Medium and Large Format Crop Factors

Analog photography has a deep and rich history of evolution and radiative diffusion going back 200 years. After an early, bewildering explosion of diversity and competition, by the late 20th century the industry had settled on a limited number of standard film types. This article serves to visually explain the most common formats and relate them to the 35mm “full frame” format for people coming from a digital photography background.

Crop Factor

Crop factor is a multiplier which allows one to compare a particular imaging area to the 35mm lens imaging area.

More generally, a crop factor can be applied to the focal length of a lens for one imaging area (or format) to provide an understanding of the angle of view that focal length will produce on a different imaging area (or format).

To be sure, it is a strange usage of this term to apply it to imaging areas which are larger than 135 format. I have chosen to use it in this fashion because the term is a familiar one to those coming from digital imaging. And mathematically, it works just as well, you simply end up with numbers which are smaller than one as the multiplier. However I understand objections to the term as conceptually, no actual crop is being applied to the larger format, rather an inverse crop (an expansion?) is being applied to the 135 format. I hope this is clear.

Part of the job of a photographer is to visualise the angle of view that a particular lens has in order to choose an appropriate focal length for a particular shot. For example, on 135 format, a 50mm lens is considered a “normal” angle of view or pretty close to the way the human eye sees. Likewise, on 135 format, a 20mm lens is considered “wide” and a 150mm lens is considered “telephoto”. However, due to the fact that focal length is an invariant property of a lens, a 150mm lens on a large format camera actually has a “normal” field of view because the imaging area is so much larger!

Hence a nominal crop factor aids in understanding and pre-visualisation of a shot, before a lens is chosen.

This discussion glosses over other important properties of a lens, such as the image circle, for the sake of simplicity.

Aspect Ratios

Crop factor is below calculated by simply dividing the diagonal of the 135 format (43mm) by the diagonal of the comparison format. However, the resulting number is only useful if the aspect ratios of the two formats are similar. Where the formats differ widely, as in 135 vs 6×17 (to take an extreme example, 3:2 vs ~3:1), the crop factor may be better calculated by dividing the two shorter sides.

All illustrations which follow are to scale for direct visual comparison (except for 8×10 large format).

135 format

The introduction of 135 format in the early 20th century enabled cameras to become much smaller and extremely portable. 135 took over from medium format as the format of choice during the post-war period. More 35mm film cameras were produced than any other film type by quite a large margin, and thus it is the format most people are familiar with.

Medium Format

Medium format is actually a collection of formats, all of which use unperforated 120 format roll film which is 62mm wide. 120 film is commonly available and can be processed in many minilabs. It produces a negative which is between 2 times and three times the diagonal of 135 format.

645 format

Example systems:

  • Mamiya 645
  • Pentax 645
  • Contax 645

6×6 format

Example systems:

  • Hasselblad
  • Most Twin Lens Reflex cameras, eg Rolleiflex, YashicaMat, Mamiya C330, etc
  • Folding viewfinder and rangefinder cameras from Zeiss, Balda, Welta, Olympus, etc
  • Mamiya 6 (1990s)

6×7 format

Example systems:

  • Pentax 67
  • Mamiya RB67
  • Mamiya RZ67
  • Mamiya 7

6×8 format

Example systems:

  • Fuji GX680

6×9 format

Example systems:

  • Mamiya Press
  • Voigtländer Bessa

Large format

Like Medium Format, there are a series of increasingly large imaging areas under the rubric Large Format. It is beyond the scope of this article to detail them all. We will simply mention two of the Large Formats, 4×5 and 8×10.

4×5 format

4-inch by 5-inch and larger formats are generally not referred to by metric numbers. (In the UK 4×5 is referred to as 5×4.)

It is found only as sheet film. It cannot be processed in a minilab, and finding a pro lab to develop your film can be a challenge, hence most 4×5 is shot and developed in black and white chemistry in home darkrooms.

4×5 is slightly more than double the area of 6×9.

4×5 is the so-called “International” format that Graflok backs and double film holders (sometimes called “dark slides”) accept. It is not to be confused with the similar European size of 9x12cm.

Example systems:

  • Pacemaker Graphic – Speed, Crown
  • Graflex
  • Linhof
  • Horseman
  • Wista
  • Toyo
  • Ebony
  • Chamonix

8×10 format

8×10 is the largest format in common use. It has double the diagonal, or four times the area of 4×5 film.

(Not to scale)

Field of View Equivalence

The values in this table are approximate and have been rounded to common focal lengths for simplicity.

1356456x66x76x86x94x5
/1.0/0.6/0.55/0.5/0.45/0.4/0.28
15242830323555
21323842484875
284550556065100
355565707880125
4570809095100160
508090100110120180
70105120140145150250
90145160180200210320
110175200220240250300
135220245270300320480
200320365400450465700

References

2 thoughts on “Medium and Large Format Crop Factors

  1. Nice work on the article but I would suggest that the diagonal is the least useful dimension to do the comparison by. Using 6×17 might be a good way to demonstrate this. The diagaonal for 6×17 is about 180mm. Compared to 6×7 which is 89mm suggests that the standard lens would be 160mm but most 6×17’s come with 90-105mm as default.
    People who convert to large format from 35mm suffer most from the diagonal conversion. Their nice comfortable 21-24mm wide converts to a 75-85mm lens which on 5×4 is very wide! If people are moving from 35mm to 4×5 it could be because they like the aspect ratio, in which case we can compare the 24mm frame height of the 35mm camera with the 96mm frame height of the 5×4. This gives a conversion of /0.25 or 4x.

    Now we convert from our 21-24mm lens and we get 84-96mm which is more like the standard wide for 5×4. Now if we compare our shot on 24mm cropped to a 5×4 ratio it will look identical to our 96mm image from 5×4.

    1. Thanks for your thoughtful comment Tim. I have updated the discussion of Crop Factor above to take this information into account.

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