In clause about charm and myths среднеформатной of a photo was shown, that on equal prints received from negatives of different size, the degree размытия is proportional to the attitude(relation):
f / N
Where
f — a focal length объектива, used at shooting;
N — диафрагменное number (1,4; 2; 2,8; 4, 5,6; 8; …).
The specified formula was received in the assumption of similarity of a corner of sight of compared photosystems. With some restrictions (in particular: without consideration пиксельную the structure of matrixes) this formula can be used and for comparison of digital and film chambers.
The size of the majority of modern digital matrixes is less узкоформатного of the staff. Hence, объективы with an equivalent corner of sight will be characterized by smaller focal lengths. But as the degree размытия on a final equal print is proportional to the attitude(relation) f / N, for maintenance of identical depth of sharpness it is necessary at shooting by the digital chamber to slightly open диафрагму, supporting an invariance of the mentioned attitude(relation) (f / N = const).
Unfortunately, not all объективы of digital chambers (especially budget) have sufficient светосилу to compensate reduction of a focal length by an equal corner of sight. The complaints of many photographers on « too large ГРИП цифровичков » are connected to this fact.
The important note
About a duality physical сущностей
Is instructive and that fact is interesting, that at change of the entry conditions it is possible to reach a completely opposite conclusion, namely: the depth of sharpness at digital chambers is less, than at film analogues.; -)
Really, we shall consider a case of use same объектива on the film and digital chamber. According to same clause: размытие on a print is proportional to product:
( K f) (f / N),
Where
K — factor of increase of the initial carrier at a seal;
f — a focal length объектива, used at shooting;
N — диафрагменное number (1,4; 2; 2,8; 4, 5,6; 8; …).
At constant f, размытие is proportional To / N. But, as for reception of an equal print digital “«negative” is increased in the greater number of time, is received, that the degree размытия on him (print) will be stronger (and depth of sharpness, accordingly, it is less). A paradoxical conclusion? At all no! Simply entry conditions for comparison have changed, and together with them the conclusions have changed also.
When the diagonal of a digital matrix in 1,5 times is less узкоформатного of a negative (and it fairly for many modern digital mirror chambers), for achievement of equivalent depth of sharpness at constant f it is necessary to cover диафрагму on the digital chamber approximately on one step (supporting an invariance of the attitude(relation) To / N). If it to not make, at same объективе and constant диафрагме the digital chamber of a considered(examined) class will show smaller depth of sharpness on an equal print.
However in most cases comparison вс ё is more correct than a message under condition of a constancy of a corner of sight, instead of constancy of a focal length (that is under condition of (K f) = const, instead of under condition of f = const).
The theory is those. And what is really possible to expect from digital chambers of a different class?
Compact digital chambers
Let’s begin from an indicative example:
If we, for example, have wanted, that the chamber CANON Digital IXUS i (f = 6,4 mm) could wash away a background as well as объектив PENTAX 43/1,9 Limited on a format of 35 mm, we should establish on it(her) объектив with светосилой f / 0,28!!! (6,4 / 0,28 is approximately equal 43 / 1,9).
Alas! The theoretical limit of a geometrical relative aperture корригированного объектива, satisfying to a condition of sine, is equal 0,5 [2]. Therefore, having taken for the prototype CANON Digital IXUS i, we never and at any conditions could not achieve such размытия, which gives PENTAX 43/1,9 Limited on a format of 35 mm!
Объектив of the chamber CANON Digital IXUS i was chosen for comparison with объективом PENTAX 43/1,9 Limited simply because his(its) equivalent focal length equally 39 mm, that is maximum close to 43 mm. In this case difference in 4 mm is not too basic, for even with е ё by the account the conclusion remains former: for achievement of equivalent washing away force in this case it would be required to us physically not sold объектив!!!
For the fans(amateurs) of accuracy nevertheless I shall notice: in this case completely equivalent on a corner of sight объектив would have a focal length not 6,4 mm, and 7 mm. For equivalent размытия he should have светосилу f / 0,3.
Now we shall consider, for example, chamber Sony Cyber-shot DSC-V1, Nikon CoolPix 5400, Canon PowerShot G5, Konica KD-410Z. To provide размытие, equivalent PENTAX 43/1,9 Limited, объективы of these chambers should have светосилу f / 0,39 (on a focal length of 8,9 mm equivalent to a focal length 43 mm in узкоформатной of a photo). Again size of the maximal relative aperture has appeared behind a side of a physical realizability (though it has turned out hardly more, than in the considered indicative example)!
Let’s lower our requirements. Let’s compare these chambers not with объективом PENTAX 43/1,9 Limited, and with объективом Индустар 50-2 (50/3,5). Provide same размытие at an equivalent corner of sight объективы of chambers from our set should to have светосилу f / 0,72. The barrier of a physical realizability would seem пройден! But to be pleased early. Photographic объектив with such светосилой basically it is possible to make. But whether he will allow to receive the image, acceptable on quality? Alas, no! If the similar optical devices could easily be made — объективы with such светосилой for a long time already laid in кофрах at all photographers! So, again it is necessary regretfully to ascertain: объективы of compact digital chambers basically are not capable to wash away a background even how it does(makes) modest Индустар 50-2.
It is not necessary to think, that for the analysis in this clause the unsuccessful models of digital compact chambers were specially selected. At all selected models at a wide corner of the review rather decent (on меркам узкоформатной of a photo) светосила 2,8. Nevertheless, е ё in any way it is impossible to consider(count) sufficient for digital compact chambers. It is necessary also to mean, that if to analyse other models digital компактов, the estimations required светосилы will be about the same order, as above mentioned.
Digital mirror chambers
At digital mirror chambers of business with размытием are is more best. First of all, it is connected that at such chambers the size of a photosensitive matrix is more in comparison with digital компактами. So, at many modern chambers of this class the diagonal of a matrix only in 1,5 times is less than a diagonal узкоформатного of the staff.
Whether there are digital mirror chambers, which size of a matrix have more? Yes. An example of such chamber is Canon EOS-1D Mark II with a diagonal of a matrix of 34,5 mm, that there is less узкоформатного of the staff all in 1,25 times.
Now in a considered(examined) class (that is, for chambers, at which the diagonal in 1,5 times is less узкоформатного of the staff) we shall find digital equivalents for ours old familiar: for PENTAX 43/1,9 Limited and for Индустар 50-2 (50/3,5). To first объективу will correspond(meet) объектив 28,7/1,3; and second — 33,3/2,3. Both объектива светосильные, but quite sold. Certainly, светосильную широкоугольную it is impossible to consider(count) optics for mirror chambers in any way accessible and mass. But, at least, to talk about physical нереализуемости already it is impossible.
Physical sense of the attitude(relation) f / N
The attitude(relation) f / N is not that other, as the effective aperture объектива (diameter of an entrance pupil), designated often in the literature by the letter D.
However in the text of given clause intentionally instead of the effective aperture the attitude(relation) f / N is discussed. To that there are some reasons. Three main are those:
First, at comparison modern объективов for film and digital chambers always it is necessary to compare объективы to different focal lengths. Hence, the specified attitude(relation) allows at once to estimate a realizability (and – or availability) this or that equivalent объектива (just as it was made above).
Secondly, the people completely incorrectly treat far from physics often effective aperture. So, for example, they often consider(count), that the diameter of a forward lens объектива — it and is a diameter of an entrance pupil, that, certainly, it is completely incorrect! The diameter of a forward lens generally is not the effective aperture.
Thirdly, the information on a focal length and светосиле/диафрагмах (that is, about f and N) is obviously displayed съемочной by the equipment, and, by virtue of it, almost always is unequivocally perceived by all photographers.
The conclusion
Thus, the large depth of sharpness peculiar to many digital compact chambers is connected that the matrixes, used in them, are much less узкоформатного of the staff. In digital mirror chambers matrixes, as a rule, cost(stand) is appreciable of the greater size (though and still less узкоформатного of the staff). Therefore for digital зеркалок the problem размытия of separate elements of the image costs(stands) is not so sharp.
Advice(council): If an opportunity to wash away a background on the photoimage for you is important, before purchase of the digital chamber estimate е ё « washing away force » with the help of the mentioned formula: D = f / N (in this formula it is necessary to substitute a true focal length, instead of equivalent).
The important final remark
It is necessary to understand, that in given clause объективы of digital chambers were compared with объективами PENTAX 43/1,9 Limited and Индустар 50-2 (50/3,5) extremely by criterion of an equivalent degree размытия on a final print. It is clear, that any объективы, including mentioned in the given clause, are characterized not only this parameter, but also set of other characteristics. Therefore from that fact, that any объектив provides the same degree размытия, as,say, PENTAX 43/1,9 Limited, at all does not follow, that this объектив will form the image, on figure not отличимое from the mentioned optical product of firm PENTAX.