MF: the equivalent 35mm format lens

buzzardkid · 01-23-2010

Recenlty I have taken up shooting MF a lot more. From way back, I knew my then standard lens on a Pentacon Six was the 80/2.8 CZJ, but what were the MF equivalents for the 35mm lenses I am used to?

This evening while browsing for the answer, I found this page, and it very simply at the bottom says:

Divide the MF length by two, and add 10% to that number. So, my Horseman 105mm lens is in fact a 58mm lens in 35mm format.

The 65mm Horseman would be a 36mm lens in 35mm format, and my 150mm is a 82.5mm.

Right. Easy as pie, no problem.

Next question now: would the DOF in the Horseman 105mm @ f5.6 equal the DOF of a 58mm SLR or RF lens @ f5.6?

If not, is there a formula to calculate MF DOF in 35mm format?

Roger Hicks · 01-23-2010

Simple, but worthless.

First, which medium format? As you're referring to a Horseman 65mm lens, I assume it's not 6x6. Of course I could be wrong.

Second, you can't do exact equivalents on formats of different shapes.

Third, it doesn't come close for most formats.

Taking the negative dagonal as a 'standard' lens, and rounding to the nearest 5mm, you've got 45mm on 35mm; 70mm on 645; 80mm on 6x6; 90mm on 6x7; and 100-105mm on 6x9 ('6x9' varies from 56x78 to 56x88). So 105 is 45mm on 6x9, 50mm on 6x7, 56mm on 6x6 and 65mm on 645.

As for the d-o-f question, it depends on how big you are going to enlarge your pictures...

Cheers,

R.

FrankS · 01-23-2010

Not to mention circles of confusion.

mfogiel · 01-23-2010

The simple answer to the DOF question is: for the same enlargement factor, the dof of a given focal length lens is the same, irrespective of the format. So if you enlarge your 35mm shots to 24x36cm and your 6x6 shots to 56x56cm (i.e. 10 times) a dof of a 50mm lens at the same aperture will be the same. However, this lens will give you a "normal" angle of view on the 35mm and a "wide" angle of view on the 6x6. As explained by Roger above, in order to be able to compare exactly the coverage between formats, one would have to compare formats with the same frame ratio (e.g. 4x5 or 2:3) So, this was the long answer. The short answer is, that on a Hassy or Rollei, the "feel" equivalent to a 35mm lens on 35mm format is a 60mm.

buzzardkid · 01-23-2010

Well, the original calculation came from a site dedicated to the Pentacon Six system, which is obviously 6x6.

With regard to the same ratio-remark, we're lucky, since I am a sucker for the 2:3 aspect ratio, i.e. 6x9. But, the calculation is useless for that, I presume?

I'll re-launch my original question then: in 35-format I shoot 28, 35, 50 and 90 lenses. What are their equivalents in 6x9-format? And, do the 6x9 lenses have a similar DOF when used at the same aperture as their 35-format equivalents?

Reason for asking: I got my act together with 35-format RF shooting with regards to predicting VOF and DOF, but I find myself in a new ball game when predicting this for 6x9, since I don't know my numbers here yet.

Roger Hicks · 01-23-2010

The diagonal of 35mm is near enough 43mm.

The diagonal of 56x84mm is 101mm, 2.35x the diagonal of 35mm.

Multiply any 35mm focal length by 2.35x and you'll get an exact equivalent. Thus 28 x 2.35 is 65.8mm; 35 = 82mm (usually 72mm or 90mm); 50 = 117.5 (110mm-127mm); 90 = 211.5 (210mm).

Cheers,

R.

buzzardkid · 01-23-2010

Thanks Roger, I think I can get to grips with it a bit easier like this.

JRG · 01-23-2010

"The diagonal of 35mm is near enough 43mm.

The diagonal of 56x84mm is 101mm, 2.35x the diagonal of 35mm.
"

Right on (and also to your earlier remark that you can't do this sort of equivalent length calculation w/o taking aspect ratio into account).

But as the 6x9 format actually has (exactly) the 2:3 aspect ratio of the 135 format, then the diagonal of the frame has little to do with the answer. Find the ratio of the shorter sides, the longer sides, or of the diagonals, and they'll all be the same. So, 56/24 = 2.333, 84/36 = 2.333, and the diagonals have that same ratio, 100.955/43.266 = 2.333. The 2.35 answer is just from finding and then rounding the diagonal lengths, an unnecessary step here.

The length of the diagonal is a customary way of declaring the "normal" length for a given format, but it plays no special role in finding equivalent focal lengths for two different formats. If the two aspect ratios are the same, convert using the long side, the short side, or the diagonal: You'll get the same answer. If the aspect ratios are not the same, then no calculation makes much sense until you've dealt with that fact. (Basically, you've got to decide how you plan to crop one format to make it resemble the aspect ratio of the other one.)