On Pentaxian discussion boards, one dispute that absolutely refuses to die is whether Pentax will (or should) introduce a “full frame,” 24x36mm sensor DSLR.

The pleading for 24×36 often comes from those who might have already invested heavily in earlier FA lenses—meaning, ones originally designed to cover the old 135 film format.

I understand the emotion here—it’s quite true that an APS-C sensor wastes some of the capabilities of those lenses. And let’s take it as given that Pentax’s engineers would be fully capable of producing an excellent 24×36 camera.

But does Pentax “owe” its existing customers such a model?

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Despite endless technological evolution, one photography convention has endured for more than a century: Our numbering system for lens apertures. That is, the familiar “f-stop” scale:

1.4   2   2.8   4   5.6   8   11   16   22

This sequence is admittedly peculiar-looking, and it always confuses beginners. Why do larger numbers mean smaller-diameter lens openings?

But as many of you are aware, these numbers are actually “f/ratios“—that is, they’re the ratio of a lens’s focal length to the aperture diameter. Setting a lens to f/4.0 means its aperture opening measures one-fourth of the focal length.

F-number Definition

F/Ratio Definition

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Something About Sharpness

February 19, 2010

Once upon a time, the way photo-geeks evaluated lens quality was in terms of pure resolution. What was the finest line spacing which a lens could make detectable at all?

Excellent lenses are able to resolve spacings in excess of 100 lines per millimeter at the image plane.

But unfortunately, this measure didn’t correlate very well with our perception looking at real photos of how crisp or “snappy” a lens looked.

The problem is that our eyes themselves are a flawed optical system. We can do tests to determine the minimum spacing between details that it’s possible for our eyes to discern. But as those details become more and more finely spaced, they become less clear, less obvious—even when theoretically detectable.

The aspects of sharpness which are subjectively most apparent actually happen at slightly larger scale than you’d expect, give the eye’s pure resolution limit.

This is the reason why most lens testing has turned to a more relevant—but unfortunately much less intuitive—way to quantify sharpness, namely MTF at specified frequencies.

MTF Chart

Thinner Curves: Finer Details

An MTF graph for a typical lens shows contrast on the vertical axis, and distance from the center of the frame on the horizontal one. The black curves represent the lens with its aperture wide open. Color means the lens has been stopped down to minimize aberrations, usually to f/8 or so. (I’ll leave it to Luminous Landscape to explain the dashed/solid line distinction.)

For the moment, all I want to point out is that that there’s a thicker set of curves and a thinner set.

The thinner curves show the amount of contrast the lens retains at a very fine subject line spacings. The thicker ones represent the contrast at a somewhat coarser line spacing (That’s mnemonically helpful, at least.)

The thick curves correspond well to our subjective sense of the “snap,” or overall contrast that a lens gives. Good lenses can retain most of the original subject contrast right across the frame. Here, this lens is managing almost 80% contrast over a large fraction of the field, even wide open. Very respectable.

The thin curves correspond to a much finer scale—i.e. in your photo subject, can you read tiny lettering, or detect subtle textures?

You can see that preserving contrast at this scale becomes more challenging for an optical design. Wide open, this lens is giving only 50 or 60% of the original subject contrast. After stopping down (thin blue curves), the contrast improves significantly.

When lenses are designed for the full 35mm frame (as this one was) it’s typical to use a spacing of 30 line-pairs per millimeter to draw this “detail” MTF curve.

And having the industry choose this convention wasn’t entirely arbitrary. It’s the scale of fine resolution that seems most visually significant to our eyes.

So if that’s true… let’s consider this number, 30 lp/mm, and see where it takes us.

A full-frame sensor (or 35mm film frame) is 24mm high. So, a 30 lp/mm level of detail corresponds to 720 lines over the entire frame height.

The number “720” might jog some HDTV associations here. Remember the dispute about whether people can see a difference between 720 and 1080 TV resolutions, when they’re at a sensible viewing distance? (“Jude’s Law,” that we’re comfortable viewing from a distance twice the image diagonal, might be a plausible assumption for photographic prints as well.)

But keep in mind that a 30 line pairs/mm (or cycles/mm in some references) means that you have a black stripe and a white stripe per pair. So if a digital camera sensor is going to resolve those 720 lines, it must have a minimum of  1440 pixels in height (at the Nyquist limit).

In practice, Bayer demosaicing degrades the sensor resolution a bit from the Nyquist limit. (You can see that clearly my test-chart post, where “40” is at Nyquist).

So we would probably need an extra 1/3 more pixels to get clean resolution: 1920 pixels high, then.

In a 3:2 format, 1920 pixels high would make the width of the sensor 2880 pixels wide. Do you see where this is going?

Multiply those two numbers and you get roughly 5.5 megapixels.

Now, please understand: I am not saying there is NO useful or perceivable detail beyond this scale. I am saying that 5 or 6 Mp captures a substantial fraction of the visually relevant detail.

There are certainly subjects, and styles of photography, where finer detail than this is essential to convey the artistic intention. Anselesque landscapes are one obvious example. You might actually press your nose against an exhibition-sized print in that case.

But if you want to make a substantial resolution improvement—for example, capturing what a lens can resolve at the 60 lp/mm level—remember that you must quadruple, not double the pixel count.

And that tends to cost a bit of money.

Where Are The Lenses?

January 23, 2010

Most DSLRs today evolved from earlier film-camera systems. (Sony’s originally came from Minolta; only Olympus started over from scratch.)

Although lens mounts stayed the same, there was a tiny problem about the sensor. Film cameras shot in a 24 x 36mm format. But making digital sensor chips of that size turns out to be quite expensive and difficult.

Sensor chips are made on costly, ultrapure silicon wafers, each about 8 or 12 inches in diameter. Obviously, increasing the area of each sensor means fewer of them can fit on the wafer.

With all the steps needed to lay down pixel electronics, it’s nearly unavoidable to get a few random, chip-wrecking defects scattered across the wafer. So the bigger each sensor is, the more likely it is to be ruined by some defect.

These two factors mean the economics of “full-frame” sensors will always be forbidding. You can read more details in a rather informative Canon PDF white paper here. (Take their marketing spin with a grain of salt; just start reading at page 11.)

By Canon’s reckoning, a finished APS-C sensor might cost 1/20th as much as a full-frame one. (That was written in 2006; today’s numbers might be a little different, with 12″ wafers more common. But still, the principle applies.)

So, despite all the wails and begging of enthusiast photographers, there are still only a handful of 24 x 36 mm format digital cameras on the market. A Canon 5D Mk II is $2500. A Nikon D700 is $2400. A Leica M9 is a whopping $7000. A Sony A850 is the “bargain,” at only $2000. These prices are without lenses, of course.

Today’s affordable DSLR models are all based on smaller, APS-C sized sensors. The origin of that cryptic name is irrelevant today; but it simply means a chip slightly under 16 x 24 mm.

There are dozens of APS-C models on the market, starting from the low $400’s—and that price includes a kit zoom. Megapixel counts range from 6 to 14 Mp. While it would be misguided to push pixel counts higher than that, the current models give satisfactory images even when set to ISO 800.

It seems apparent that APS-C is today’s sweet spot for digital-camera value. And because of the chip economics I mentioned, that is not likely to change anytime soon.

So let me (finally) get to my real point.

Where are the lenses?

Where Are the Lenses?

Missing in Action: Interesting APS-C Primes

Back in the olden days of 35mm SLRs, the “kit lens” was typically a 50mm standard one, with an aperture f/1.8 or so. A photographer more serious about low-light shooting could buy the f/1.4 version. You could get a nice inexpensive wide angle or portrait lens of f/2.8 or faster.

So, where are the equivalents for APS-C?

Lots of old lenses designed for film bodies are still being sold. But when used on APS, these make you to pay a premium in size, weight, cost, and maximum aperture. Cameramakers have dragged their feet on creating interesting, new, fast lenses dedicated to APS-C bodies.

Today, of course, the default is to offer zooms instead of primes;  the APS-specific lenses you are able to buy are mostly zooms.

Yes, zooms are convenient. But you typically lose two f-stops of light-gathering power. Some say modern image stabilization gives back those two stops—but that’s true only if you don’t care about viewfinder dimness, or blur when the subject moves. Zooms are larger and heavier than primes, too.

The normal lens for an APS-C camera would be about 32 mm (48e on a 1.5x sensor; 52e on a 1.6x Canon). The only camera maker so far to “get it” with an APS-specific normal is Nikon, with their 35/1.8. Sigma sells a 30mm f/1.4 in various mounts—but it’s mystifying that they’re all alone in that market.

For portraits we generally want a nicely-blurred background—meaning we’d like a wide maximum f/stop. This is especially true when using a smaller sensor, because depth of field increases slightly compared to 24 x 36 format. So where are the APS-specific portrait lenses, at f/2.0 or faster? In the range of 60 to 70mm (giving 90-105e), there’s only this Tamron—intended more as a dedicated macro lens.

Yes, there’s oodles of 50mm’s around, recycled from the film era. Canon is well known for their “thrifty 50” —which apparently they’re able to knock out for a hundred bucks, despite it covering a larger format. Why on earth should APS-specific lenses be more expensive? The image circle they cover is only 2/3rds the width!

Shooting film, my most-used wide-angle is a 24mm f/2.8. And back in the day, cheap 28mm f/2.8’s were a dime a dozen. But convert that to APS-speak. Are there any f/2.8 lenses of roughly 17mm? Is your sole available choice one chubby $600 zoom? I sure can’t find anything else.

I’ll give credit to Pentax, for creating the widest lineup of APS-specific lenses—including several beautifully-finished primes. But their prices are high, and their widest apertures are really nothing to get excited about.

Finally, lets take a glance at the Micro Four Thirds universe, too. Panasonic’s new 20mm f/1.7 pancake (40e on the µ4/3 sensor format) has indeed made quite a splash.

The test reports are excellent. So I suppose it would be snarky to observe that Panasonic’s 20 just revives a lens style that numerous snapshot cameras offered in the 1970s—and at a much higher price.

So, where are the lenses?