Technicalities

A Comparison of 35mm, 6x4.5, and 6x7

This is a comparison of the three most common rectangular film formats: 35mm (135), 6x4.5 (645), and 6x7 (67). It is both quantitative and subjective. I present numbers about each film format and theoretical equivalencies with regard to enlargement, then present side by side examples, and conclude with a personal opinion regarding print quality from the different formats. This "article" is fairly long and has a lot of numbers in it, but there is a lot of potentially useful information in this page. I have a B.Sc in Mathematics so the calculations are reliable. If you are going to refer to or quote information you've found on this page, please reference it with a link. There is a link at the bottom of this page to the examples.

The Contenders:

	Film	Format	Camera	Lens
A	Kodak Technical Pan	135 (35mm)	Pentax LX	SMC Pentax 1:3.5/28mm
B	Kodak T-Max 100	135 (35mm)	Pentax K1000	SMC Pentax 1:3.5/28mm
C	Kodak T-Max 100	6x4.5 (645)	Pentax 645	SMC Pentax 645 1:2.8/45mm
D	Kodak T-Max 100	6x7 (67)	Pentax 6x7 MLU	Super-Multi-Coated TAKUMAR 6X7 1:3.5/55mm

The SMC Pentax 1:3.5/28mm lens used for the two 35mm cameras is the sharpest of the lenses used, and it is the 2nd sharpest 28mm lens Pentax has produced. The lens for 645 is very good, and the lens for 67 is good but not in the same league as the other two.

The Testing Procedure

Identical pictures were taken with the four cameras from either the same tripod position, or the cameras resting on a very solid object.
The scene was metered with the Pentax 645, and each camera containing TMX received the same exposure (1/250 or 1/125 at f/8). The Technical Pan I used is very old and has lost some speed, so two pictures were taken: one with an ISO of 10, and one with an ISO of 5.
Mirror lockup was used for the Pentax LX and Pentax 6x7. The K1000 does not have mirror lockup, and the 645 has an excellent mirror dampening system that reduces mirror vibrations.
The TMX film was developed in D-76 diluted 1:1 for 9.5 minutes at 20° C (68° F) with agitations every 60 seconds.
The Technical Pan film was developed in Technidol for 9 minutes at 20° C (68° F) with agitations every 60 seconds.
Each negative was enlarged until the long side measured 20 inches. An 8" x 10" print (with 0.5 inch borders) was made from each negative so that each print had similar density & tones.
The enlarger was a Saunders LPL 670DXL. For the 35mm negatives I used an EL-Nikkor 1:2.8/50mm lens stopped down to f/5.6, and for the medium format negatives I used an EL-Nikkor 1:5.6/80mm lens stopped down to f/11. The filtration was 70M 0Y for all prints (producing fairly flat prints).
For web viewing, each print was scanned in at 600 PPI on a cheap flatbed scanner. The scans would look better if they were drum scanned at the resolutions listed below.
- 8500 ppi, creating a 91.5 mega-pixel image for 35mm.
- 5500 ppi, creating a 102 mega-pixel image for 645.
- 4400 ppi, creating a 108 mega-pixel image for 6x7.

The Numbers Behind the Test:

Format	Height	Width	Area	% of 135	% of 645	% of 67
135	24mm	36mm	864 mm²	100	37.9	22.4
645	41.5mm	55mm	2282.5 mm²	264.2	100	59.3
67	55mm	70mm	3850 mm²	445.6	168.7	100

If you look at the table above you can see that a 67 negative is about 1.7 times the area of a 645 negative. This doesn't mean that each dimension of your pictures can be 1.7 times as long, since the area would be almost 3 times as large. The table below give a rough method of figuring out "equivalent" print sizes for each format, assuming equivalent aspect ratios, lens quality, film type, and enlargement magnification. Variations in lens quality, film type, and tripod usage can have more impact on print quality than negative size. A tripod image with 135 film will look better than a blurry handheld image from a 67 camera.

Format	Side Length Compared To 135	Side Length Compared To 645	Side Length Compared To 67
135	100 %	62 %	47 %
645	163 %	100 %	77 %
67	211 %	130 %	100 %

Using the table above, if you're happy with the print quality of an 11" x 14" for 135 film, with 645 those numbers get multiplied by 1.63 to give about 18" x 23", and with 67 those numbers get multiplied by 2.11 to give about 23" x 29.5". If you're happy with 16" x 20" from a 645 system and want to know how your horizons will expand with a 67 system, multiplying by 1.3 gives about 21" x 26". Unfortunately, the table doesn't take into account that the aspect ratios of each negative size are different, so some cropping must occur for different print sizes. Since all three formats deliver superb 8" x 10" prints, I will deal with 11" x 14", 16" x 20", and 20" x 24" prints.

The table below shows the minimum crop that must occur to completely fill each print size, and in brackets are the size of the print if no cropping were to occur. All crops are off the long side, and are rounded to one decimal place.

Print Size	Crop for 135	Crop for 645	Crop for 67
11" x 14"	2.5" (11" x 16.5")	0.6" (11" x 14.6")	None
16" x 20"	4" (16" x 24")	1.2" (16" x 21.2")	0.4" (16" x 20.4")
20" x 24"	6" (20" x 30")	2.5" (20" x 26.5")	1.5" (20" x 25.5")

Perhaps the most useful table is the one below: it shows the minimum enlargement necessary to produce a print on each paper size with no borders. Assuming each film format can withstand a similar enlargement factor, it helps demonstrate what might be a comparable quality print size from format to format. Keep in mind that lenses for 35mm cameras are almost always sharper than those for medium format.

Format	Enlargement Factor 8x10	Enlargement Factor 11x14	Enlargement Factor 16x20	Enlargement Factor 20x24
135	8.47	11.64	16.93	21.17
645	4.90	6.73	9.79	12.24
67	3.69	5.08	7.39	9.24

I've heard that up to 8 times enlargement, the quality of a 100 speed film will hold up, but the image quality starts to degrade once you enlarge past that. I'm often happy with higher enlargement factors than that (and sometimes significantly higher), but if you attend grain sniffers' anonymous every Thursday night and are happy with only the finest image quality, I recommend you get (or use) an 8" x 10" view camera and stop fooling around with the puny negatives I'm discussing here.

Example 1: The Thrift Store