About Fuji's "Value Angle", the importance of center forward and why Arjen Robben is not an option

Yes, yes, it's been about photography. The football is, however, produces the clearest explanations for why a camera system should have certain basic parameters to maximize the achievable image quality. Fuji recently the concept of Value Angle introduced to characterize camera systems, but what's this all about and what can us the Value Angle say?

Within a camera system you can use a camera body with a variety of lenses and combine a lens with different housings. And thus these components are interchangeable, certain characteristics of the system must be specified. These are primarily the Connection diameterThat flange (The distance between bayonet and sensor), the minimum back focus (The distance from rear lens and sensor) and the sensor diagonal, These parameters limit the scope of the lens designers as well as the camera's designers, and since they can not be changed after the introduction of a new system, you have to specify with a perspective of several decades if you obstruct any conceivable only in the future developments want.

Value Angle
The connection diameter (Mount diameter), The flange (Flange back), The minimum focal distance (Back focus) and the sensor diagonal are the characteristics of a camera system. Source: Fujifilm

Fuji's concept of Value Angle (That is literally a value angle) Now holds the characteristics connection diameter, sensor size and back focus to a handy number together. The idea is that the largest possible angle is generally advantageous and construction of lenses can accommodate that provide a particularly high image quality.

The formula for calculating the value Angle is

  Value Angle = 2 × arctan ((Anschlussdurchmesser – Sensordiagonale) / (2 × Auflagemaß))

To calm for math phobics: Man, this formula does not have to know, because the value angle can be represented graphically by inscribing it in a cross-section through the camera Also: You draw two lines, each containing a corner of the sensor and an edge connecting the lens mount, and the angle formed by these lines is precisely the searched value angle.

Wie man den Value Angle konstruiert, indem man in den Querschnitt der Kamera zwei Geraden einzeichnet, die jeweils eine Sensorecke und die Kante des Objektivanschlusses berühren.
For the Value Angle structure to draw a two straight lines each connecting a corner of the sensor with an edge of the lens mount. Source: Fujifilm

Thus the value Angle now assumes a very large value, the port diameter must be as short as possible significantly larger than the sensor diagonal and the flange. Reflections on the optimal parameters of a camera system are now not new. Olympus, for example, had analyzed the relationships between port diameter and sensor diagonal for the development of the introduced 2003 FourThirds system and had come to the conclusion that the lens mount should be twice the size of the sensor. That the flange did not matter, is easy to understand: Four Thirds was indeed a mirror reflex system, so that the distance between sensor connection and was largely determined by the mirror, which had to find space in between. It would have been pointless to think about optimal values ​​of a size thoughts, which is predetermined anyway.

The situation is different in the mirrorless Micro Four Thirds system, and also in all other mirrorless camera systems that followed it. In these systems, the flange is usually between 16 and 20 mm, and would not have still a mechanical focal plane shutter in front of the sensor are housed, one could reduce the mechanical focus on 10 to 15 mm. But why the lens mount should now be as short as possible significantly larger than the sensor and the flange back? The often given to the latter question answer is that you can then build smaller cameras, but in practice, this bill is not really on. The sharp image after all created at a certain distance behind the lens, and the optical path needs space - if not in the camera body, then just in an extended lens. measured from the front lens to display the camera lens combination is not small with a short back focus.

The real reason is to be found in optics, and thus we come to the football. As a photographer, one desires a lens that is substantially free of aberrations, the thus all light beams deflects the sensor pixels in each case, in which they contribute to a perfect image. A soccer player hopes to zirkeln the ball untenable between bar and post and goalkeeper into the net, and this task is comparable in many ways. A lens has aberrations on why light rays from a single point of the subject land in various sensor pixels and a canted lens directs all the light in the wrong direction. The football player can shoot the ball with a limited accuracy in the desired direction, so that this bar or pole happens often on the wrong side.

Now suppose a player aims at a certain point, for example, the upper right corner of the goal. However, the shot is off by a degree from that direction, and if we draw a line from the foot of the player think to destination, from the actual trajectory of the ball the farther from this line, the farther it flies. A hopeful attempt to achieve from the center line of a goal could miss the gate by one meter, while a shot from a position "to have to make" falls despite the deviation in the category directly in front of goal and fails just unlucky. So: The shorter the distance to the goal, the greater the chance to hit it despite not quite accurate shot.

Sure, everybody knows that, but same applies to the light rays exiting the lens. A small error affects more strongly from the farther is the distance the light must travel to the sensor. If the flange sufficiently short, the light often meets even the right sensor pixels, although the light beam deviates slightly from the ideal line. Thus, it would explain why a short back focus is so desirable.

For the chance to score a goal, the angle plays a role in which you attempt on goal. If the player in the middle front of the goal, making a slight deviation of the weft little difference. he attempts on goal but from a point near the sideline, the same deviation has much greater impact. It can also explain conversely: From the sidelines the gate appears foreshortened; the angle between the goal posts is much smaller than from the position in the middle front of the goal, and therefore it is more difficult to meet. The best position for a shot on goal is located in a central position as possible in the penalty area, ie where a center forward should be capable of directing an edge of a goal into the net. A direct shot of the goal would be far less promising, and therefore one should - contrary to what has often been said in recent years - not without a center forward.

In football, there are alternatives, however. A typical goal from Arjen Robben for example, becomes such that he gets near the center line of the ball, crosses on the right wing at high speed the opposition half, then pulls into the center and from there attempt on goal. In the square that looks very elegant, but in appearance, it is unfortunately not an option, because the light can not hit like Arjen Robben hooks, but spreads out in a straight line. Thus, the incident angle of the light beams is everywhere low, and the impact of imaging errors remain imperceptible, each light beam must leave the lens on a position that is the direct opposite to be taken sensor pixels. To guarantee this for all the pixels, the rear lens must be larger than the sensor to be, and accordingly applies the lens for the port, which gives the necessary space this large rear lens.

Value Angle
The course of the light rays in Fuji XF 56 mm: Thanks to the large rear lens, the light beams meet in the edge region of the sensor still nearly perpendicular to the sensor, whereby the effects of aberrations can be kept low. Source: Fujifilm

If criteria for an optimal camera system are explained: The flange should be short so that the lens delivers the light by the shortest route and with minimum deviations in the sensor and the connection diameter should be significantly larger than the sensor (a large rear lens and the to give them leave) place rays. A large value of the Value Angle indicates that these conditions are met. And how now the various mirrorless systems cut off in this regard?

Value Angle

Although I expected for this table using the same formula for the value angle as they used Fuji, my values ​​of which are different from that Fuji has presented itself. The reason is that Fuji expects when connecting diameter to the effective diameter which is slightly smaller than the inner diameter. But this effective diameter is not known in all systems; even the inner diameter, the one about listed at Wikipedia place is often not specified by the manufacturers themselves, but had to be measured. I have therefore expected the inner diameter, thus resulting in a somewhat larger value Angle. It's about the value itself as the comparison of the values ​​for different systems less makes no difference.

It does not now surprised that the value Angle for Fuji's own X system is relatively large (48.1 degrees). but he is not the greatest, but is supported by Canon's EOS System M (55.1 degrees), Leica T / TL models (54.5 degrees) and Sony's NEX cameras (52.8 degrees) still exceeded. In general, we find the greatest values ​​for cameras with sensors in the formats APS-C and Micro Four Thirds. Theoretically, one could also design systems with small and medium format sensors for a larger value angle but to a very large port and correspondingly bulky camera body would be necessary that you'd rather avoid.

Interestingly, the values ​​for those connections lens where there are cameras and lenses for two sensor formats, small picture and APS-C, the L-mount Leica, Panasonic and Sony and Sigma E-mount are. Both reach a large value Angle for APS-C, but only a very small value for miniature cameras. They are almost perfect so - but only for APS-C; for the small picture format, these connections are actually too tight. Although Fuji GFX models with medium format sensor miss the high value of the X-system from the same manufacturer, but perform better than L- and E-mount and Hasselblad competing XCD. What is achievable in the small image area, in particular shows Nikon, whose Z-system comes to a value angle of 40.3 degrees.

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Michael J. Hußmann

Michael J. Hussmann is a leading expert in the technology of cameras and lenses in Germany. He studied computer science and linguistics and worked for some years as a scientist in the field of artificial intelligence.

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    1. Die Berechnung des Value Angle macht eigentlich nur für spiegellose Systeme Sinn (siehe meine Anmerkung zu FourThirds); daher kommen keine Spiegelreflexsysteme in dieser Tabelle vor. Es gab zwar vor sieben Jahren mal eine spiegellose Pentax mit K-Bajonett (die wenig erfolgreiche K-01), aber ich sehe das System insgesamt als Spiegelreflexsystem an. Wie auch immer: Für Kleinbild läge der Value Angle bei 0,9 Grad, für APS-C bei 19,7 Grad.

  1. Vielen Dank für den Artikel! Als nicht Fußball-Fan ist es etwas mühselig der Analogie zu folgen.
    Nach dem “Value Angle” wäre also Sony E Mount (Vollformat) das ungünstigste Bajonett.
    Genau in diese Richtung argumentiert auch Nikon in seiner aktuellen Werbung.
    Sony hat auf einer Pressekonferenz behauptet, dass sei alles quatsch.
    Gibt es denn irgendwelche konkreten “Beweise”, inwieweit die Hersteller mit besserem “Value Angle” überlegene Leistungen abliefern?
    Als Sony-Nutzer ist meine Hoffnung, dass der geringe Bajonettdurchmesser, insbesondere bei Objektiven mit Blende 1,4, noch keine messbaren Nachteile produziert. Bei noch lichstärkeren Objektiven (1,2 / 0,95) mag ein größeres Bajonett vielleicht Vorteile bringen. Aber solche Objekte sind dann sehr schwer, haben einen langsamen Autofokus (schwere Linsen die bewegt werden müssen) und sind sehr teuer. Bevor ich in solche Objektive (mit den entsprechenden Nachteilen) investiere, würde ich dann lieber konsequent ins Mittelformat wechseln. Deshalb sehe ich für Sony keinen Nachteil, da die “normalen” Objektive wie z.B. 35er und 85er mit Anfangsblende 1,4 aus praktischer Sicht bereits das Optimum darstellen.

  2. toll.
    schon wieder muss ich loben.
    ich habe ganz genau verstanden, worum es geht.
    bisher waren mir solche technischen sachen immer egal – ich habe das jeweilige zeugs so verwendet, dass möglichst ordentliche bilder herauskommen.
    aber jetzt sehe ich viele dinge klarer und verstehe, weshalb es manchmal eben doch sinnvoll ist, sich mit technik-zeugs zu befassen.
    dankeschön für das anschauliche erklären – mehr davon!

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