In an era of total victorious march of digital technology, many supporters of traditional film photography are increasingly turning their gaze to the middle and large format. After hearing about the benefits of these formats, newcomers are often imbued with a confidence that even small footprint obtained with medium format will be strikingly different from similar-sized prints obtained with a narrow film. How justified these expectations?
To answer this question we must first learn to properly compare the different formats together.
First, we must compare the system with lenses having the same angle. This means that we must move on lenses with longer focal lengths in the transition from a narrow format to the average. For example, often believe that the narrow screen normal “half a ruble” corresponds to the medium format lens with a focal length of 80 mm.
However, the exact figure is not important. Without rigor, we can assume, for example, that the normal medium format lens should have a focal length of 100 mm.
Secondly, the need to compare the system with the same depth of field (DOF) or, in other words, with the same degree of fuzziness of various elements of the image. Let us examine this issue in more detail.
In an article devoted to understanding the physical meaning of sharpness, a formula that allows to assess the degree of blur on the negative. Blur is directly proportional to the square of the lens focal length divided by the aperture value: f 2 / N, where N = 1,4; 2, 2,8, 4, 5,6, 8, …
The formula for assessing the degree of blur is easily obtained from the classical formulas for determining the depth of field, if we express the diameter of the circle blur terms of the other options.
In this case we are interested in the degree of blur of the prints, not negatives. Therefore, in the Formulas you need to add another factor K, which shows how many times would increase the negative when printing. The result is that the degree of blurring the point on the print is directly proportional to the following value:
C * = (K f 2) / N = (K f) (f / N),
where
K – coefficient of increase of the negative when printing;
f – focal length lens used in the shooting;
N – aperture number (1,4, 2, 2,8, 4, 5,6, 8, …).
Within the same DOF of the prints obtained from the negatives of various formats, can be written as:
C * = const.
This condition is satisfied, while respecting the two conditions:
A. (K f) = const
B. (f / N) = const
Condition A is carried out almost automatically. Indeed, on the one hand, the transition from the narrow format on average, we use lenses with long focal length (subject to the uniformity of the angle of view – see above). On the other hand, for equal area poster-size prints have increased in the smaller number of times (in proportion to the increased focal length). As a result, the product of K, f remains unchanged.
Condition B means that when taking a long-focus optics must be stronger than the spinning aperture. Only in this case the following condition is equivalent GRIP.
So, while respecting the two conditions (A and B) all points on the print (both within and outside the permissible area of field) will be equally blurred. This equal-prints obtained from the negatives of various formats, will not differ greatly from each other (ideally in general will not).
But this does not mean that all the formats are equal. So it would be if we were dealing with an ideal foil and ideal optics (the word “perfect” here, above all, means “with infinite resolution”). Yet the real combination of optics and the recording medium (film) is always far from ideal. And this non-ideality is more pronounced, the more we will increase the negative when printing.
Characteristics of lenses, often united in the concept of “image”, also be interpreted as a set of individual imperfections. Ideal optics has no specific figure.
CONCLUSION:
So, if the transition from the narrow format on average
1. using lenses with long focal length to maintain constant viewing angle;
2. tighten the diaphragm in proportion to the focal distance (f / N = const), thus providing the same depth of field on the print;
3. print with different negatives prints of the same size (K f = const), for example 20 x 30 cm;
4. use an ideal foil and ideal optics;
the obtained image will not differ from each other.
However, if the execution pp. 1 – 3 of great complexity does not matter, then the condition 4 is simply not realizable. All the observed differences were due to practice is non-compliance with this condition. The more the final print, the more obvious will be for the observer all imperfection and characteristics of a particular combination “lens + film.
By the way:
1. From the above considerations, in particular, that the shooting medium format lens with f = 80 mm at aperture f / 6,7 is equivalent to the narrow screen shooting lens with f = 50 mm at f / 4. However, most often, the lens with a relative aperture of 6.7 have higher rates of real resolving power compared with the lenses in which shooting is set to aperture 4. In some cases, this factor may be another explanation of the observed in practice, the benefits of shooting on medium format. In other words, to speak about the characteristic features were not the combination “lens + film”, and the combination “lens + iris + film”.
2. Another important factor – the projection error. They appear when you print using the magnifier, and when you use the projector for viewing slides. Errors of this type the more noticeable the greater the increase. In some cases, the advantages of medium format is due to a smaller degree of increase in the projection device. However, one must bear in mind that not all cases, the viewer is faced with this phenomenon. An example of such a situation can serve as a scanning negatives (or slides) in preparation for printing polygraphic methods.
From Theory to Practice
Let’s see whether our findings are consistent with the practice.
First consider the test image with a relatively small increase (ie, submitted photofragments will meet a large area of negative). Fig. 1 shows the photographs obtained in accordance with the formulated above theoretical conditions.
Fig. 1
Yes. narrower; f = 50 mm. @ F / 8; b. medium format; f = 80 mm. @ F/13
In general we can say that the theory is well confirmed by experiment. Both images in Fig. An approximately equal. Here, it is necessary to make a reservation, that in making such a conclusion, we deliberately ignore the subtle differences caused by the imperfection of optics and experimental errors. But if you wish to find such differences can be without special efforts.
Among the experimental errors should, above all, mention the following. First, the focal lengths are not equal in accuracy of 50 mm and 80 mm. Secondly, 50 / 8 = 6.25 and 80/13 = 6.15. When viewing a need to make allowances for all these factors. But again, as a whole must admit that the images do not differ greatly, ie, the results agree well with theory.