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 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.
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).
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 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.
- Mamiya 645
- Pentax 645
- Contax 645
- 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)
- Pentax 67
- Mamiya RB67
- Mamiya RZ67
- Mamiya 7
- Fuji GX680
- Mamiya Press
- Voigtländer Bessa
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-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.
- Pacemaker Graphic – Speed, Crown
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.