Charms and Myths of the Medium Format
that digital technologies are getting more and more widespread and
the good old 35mm photography is going to disappear, some old-fashioned
photographers and/or beginners turn to the medium format. Some of them
claim that it outperforms 35mm film even if the frames are not significantly
enlarged. But is it really so? To answer the question, we have to learn
how to compare different formats with each other correctly.
The conditions of fair comparison
Firstly, one must compare photo systems with equal angles of view. This means we should deal with larger focal distances when we switch from 35mm photography to the medium format. For example, a normal 50mm lens on a 35mm camera corresponds to an 80mm lens on a medium format camera.
The exact figure is not important here. For the purposes of this article, we may for example assume that the normal lens for a medium format camera should have a focal length of 100mm.
Secondly, the comparable photographs must have the same depth of field (DOF). In other words, the degree of fuzziness should be equal for the corresponding elements of the photos. Let us discuss this requirement in detail.
The formula that describes fuzziness in the picture can be found in my article that explains physical meaning of sharpness. That formula shows that the degree of fuzziness is proportional to f 2 / N, where N = 1.4; 2; 2.8; 4, 5.6; 8; …
When we speak about final prints, we must allow for the enlargement. Thus, we may say that the fuzziness in the photograph is proportional to C*:
C* = (K f 2) / N = (K f) (f / N),
The unchanged depth of field means C* is costant in our prints:
C* = const.
This condition can be split up into the following two conditions (which must be simultaneously met):
f) = const
Condition A is met almost automatically. On the one hand, when we switch from the 35mm photography to the medium format, we have to use lenses with larger focal distances to preserve angles of view. On the other hand, medium format negatives do not have to be enlarged that much to produce the final picture of the same size. As a result, (K f) is maintained constant.
Condition B means we must stop down our medium format lens to maintain the same degree of fuzziness in our prints.
Thus, if both conditions are met (A and B), the resulting fuzziness is maintained constant. If our lenses and films were ideal, the final photographic prints produced by different formats would be absolutely identical. But unfortunately there are no ideal lenses and films. When our prints become larger, the imperfections of any given combination of a lens plus film become more evident.
Both gurus and novices like to compare lenses and to talk about specific peculiarities of optical devices. However, mention should be made that all such peculiarities result from imperfections. Ideal lenses do not have any peculiarities at all.
Now let us check our conclusions with real life images.
On the whole, the theory works well. Both pictures (Fig. 1) are basically the same.
Of course, when saying that, we have to ignore some subtle differences. But they are not difficult to detect.
First, there are some experimental faults. The focal distances of the lenses were not exactly equal to 50mm and 80mm. Moreover, 50/8 = 6.25, while 80/13 = 6.15. All such factors should be taken into account, when comparing one picture to another.
Second, there is some additional fuzziness in Fig. 1b. It happened because the wind stirred some elements of the picture during exposure. The results can be seen only locally (mainly in the left lower corner). Although this effect is easy to detect, it does not prevent us from judging the picture as a whole. On the other hand, this extra fuzziness shows that the medium format has its own specific drawbacks. In this case, I had to increase exposure to 1/2 seconds, after the lens was stopped down to f/13. The camera was on a tripod, and I waited for the moment with minimal wind. Still, some twigs and leaves got stirred.
Now lets us look at another pair of pictures. In this case, magnification is large. Both images account for a small part of the initial frames (Fig. 2).
Again we can see that the pictures are basically the same. However, some subtle differences became more obvious now.
CRT monitors make them more distinct than TFT monitors.
Well, what about more evident differences? To see them, we will need to analyze greater enlargements (Fig. 3).
At such a high magnification, it becomes evident that the 35mm photography produces pictures with larger grain and lower resolution compared to the medium format photography.
Does the medium format justify our hopes? Is it substantially better? It depends. There is no simple and definite answer to this question.
Of course, the formats differ from each other in many aspects. Sometimes, such differences are important. Sometimes we may neglect them. However, to detect and analyze any differences we must stick to the rules mentioned in this article.
If our lenses and films were ideal, the different formats would produce the same result.
Thus, the advantages of the medium format (compared to the 35mm photography) are mainly as follows:
In its turn, the 35mm photography also has an advantage. We will not have to stop down our lens that much for any given degree of fuzziness. As a result, our exposures will be shorter.
Yefremov. Understanding physical meaning of sharpness. Is Harold
Merklinger's theory correct? (In this article, you may find a
formula that describes fuzziness in photographs.)
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