Depht-of-field and Camera Format

[hjoseph7]

Ok so the aperture f2.8 on a FF 35mm format camera is the same as on an APS-C camera and  is the same as on a Medium Format camera and is the same f2.8 on a Large format camera. But is the DOF any different ? What I mean is, would the DOF on a Medium format camera be more shallow at an aperture of F2.8 than on a FF camera ? 

[Ed_Ingold]

Depth of field is inversely proportional to the format size (cropping factor) at the same relative (ie, f/stop) aperture.

[Ricochetrider]

@Ed_Ingold so how does this translate in real world terms? As in the difference between formats and image/negative/sensor sizes and results? 
 

IOW does the depth of field increase or decrease as the image size increases? 

[William Michael]

One of the best papers I have read on this topic is by Joseph James, Equivalence - LINK

Bob Atkins also has written two (probably more) papers on this and associated topics - the two to which I refer now, touch on (the variances of) the (type of) Background Blur and Bokeh with variances in Camera Format. Obviously, Background Blur and to a larger Bokeh are somewhat subjective; moreover are functions of Lens and Iris however, from my memory, Bob's papers touch on a reasonable detailed factoring of Camera Format. The Background Blur paper has a user friendly on line spreadsheet to input data. I don't have the links readily available to Bob's efforts.       

1 hour ago, Ricochetrider said:

so how does this translate in real world terms?

My one sentence answer is - in this Digital Age  - in 2023 - 135 Format(*1) offers the Photographer the options to attain the Shallowest DoF possible for any Shot.

*

This is because, to fit the 135 format size, there exists lenses which are exceptionally fast: creating a format size/lens speed combination which other formats and lenses to suit, cannot compete.

The larger the Format the bigger the hunk of glass required to cover the image circle; the smaller the Format, the faster the lens required to compete - to attain the 'equivalence' in shallowness of DoF.

* 

A "real world" comparison/example -

Obviously not every Photographer hunts for Shallow DoF Lenses (nor necessarily wants light sucking lenses simply for the sake of light sucking).

However as an example, I have a 35mm f/0.95 for my APS-C Format Camera. I would need an 35mm f/0.55 lens for my camera to achieve the 'equivalent' shallow DoF which Marc Williams can achieve using his 50mm f/0.95 lens on his 135 Format Camera.

As a bonus - here is one of the shots Marc made with that lens - not necessarily highlighting the Shallow DoF capacity - however it is one of my favourites of Marc's work - LINK   

WW

*1 - '135 Format' or in (more) archaic terms 'Miniature Format' is what is now commonly termed 'Full Frame' - how ironic are these the name changes.

[William Michael]

3 hours ago, hjoseph7 said:

What I mean is, would the DOF on a Medium format camera be more shallow at an aperture of F2.8 than on a FF camera ?

(please read the bullet points under the chapter "Equivalence on the Quick" in the link I provided to Joseph James' paper)

Then - if all other factors are 'equivalent':

Yes. Using an 80mm F/2.8 lens on Medium Format Camera (e.g. either 645 Format or Mamiya ZD) will allow a Shallower DoF than using a 50mm F/2.8 Lens on a 135 Format (or 'Full Frame') Camera.

To achieve an 'equivalent' Shallow DoF using the 135 Format Camera, one would need to use a 50mm F/2.2 or 50mm F/2 lens. (respectively)    

WW

[httpwww.photo.netbarry]

3 hours ago, hjoseph7 said:

What I mean is, would the DOF on a Medium format camera be more shallow at an aperture of F2.8 than on a FF camera ? 

Yes

[Rick Helmke]

I think everyone is over complicating the question. An 80/2.8 will give less depth of field than a 50, 35, 28 and so on at a given aperture. A 180 will give even less, a 300 less than the 180 and a 500 even less. Format doesn’t change that. The problem is that a ‘normal’ focal length lens may be an about a 28 or 35 on a crop format but is a 50 on a 35mm size frame. The 80 is ‘normal’ on a medium format, 2 1/4 square or 2 1/4 by 2 3/4. The 80 gives less dof at a given aperture whether it’s on  crop frame or medium format. The 150-180mm is a medium format portrait lens and gives less dof than shorter lenses. I love the Mamiya 180 on an RB67 the same way I like the 105/2.5 Nikkor on 35mm film/full frame sensor. With either setup the distance to the subject at that focal length causes backgrounds to blur out while delivering razor sharp images of the subject. It’s pretty straightforward really, just takes some practice.

 

Rick H.

[William Michael]

11 minutes ago, Rick Helmke said:

An 80/2.8 will give less depth of field than a 50, 35, 28 and so on at a given aperture. A 180 will give even less, a 300 less than the 180 and a 500 even less.

Not arguing.

However, over simplifying may also present its potholes.

In the spirit of keeping it simple, I reckon it's relevant to include your (Rick's) response assumes those various lenses are at the same distance from the Subject - i.e. the DoF changes, also the Framing changes ? Yes?

WW   

[Rick Helmke]

I never actually measured but always seem to be about the same distance from the subject using those format/lens combinations. The framing remains the same or close to the same. The 105 will give a little less background blur than the 180 all else being equal but the longer the lens the more the background drops out of focus at a given distance. At a distance of say 15 feet the image will be about the same crop ie a headshot with the 105 on a 35 format and a 180 on an RB67 a which is 2 1/4 x2 3/4 format. The 180 will have less dof and a more blurred background. 
 

Rick H.

[William Michael]

4 hours ago, Rick Helmke said:

I never actually measured but always seem to be about the same distance from the subject using those format/lens combinations. The framing remains the same or close to the same.

Agree.

I concur both statements bold and underlined are correct.

***

Regarding my previous response -  I think there was a misunderstanding. Either I misunderstood your second sentence, which I quoted, or you misunderstood my reply, or both.

I was only addressing that sentence in my reply.

Anyway, not wishing to confuse the issue: I hope Harry has his answer.

WW

[Sanford]

Just turn the dial to "P" and forget about the rest of that stuff.

[Niels - NHSN]

2 minutes ago, Sanford said:

Just turn the dial to "P" and forget about the rest of that stuff.

My Rolleiflex does not have “P”. Can I use “B” instead?

[JDMvW]

This issue of whether depth of field is different for different formats is surprisingly contentious. If the search engine here worked better you might be able to find "A tedious essay on Depth-of-Field" by  Alan Marcus. It may still be here somewhere, but the old address pointers don't work...

LATER Against all odds I found it:

 

Edited April 1, 2023 by JDMvW

[paddler4]

The most straightforward explanation I have ever come across was an article written by Bob Atkins, http://www.bobatkins.com/photography/technical/digitaldof.html. In addition to providing an explanation, he starts with 5 simple summary statements.

[rodeo_joe1]

If you keep the subject distance and angle-of-view the same, then the lens focal length has to get shorter or longer in line with the format size. And depth-of-field is more dependent on focal length than it is on relative aperture - Consider; f/2.8 on a 20mm lens is physically a lot smaller than on a 200mm lens.

Visual proof. All taken at the same aperture and from the same distance. 

Full frame - 

DX format - 

Tiny compact camera (sensor size about 6x8mm) - 

Same happens if you crop a wide angle shot to emulate a telephoto one. The D-o-F from the wideangle lens is much greater than that from a telephoto lens giving the same cropped field of view. 

This is depth-of-field 101 and must have been covered dozens of times before on PN alone.