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'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 lens35 mm format lens having the
 same horizontal angle of viewsame vertical angle of viewsame diagonal angle of view
38 mm24 mm16 mm21 mm
40 mm25 mm17 mm22 mm
50 mm32 mm21 mm27 mm
60 mm38 mm25 mm32 mm
80 mm51 mm34 mm43 mm
100 mm64 mm42 mm54 mm
105 mm67 mm45 mm57 mm
110 mm70 mm47 mm60 mm
120 mm76 mm51 mm65 mm
135 mm86 mm57 mm73 mm
150 mm96 mm64 mm81 mm
180 mm115 mm76 mm97 mm
250 mm159 mm106 mm135 mm
300 mm191 mm127 mm162 mm
350 mm223 mm149 mm190 mm
500 mm319 mm212 mm271 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 lens35 mm format lens having the
 same horizontal angle of viewsame vertical angle of viewsame diagonal angle of view
38 mm24 mm22 mm23 mm
40 mm25 mm23 mm25 mm
50 mm32 mm29 mm31 mm
60 mm38 mm34 mm37 mm
80 mm51 mm46 mm49 mm
100 mm64 mm57 mm61 mm
105 mm67 mm60 mm65 mm
110 mm70 mm63 mm68 mm
120 mm76 mm69 mm74 mm
135 mm86 mm77 mm83 mm
150 mm96 mm86 mm92 mm
180 mm115 mm103 mm111 mm
250 mm159 mm143 mm154 mm
300 mm191 mm171 mm184 mm
350 mm223 mm200 mm215 mm
500 mm319 mm286 mm307 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 lens35 mm format lens having the
 same horizontal angle of viewsame vertical angle of viewsame diagonal angle of view
38 mm33 mm22 mm28 mm
40 mm35 mm23 mm30 mm
50 mm44 mm29 mm37 mm
60 mm53 mm35 mm45 mm
80 mm70 mm47 mm60 mm
100 mm88 mm59 mm75 mm
105 mm92 mm61 mm78 mm
110 mm97 mm64 mm82 mm
120 mm105 mm70 mm90 mm
135 mm119 mm79 mm101 mm
150 mm132 mm88 mm112 mm
180 mm158 mm105 mm134 mm
250 mm220 mm146 mm187 mm
300 mm263 mm176 mm224 mm
350 mm307 mm205 mm261 mm
500 mm439 mm293 mm373 mm

Figure 6
Relative sizes of 6x6 (black), 6x4.5 (blue), superslide
(green) and 35 mm (red) formats superimposed
Online calculator

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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