Different formats and DOF

[Matus]

Obvious to some and complete magic to others. But how do the different formats compare concerning the DOF? Event though I am a physicist, I was just too lazy to sit down and compute, so I have just used one of the online DOF calculators. Given the instructions of this forum - all of the bellow is dry and technical and had little to do with visual arts and will improve you skills as little as the new lens you have just won on the eBay 😉

So - to demonstrate what I have in mind, lets have a look at 3 different formats: 35mm, 6x4.5 and 6x7. If we compare the latter two the the 35mm in the terms of the Field Of View (FOV) based on the longer side of the film frame, than the conversion factor are (approximately) 1.5 for 6x5 and 2 for 6x7. In other words if we start with a 50mm lens on the small format we would consider to get the same FOV with 75mm lens on 6x4.5 and 100mm lens with 6x7.

So far so good. Now what about the DOF?

Now let's consider that with such 3 camera setups we would take a picture of the same subject at a distance say 10 feet and we want to make prints. Let us consider 2 different cases:

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1) We want to make prints with the same ENLARGEMENT FACTOR, in other words if we would use the 35mm negative to produce a print of approx 8x10" size, the print from 6x4.5 neg would be 12x15" and the one from 6x7 neg. 16x20" (let us forget the slightly different aspect ratios of our 3 negatives).

now comes the important part - because the enlargement factor is constant - we need to take THE SAME size of the circle of confusion for all 3 cases. Iw we do so (let me take the one commonly used for 35mm cameras of 0.03 cm). But as we wanted to get THE SAME looking prints - we will need DIFFERENT f-stops for each lens to reach the same DOF.
if we use the DOF calculator we will get following numbers:
- Focus distance: 10 feet
- circle of confusion: 0.03cm
- DOF 50mm lens @ f/2.8: 2.06 feet
- DOF 75mm lens @ f/5.6: 1.81 feet (would be 2.04 if we would stop down 1/3 stop more
- DOF 100mm lens @ f/11: 2.02 feet

First conclusion: We needed to stop by (approx.) 2 stops more for the 6x4.5 neg and by 4 stops more for the 6x7 neg to reach the same DOF in our prints (which have different size). So here the rule would be :
Number of f/stops more = Cf^2 , where the Cf is the conversion factor between the formats

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2) We want to make prints of the SAME SIZE for our 3 negs. That means (contrary to the case (1)) that we will take different circles of confution for each format and these will differ by the same factors as does the FOV, so we will end up with following circles of confusion: 0.03cm for 35mm, 0.045cm for 6x4.5 and 0.06cm for 6x7.

Now - if we again want to have the same DOF in our prints, we will end up with following f stops:

- DOF 35mm @ f/2.8: 2.06 feet
- DOF 6x4.5 @ f/4 : 1.92 feet
- DOF 6x7 @ f/5.6 : 2.02 feet

Second conclusion: In this case we needed to stop by one stop more for the 6x4.5 and by two stops more for the 6x7. So the rule in this case is:
Number of f/stops more = [ Cf^2 ] / 2

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Bottom line: The above is only a bit of sloppy math, but does this makes sense to you? What is your practical experience? It looks nice, as most of MF lenses (and especially RF MF lenses) are rather slow, so we still get nice shallow DOF for portraits or those moody shots.

 

[payasam]

In the 1960s it was common knowledge that as one moved from 6x6 to 35mm, using normal lenses in both formats, depth of field would jump up. The "miniature", as 35mm was known (in fact earlier even 6x6 was called that), became famous initially not just for its small size but also for its considerable depth of field.

 

[Nikon Bob]

I was always told that as you move from 35mm format to medium format and on to large format you lost DOF if using lenses with the equivalent FOV in each format. There are movements in large format cameras to help compensate for their relatively limited DOF. From limited experience with larger formats I would think this is so. The opposite occurs when moving down in sensor size from 35mm in digital. I used f4 on my C5050 instead of f8 for what appeared to be similar DOF.

Bob

 

[Roger Hicks]

It's all down to magnification.

Smaller format = smaller magification (at a given distance/field of view) = more DoF.

Longer lens = more magnification = less DoF.

BUT

Bigger enlargement = less DoF, so 30x45cm off 35mm (x12) = less DoF than a 20x10cm enlargement (8x) and less still than postcard (5x).

AND

Greater viewing distance (normal with bigger prints) = more DoF.

Hope this helps.

Tashi delek,

R.

 

[bsdunek]

It was always my understanding, and experience seems to back me up, that DOF was a function of focal length. What the film or sensor size is doesn't matter. I know the standard 'circle of confusion' for 35mm cameras is usually 0.03mm. Perhaps larger formats allow a larger circle of confusion because you don't enlarge so much, and so can get away with it.

 

[Pickett Wilson]

Not specifically related to different format sizes (but may help), here's way more than you ever wanted to know about DOF...and it's even understandable. 🙂

http://toothwalker.org/optics/dof.html