...
Sheradenin> stas27>> Вкратце, вся соль в абсолютном (в мм) размере дырки объектива)
Balancer>> На пальцах поясни. Экспозиция зависит от диафрагменного числа, выдержки и чувствительности. В какое место вставляется абсолютный размер дырки?
Sheradenin> Ну мы же тут о ГРИП? ...
На самом деле не только. В конце концов качество картинки зависит от числа фотонов, которые были зарегистрированы светоулавливателем (о как я придумал - ничуть не хуже "думателя"
ТМ ). Это количество в конечном счёте определяется абсолютным размером дырки диафрагмы и площадью этого самого светоулавливателя. Отсюда проистекают более интересные зависимости, которые есть в ссылке, где слишком много латиницы (и там и
есть разъяснения [показать]Let's now demonstrate the DOF equivalence mathematically. As stated earlier, the DOF is the distance from the focal plane where objects in this zone are considered to be critically sharp. However, the distance from the focal plane is not always an even split. When the subject distance (d) is "large" compared to the focal length of the lens (non-macro distances), the far limit of critical focus (df) , near limit of critical focus (dn), and DOF can be computed as:
df ~ [H·d] / [H - d]
dn ~ [H·d] / [H + d]
DOF = df - dn ~ [2·H·d²] / [H² - d²]
where d is the distance to the subject and H is the hyperfocal distance. We can now compute the DOF behind the subject and the DOF in front of the subject:
DOF behind = df - d = d² / [H - d]
DOF in front = d - dn = d² / [H + d]
Note that the smaller the subject-camera distance (d) becomes in comparison to the hyperfocal distance (H), the more evenly the DOF is split in front and behind the subject, since (H - d) and (H + d) are nearly equal for values of d that are small compared to H. In other words, the common wisdom that 1/3 of the DOF is in front of the subject and 2/3 of the DOF is behind the subject is not always true. This "rule" is valid when only when the subject-camera distance, d, is equal to 1/3 the hyperfocal distance, H. As the subject distance changes from that particular value, the 1/3 - 2/3 DOF split becomes a progressively less accurate description of the split of the DOF in front and behind the subject. In another scenario, it is also interesting to note that as subject distance approaches the hyperfocal distance, the far distance of critical focus approaches infinity, and the near distance of critical focus approaches half the hyperfocal distance, thus giving infinite DOF beyond half the hyperfocal distance.
Another interesting scenario to consider is that when the subject-camera distance, d, is small compared to the hyperfocal distance, H, then, for the same format, the DOF will be essentially the same for the same framing and f-ratio. For example, 50mm at 10 ft has the same framing as 100mm at 20 ft on 35mm FF. If we shoot the scene at f/2 in each case, we will get the same DOF since the hyperfocal distance is 137 ft for a CoC of 0.03mm (the value used in most DOF calculators for 35mm FF, which corresponds to an 8x10 inch print viewed from a distance of 10 inches), which is much larger than the subject distance of 10 ft. However, were we instead to compare 24mm f/2 at 30 ft to 48mm f/2 at 60 ft (same framing), we would get a different DOF since the hyperfocal distance works out to 30 ft (for a CoC of 0.03mm), which is the same, rather than much larger, than the subject-camera distance.
In any case, we can see that the DOF is a function only of the hyperfocal distance (H) and the subject distance (d). The role of the focal length (FL), f-ratio (f), and CoC © are contained in the hyperfocal distance:
H ~ FL² / (f·c)
If we scale the focal length, f-ratio, and CoC by the sensor ratio (SR), the hyperfocal distance remains the same:
H' ~ (FL·SR)² / [(f·SR) · (c·SR)]
= [FL²·SR²] / [(f·c) · SR²]
= FL² / (f·c)
= H
Consequently the DOF is invariant for the same perspective, framing, and aperture diameter. By expressing H in terms of aperture diameter (a), angle of view (AOV), and the proportion of the sensor diagonal that the CoC covers (p), we get a format independent expression for the hyperfocal distance, and consequently DOF:
H ~ a / [2·p·tan (AOV/2)]
Thus, for non-macro situations, the DOF for the same perspective, framing, and output size is also the same.
, откуда берётся и куда суётся абсолютный размер дырки диафрагмы и сенсора):
Ещё раз призываю всех интересующихся этим вопросом посмотреть на экспериментальное подтверждение этих результатов в ссылке на ДПРевью, которую я тоже приводил чуть выше
.