Question regarding depth of field on Medium Formats

[paolo_avenir]

I recently bought a Yashica Mat124G TLR, it has a fixed 80mmm f/3.5 lens. I only

use 35mm film so I am not familiar with medium formats especially when it comes

to aperture/DOF.

 

I've looked around and so far I have to stop it down by two stops to make it

equivalent to 3.5.

 

Is this a rule or is it a guess?

 

Many thanks in advance.

[russ_britt3]

The depth of field for a 80mm on a 6X6 is the same as the depth of field of a 80mmon a 35mm camera. Its just the field is a little wider because the film is larger. You will see more details, and maybe find to get sharper images you may need a tripod more often.

[Ed_Ingold]

The depth of field is the same for any lens of any focal length on any format of film IF the relative aperture is the same, and the image is the same absolute size on the film plane.

 

That may be a trivial answer. Since medium format film is larger, you'd have a lot more space around the subject if the subject were the same size. You would then move in closer so that the composition were the same, in which case the depth of field at the same aperture is less than for 35mm.

 

That proves to be a problem for landscapes but a blessing for portraits. It's hard, with a small-format camera, to keep from showing all the threads and wrinkles in the backdrop.

[michael_madio]

See http://www.dofmaster.com/

[helenbach]

Depth of field is a tricky thing to make rules about, unless you fix some of the variables. If you take two pictures from the same place that result in the same horizontal angle of view and the same size of print then the following holds true, supposing that one is made on 36 mm wide film and the other on 56 mm wide film:

 

The depth of field varies with N.C/f^2 (except for close-up) where N is the f-number, C is the diameter of the acceptable circle of confusion and f is the focal length of the lens.

 

If you want the same DoF (in this case) with MF as with 35 mm, then the f-number for MF needs to be 56/36 times that for 35 mm, or 1.56 times - so f/8 in 35 mm is equivalent to f/12 in MF. Therefore a stop and a bit is exactly right, or it would be if a bit was a quarter, which it isn't.

 

Best, Helen

[paolo_avenir]

Thanks everyone.

 

Helen,

 

So you mean a f/3.5 on a MF is f/5.6 on a 35mm?

 

Thanks.

[paolo_avenir]

OOPS!

 

I meant a 5.6 on a MF is 3.5 on a 35mm.

[helenbach]

With all the assumptions I gave, and as a general approximation, yes. If you use a 50 mm lens on a 35 mm camera, and an 80 mm lens on a 6x6 for two pictures taken from the same place, and focused at the same distance, then print the 35 mm one to 8x12 and the 6x6 one to 12x12 (for example) the theoretical depth of field will be the same if the 50 mm is at f/3.5 and the 80 mm is at f/5.6.

 

The dofmaster online calculator gives slightly different values, because of a slightly different relationship between the acceptable circles-of-confusion for the two formats.

 

Best,

Helen

[john_stockdale2]

But if you're using 6x6 instead of 35mm to get better quality or to make bigger enlargements you might want your circle of confusion to be more stringent. I generally use the DoF marks on the lens barrel, but I use the marks for the aperture two stops more open (=less DoF) than I'm actually using. I might relax this stringent criterion if the subject allows it or if there's no choice.