6x6 and 75mm, what's the equivalent in square cropped 35mm as far as focal length?

[mossscott]

6x6 and 75mm, what's the equivalent in square cropped 35mm as far as focal length? This is one for the math folks. I would like to get a lens for my 35mm camera that, when cropped square, would have the same field of vision as 6x6 and 75mm.

Thanks in advance!!!

 

[Oren Grad]

6x6 is generally 56x56 mm. A 35mm frame cropped to square is 24x24 mm.

(75/56)*24=32.1

 

[Beemermark]

A squared + B squared = C squared.
Where A is one side of the rectangle, B is the other side, and C is the diagonal. So for 6x7 cm film:
36+49=85. The Square root of 85 is 9.2 cm or 92mm. Call it 80 mm which is the normal lens lens for most 6x6 and 6x7 cameras.
But to cut it short, a 35mm (for a 35mm camera) finder works well for 75mm lenses on medium format and a 50mm finder works just dandy as a normal finder.

 

[Godfrey]

6x6 and 75mm, what's the equivalent in square cropped 35mm as far as focal length? This is one for the math folks. I would like to get a lens for my 35mm camera that, when cropped square, would have the same field of vision as 6x6 and 75mm.

Thanks in advance!!!

Rui Salguiero's Field of View Calculator is an excellent resource for this kind of thing. For example, I ran calculations for 6x6 cm and 35mm square crop (24x24mm) and got this list:

Width = 56 mm, Length = 56 mm, Diagonal = 79.196 mm
f Hor Vert Diag H/V
75.0 40.9446 40.9446 55.6657 1.0000

Width = 24 mm, Length = 24 mm, Diagonal = 33.9411 mm
f Hor Vert Diag H/V
28.0 46.3972 46.3972 62.4394 1.0000
31.0 42.3225 42.3225 57.3959 1.0000
35.0 37.8493 37.8493 51.7350 1.0000
40.0 33.3985 33.3985 45.9795 1.0000
43.0 31.1856 31.1856 43.0748 1.0000
45.0 29.8628 29.8628 41.3253 1.0000

From that you can see that you'll likely want a 31 or 35 mm lens on your 35mm camera (square cropped) to achieve the same angle of view as a 75 on 6x6cm. It won't be exact but it will be close enough...

G

 

[spijker]

The multiplication factor for the focal length is the ratio of the image diagonals:
sqrt(24^2 + 24^2)/sqrt(56^2 + 56^2) x 75mm = 33.9/79.2 x 75mm = 0.43 x 75mm = 32mm

The diagonal view angle for both cases is 54 degrees

 

[sojournerphoto]

The OP was comparing square format images. So, as the aspect ratio of each is the same, you can simply use the ratio of side length to compute equivalent focal lengths - as pointed out in the first response 75mm x 24 / 56 = 32mm (to the nearest whole mum).

It’s more complex when comparing different formats, e.g. 3:2 cf 1:1 or 5:4. Convention uses the format diagonal to describe a broad equivalence, but it can be valid to consider format width or height as well. I often do this for a couple of reasons - first the feel of a picture changes with aspect ratio and what’s included horizontally may be the most important consideration and, related, second panoramic pictures effectively exclude large amounts of foreground/sky compared to more square formats for the same width.

At the end of the day, I think the change in feel means equivalence is helpful but not a complete description of the effect of focal length with different formats.

 

[shawn]

A 35mm f1.2 lens on 35mm will give you almost the same FOV and DOF as an 80mm f2.8 on 6x6.

From https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/

 

[Retro-Grouch]

A very useful chart from Jake Horn: Lens Focal Length Equivalents — Jake Horn Photography
My only minor gripe is that he bases these figures on the longest side of the frame, thereby giving the same focal length equivalents for 6x6 and 6x4.5. It's certainly close enough in the real world, however.

 

[Darthfeeble]

I'm not a math guy, I'm a KISSguy, wouldn't a shot taken on a 35mm camera with a 70mm lens and cropped square be the equivalent??? What else could it be?

 

[wes loder]

I did some calculating of focal lengths several years back when comparing the Tenax II camera's 24X24 format to the usual 24X36 format. My thoughts below:
Comparison of format diagonal to traditional normal focal length.

24X24=34 34/40

24x36=43 43/50

24x24 use of 40mm matches the 24x36 format perspective using a 50mm lens.

57x57=80.6 80/80 ????

For a 21/4x21/4 format camera to yield the same perspective as 35mm film cameras, the “normal lens” should be 94mm focal length. 80mm in the 120 film square is the equivalent of just a 42.5mm lens in 24x36 format.

57x45=72.6 72.6/70

The 70mm lenses on the early 2&1/4x1&3/4 format cameras are equivalent to a 41.5mm lens. The 75s on later cameras is equivalent to a 44.4mm lens. Closer to the 35mm standard, but still wider.

18x24=30 30/38 or even 42 ????

For a “half-frame” format camera to yield the same perspective, its “normal lens” should be 35mm focal length.

So, medium format reflex cameras use short lenses, while as half-frames, like an Olympus Pen, use longer focal lenses.

Using the same criteria, the 2.7cm Orthometar is equivalent of a 34mm lens in 24x36. The 7.5cm Sonnar is the equivalent of a 94mm lens in 24x36.

 

[shawn]

With the different aspect ratio formats I find it easier to think about the vertical FOV equivalent (image height) and then adjust to the differences in width.