'Equivalent' 35 mm format (24x36 mm) focal lengths
By Q.G. de Bakker
A very frequently asked question is one about the 35 mm format focal length equivalent of the lenses used on
Hasselblad's 6x6 format.
It is often asked, perhaps because there is no single answer. The aspect ratios of 6x6 format (1:1) and
35 format (1:1.5) differ considerably, and an answer will depend on what aspect is used in the comparison.
Figure 1
Same diagonal angle of view
Diagonal
Manufactureres usually only state the diagonal angle of view of the lenses they
produce. As a result, the diagonal often has to be taken as the basis for comparisons.
In Figure 1, 6x6 format (black outline) and 35 mm format (red outline) frames that have the same size diagonal
are superimposed.
As Figure 1 shows, the different aspect ratios of both formats lead to a rather different framing: the 6x6
frame is taller than the 35 mm format frame, yet less wide.
A subject just fitting inside the 35 mm frame horizontally will not fit inside the 6x6 frame, and conversely, a
subject just fitting inside the 6x6 frame vertically will not fit inside the 35 mm frame.
Framing in both formats is equivalent only in the rare case that subjects have to be framed diagonally.
The differences in both horizontal and vertical aspects then still make it very hard, if not impossible, to
crop an diagonally composed image in either format to the image produced by the other format.
Figure 2
Same vertical angle of view
Vertical
Another option would be to select lenses so that their vertical angles of view
are the same (Figure 2).
The difference in aspect ratios of the two formats then shows itself in the horizontal angle of view, which is
1.5 times as large on 35 mm format as that of the equivalent lens on 6x6 format.
It obviously is very easy to crop the larger frame to produce an image exactly the same as that produced inside
the smaller frame. The (vertical) equivalence is true.
Figure 3
Same horizontal angle of view
Horizontal
Conversely, when the lenses are chosen so that the horizontal angles of view are
the same (Figure 3), the difference in aspect ratio leads to a 1.5 times larger vertical angle of view on 6x6 format.
Again, cropping the taller frame to produce an image exactly the same as that captured in the smaller frame is
not a problem, i.e. the (horizontal) equivalence is true.
The Hasselblad focal lengths given in the table to the right are nominal. The actual focal lengths differ,
not only from this nominal focal length, but also from that of other Zeiss/Hasselblad lenses having nominally
the same focal length.
To keep the table simple, the choice was made to use the nominal focal lengths instead of the true focal
lengths.
As a result the table only gives an approximation of the equivalent 35 mm format focal lengths.
Since the known focal lengths of 35 mm format lenses are also nominal, with their true focal lengths rarely known,
an approximation will be the best a general comparison can achieve. For critical purposes, an actual test of all
the lenses involved will be necessary.  6x6 format lens  35 mm format lens having the   same horizontal angle of view  same vertical angle of view  same diagonal angle of view  38 mm  24 mm  16 mm  21 mm  40 mm  25 mm  17 mm  22 mm  50 mm  32 mm  21 mm  27 mm  60 mm  38 mm  25 mm  32 mm  80 mm  51 mm  34 mm  43 mm  100 mm  64 mm  42 mm  54 mm  105 mm  67 mm  45 mm  57 mm  110 mm  70 mm  47 mm  60 mm  120 mm  76 mm  51 mm  65 mm  135 mm  86 mm  57 mm  73 mm  150 mm  96 mm  64 mm  81 mm  180 mm  115 mm  76 mm  97 mm  250 mm  159 mm  106 mm  135 mm  300 mm  191 mm  127 mm  162 mm  350 mm  223 mm  149 mm  190 mm  500 mm  319 mm  212 mm  271 mm 

Figure 4
Same horizontal angle of view
Figure 5
Same vertical angle of view
6 x 4.5 format
Ever since Hasselblad introduced a film magazine producing 16 frames in 6x4.5
format in 1956, 6x6 is not the only Hasselblad film format.
The difference in the aspect ratios of 35 mm format and 6x4.5 format is small. The 6x4.5 format is a little
bit taller than the 35 mm format. Consequently, the spread in equivalent focal lengths depending on what
aspect is chosen to be equal is also smaller.
Figures 4 and 5 show the difference in framing when lenses are chosen so that either the horizontal or
vertical angles of view are equal (blue outline = 6x4.5 format).
6x4.5 format lens  35 mm format lens having the   same horizontal angle of view  same vertical angle of view  same diagonal angle of view  38 mm  24 mm  22 mm  23 mm  40 mm  25 mm  23 mm  25 mm  50 mm  32 mm  29 mm  31 mm  60 mm  38 mm  34 mm  37 mm  80 mm  51 mm  46 mm  49 mm  100 mm  64 mm  57 mm  61 mm  105 mm  67 mm  60 mm  65 mm  110 mm  70 mm  63 mm  68 mm  120 mm  76 mm  69 mm  74 mm  135 mm  86 mm  77 mm  83 mm  150 mm  96 mm  86 mm  92 mm  180 mm  115 mm  103 mm  111 mm  250 mm  159 mm  143 mm  154 mm  300 mm  191 mm  171 mm  184 mm  350 mm  223 mm  200 mm  215 mm  500 mm  319 mm  286 mm  307 mm 

The Hasselblad focal lengths given in the table to the left are nominal. The actual focal lengths differ,
not only from this nominal focal length, but also from that of other Zeiss/Hasselblad lenses having nominally
the same focal length.
To keep the table simple, the choice was made to use the nominal focal lengths instead of the true focal
lengths.
As a result the table only gives an approximation of the equivalent 35 mm format focal lengths.
Since the known focal lengths of 35 mm format lenses are also nominal, with their true focal lengths rarely known,
an approximation will be the best a general comparison can achieve. For critical purposes, an actual test of all
the lenses involved will be necessary. 
4 x 4 'superslide' format
The third and last Hasselblad format to consider is the 4x4 cm 'superslide' format.
It was introduced in 1957, to produce slides that could be projected using normal 35 mm format projectors.
The aspect ratio is the same as that of the original 6x6 format, so the same considerations apply. Though
being much smaller than the 6x6 format, the angles of view will be smaller too. The equivalent 35
mm format focal lengths will be longer accordingly.
The Hasselblad focal lengths given in the table to the right are nominal. The actual focal lengths differ,
not only from this nominal focal length, but also from that of other Zeiss/Hasselblad lenses having nominally
the same focal length.
To keep the table simple, the choice was made to use the nominal focal lengths instead of the true focal
lengths.
As a result the table only gives an approximation of the equivalent 35 mm format focal lengths.
Since the known focal lengths of 35 mm format lenses are also nominal, with their true focal lengths rarely known,
an approximation will be the best a general comparison can achieve. For critical purposes, an actual test of all
the lenses involved will be necessary.  4x4 format lens  35 mm format lens having the   same horizontal angle of view  same vertical angle of view  same diagonal angle of view  38 mm  33 mm  22 mm  28 mm  40 mm  35 mm  23 mm  30 mm  50 mm  44 mm  29 mm  37 mm  60 mm  53 mm  35 mm  45 mm  80 mm  70 mm  47 mm  60 mm  100 mm  88 mm  59 mm  75 mm  105 mm  92 mm  61 mm  78 mm  110 mm  97 mm  64 mm  82 mm  120 mm  105 mm  70 mm  90 mm  135 mm  119 mm  79 mm  101 mm  150 mm  132 mm  88 mm  112 mm  180 mm  158 mm  105 mm  134 mm  250 mm  220 mm  146 mm  187 mm  300 mm  263 mm  176 mm  224 mm  350 mm  307 mm  205 mm  261 mm  500 mm  439 mm  293 mm  373 mm 

Figure 6
Relative sizes of 6x6 (black), 6x4.5 (blue), superslide
(green) and 35 mm (red) formats superimposed
Click here for an online calculator, comparing the angles of view for a number of popular formats.
Copyright 2009  Q.G. de Bakker. All rights reserved.
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