 |
Medium Format Lenses
(Photo Asia Magazine, Singapore, May 1993 Issue)
|
|
Medium Format Lenses
(Photo Asia Magazine, Singapore, May 1993 Issue)
|
Through frequent contacts with serious amateur photographers, I have encountered a lot of fallacies, which I believe are created by those photo enthusiasts knowing some facets about photography. Those fallacies which are stimulating and at times appalling can spread out very fast and wide. After a couple of years, they become the golden rules upon which people choose their equipment.As an enthusiast in photographic optics, I would like to smash up these fallacies so that our readers will no longer be bewildered by those seemingly right theories.
Let us name them one by one (other fallacies are discussed in seperate columns).
Medium format lenses perform better than original lenses in 24x36mm format.
It is a very common practice for serious amateurs, particularly middle aged salon fans, to fit an adaptor to a Hasselblad 6x6 format lens for use in a 24x36mm SLR body e.g. Canon New F1 or Nikon F2. The results, they claim, are better than the original Canon L series or Nikkor lenses.
The 'second hand market' salesmen exploit this fallacy (to promote sales of medium format lenses) so much that it now becomes almost a Commandment.
They claim that the central part of a lens has higher resolution than the edges (which is quite true) and hence the best part of the medium format lens is employed if used in a smaller format body, whilst the original 24x36mm format lens has to include the edges (which has lower resolution) to form the full image and hence the image is inferior.
This is true if, and only if, the resolutions of medium and small format lenses are the same. But in fact the average resolution of a 24x36 mm lens is around 100 line pairs per mm and that of 58x58 mm lens is 80 or lower. The resolution goes lower and lower as the format goes larger and larger.
Why then a large format lens gives a much sharper image ?
The reason is that sharpness of an image depends not only on resolution, but also on area of the recording film.
Although the resolution of a 58x58 mm format lens is 20% (80/100) less, compared with the 24x36 mm format lens, the area is 3.89 (58x58/24x36) times bigger, and hence the overall gain in resolution is 80/100 x 3.89 or 3.11 times. That explains why even when the 58x58 square negatives are cropped to rectangular 8x10, 16x20 sizes, the images are still sharper.
If we use the 58x58 mm lens in 58x58 mm format, the total resolution is 269,120 (80 x 58 x58) lines.
If we use the 58x58 mm lens in 24x36 mm format, the total resolution is 69,120 (80 x 24 x36) lines.
If we use the 24x36 mm lens in 24x36 mm format, the total resolution is 86,400 (100 x 24 x36) lines.
Hence if we use medium format lenses on 24x36 mm format, we lose 20% (69,120/86,400) resolution.
Another point I would like to make here is that the centre part of all lenses should have better resolution performance if it were not necessary for the lens designer to compensate the poor performance over the edges by making some sacrifices in the centre part of the lens element. These scrifices are necessary not only arising out of the resolution issue alone, but also covering other issues such as chromatic, curvature of field, barrel, pin-cushion distortions which we are all familiar with. I have to stop here or else I might step into a subject which may fill up a few more pages.
To simplify and also to generalise my calculations, I take the average resolution figures of 100 and 80 as basis of reference.
Then why some of us really 'see' that the medium format lenses do perform better than the original lenses in 24x36 mm format ?
This reasons behind might be very complex.
First 'sharpness' is defined by resolution, contrast and acutance (similar to acuity) (you might not be able to find this word in an ordinary dictionary and hence I have to give a little bit of explanation here. Imagine you tear off a page of this magazine into two halves with your fingers and you also cut the same page into two halves with a razor blade. The edges made by a razor blade are much smoother than those made by your fingers. We can say the razor blade creates better acutance or smoother edges.) Without a pair of trained eyes, most people can easily be fooled by an image of good contrast but poor resolution and these people will say it is 'sharper' than an image with poor contrast but better resolution. The combination of contrast and acutance can also fool our untrained eyes. Books on optics show you the different combinations of resolution, contrast and acutance to prove the above statements. Other factors such as tone scale, colour balance, spectrum coverage, personal favours etc. can play an important part in our visual judgement. We should not forget that Photography is a visual art, and (like all fine arts) is judged subjectively, but photographic optics is an objective science, and we can measure it.
Before I studied photographic optics, my judgements on 'sharpness' were all wrong. We all make mistakes, somewhere, sometime. The important thing is, can you learn from your mistakes? And will you never make the same mistake twice?
 |
(416) 298-5881 |  |
(416) 292-5535 |
|