way back to “Hey why don’t we put this baby on the desk?”

Figure 1 – A floating blob of water passes in front of Astronaut Chris Hadfield’s face on the International Space Station on January 27, 2014, showing how a lens inverts or rotates an image by 180 deg.. Note also the demagnification. Image from NASA and in the public domain.

The next camera optical element to consider is the lens.  Figure 1 is an image of Canadian astronaut Chris Hadfield’s floating in the International Space Station on January 27, when a spherical blob of water passes in front of him and acts like a lens.  His image is inverted and demagnified as a result of refraction. Right and left are flipped as we see them, which is to say they are preserved for the inverted image.  It is as if Hadfield’s face was rotated by 180 degrees.

From my point of view this image, posted by Hadfield on Twitter, is both whimsical and genius.  In the spirit of a picture is worth a thousand words, Figure 1 pretty much says it all!

Recently I was discussing with a reader whether she could use her IPhone to photograph comet ISON.   This was back in the glory days when comet ISON still had promise.  Well those days appear to be gone, but tonight my wife brought to my attention some very interesting cell phone camera accessories.  So if there were a very bright comet one might just be able to take cool pictures of it with your cellphone.  I need to make two important points: 1. I have not tried any of these gadgets out.  So this is not a product endorsement.  But they are pretty cheap so I may try one or two of them. 2. You almost always get what you pay for; so they are probably crap.  But here goes.

First we have changeable lens for your IPhone or Android.  You can get a 2 x telephoto lens, a macrolens, a polarizer, and a wide angle fisheye lens –  all for around $20 a piece.  Then there is a turret that mounts to your phone; so that with a simple rotation you can select between lenses.  Weep your eyes out Leica owners.  Back in the dinosaur ages this was a coveted M series accessory!  There is also a film copier to digitize slide film onto your camera.  How many people want to do that? At $54 it’s a bit pricey.

Want to turn you smartphone into a microscope for about $25 – no problem.  How about a smartphone telescope?  This too is no problem. And for those of you who already have microscopes or telescopes finding an cell phone adapter is also not a problem.

Mostly we are talking gimmicks, toys, and science fair projects here.  But, the possibilities are definitely endless.  I have a colleague who launched a weather balloon armed with a cellphone.  It took pictures and beamed back its gps location.  Also, and luckily, it didn’t hit anyone on the way down.  Did I mention that the parachute failed to deploy?

I read today that Amazon is planning on delivering packages by drones to your doorstep in about five years.  So of course, David is thinking about two of his favorite blog themes: robotic eyes and the technological singularity.  Technical innovation is driven by cost and the availability of such “toys” for twenty dollars or so opens up a very exciting realm – not to mention a potentially very dangerous one.  Stay tuned.

Figure 1 – From the Bayeux Tapestry (c 1070), the appearance of Halley’s Comet in 1066 AD brings fear to the hearts and minds of the troops of William the Conqueror. Image uploaded by Mirabella to the Wikipedia Commons and in the public domain.

Well here’s something unusual – a technical blog.  What can I say, I’ve gotten just a bit lazy.  But I do want to point out that on Thanksgiving Day, November 28, 2013 comet ISON is going to whip around the sun, aka reach its perihelion.  If it doesn’t completely vaporize, which is pretty unlikely since this is what comets do, it may produce a truly spectacular show in December and January.  This means, of course, that you are going to want to photograph it, and I am going to tell you how.

First of all, I am going to assume that you do not have a telescope with celestial clock drive.  A celestial clock drive is a computer driven motor that enables you to point your telescope at a sky object, and it will follow the object as the sky moves.  BTW – they worked pretty well in the analogue days before computers – better living through physics.  If you have one of these, you probably don’t need me to tell you how to use it.  Second, this could turn into a pretty big object, in which case a telescope isn’t necessary.  It all depends, and that’s the key.  Nobody really knows what we are in for.  But it is likely that the best shots are going to be taken with a moderate telephoto or zoom lens.  I’m hesitant to conjecture, but maybe 200 to 400 mm.

So here you go:

1. Find a place away from city lights.  That’s the tricky part because these are going to be several second exposures, and you don’t want the background of the sky to outshine the comet.  This also makes just after perihelion a bear of a time to photograph.  At that point comet ISON is going to be in the early morning sky, hovering below the horizon.  Also at these early points, do I have to say this, DON’T LOOK DIRECTLY AT THE SUN.  I have a feeling that the best times are going to be mid-December to early January.

2.Moderate zoom lens

3. Lens wide open aka smallest f-number

4. You need a tripod

5. Image stabilized lens recommended because of wind.

6. Manual focus preferred – despite the fact that the thing is at infinity

7. ISO of at least 800

8. Lock your mirror up to avoid shake

9. Exposure will be several seconds, depending on events again.  This is probably best done with manual shutter speed and bracketing.

This should be all you need to know.  Happy shooting.

Figure 2 – Comet ISON photographed by the robotic eyes of the Hubble Space Telescope on October 9, 2013. From NASA and in the public domain.