Equivalent Depth of Field

Pieter12 · Mar 18, 2021

I use a FF digital camera as a Polaroid of sorts to test lighting and composition when shooting table-top still life set-ups. Because my set-ups are modest in size the camera-to-subject distance is short and a small difference in depth of field can change the nature of the shot. I use macro lenses but assume that does not factor in except for the close coccus distance.

My question is, is there a formula or direct correlation to get the same depth of field on the equivalent lenses in each format? As an example my experience is that a 50mm ("normal") lens on a 35mm/FF camera, set at f16 would have similar depth of field as an 80mm ("normal") lens on a MF camera set at f32 (2 stops difference). Both camera+lenses focused on the same subject, although the distances will be different due to the different focal lengths. Using DOF preview can be hard to judge (even with additional light) at such small apertures so I am looking for a chart, calculator or rule of thumb for this.

Sirius Glass · Mar 18, 2021

John Will · Mar 18, 2021

Something like this:- https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#{%22c%22:[{%22f%22:13,%22av%22:%228%22,%22fl%22:50,%22d%22:3048,%22cm%22:%220%22}],%22m%22:0}

Pieter12 · Mar 18, 2021

John Will said:
Something like this:- https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#{%22c%22:[{%22f%22:13,%22av%22:%228%22,%22fl%22:50,%22d%22:3048,%22cm%22:%220%22}],%22m%22:0}

Yes! Exactly what I was looking for. Now if it only were an app! But I can bring a laptop to the studio, I guess.

John Will · Mar 18, 2021

These are also interesting:- https://dofsimulator.net/en/ https://www.photopills.com/calculators/dof

reddesert · Mar 18, 2021

Try to scale the focal length of the lens to have the same angle of view in the two formats, and the physical diameter of the aperture (really the entrance pupil) to be the same in millimeters. In practice, this means that both focal length and f-number will scale up with the format diagonal. Thus, if you use a 50mm on 35mm format, 100mm on 6x7, and 150mm on 4x5, use apertures of roughly f/8, f/16, and f/24 (I know there's no f/24 marking, so just use f/22).

When you keep the diameter of the aperture the same, the cone of light from some point in the subject plane that enters the lens is the same. Where this cone intersects the plane of perfectly sharp focus, you get a circle; this circle is the blurred out of focus image for points ahead of or behind the plane of sharp focus. So keeping the diameter of the aperture the same preserves the relation of in/out-of-focus areas.

wiltw · Mar 18, 2021

Part of the problem of comparing DOF is the fact that the aspect ratios of different formats varies.

135 format 1.5:1
645 format 1.3:1
5x4 format 1.3:1

...and then the usual 'normal' for each format is NOT the same AOV! But if you realize the short dimension of the frame can express 'equivalent FL' best, you can start to equalize calculations

24mmV in 135, 'normal' = 2*24 =48mm
43mmV in 645, 'normal' = 2*43 = 86mm
93mmV in 5x4, 'normal' = 2*93 = 186mm

Now if we stand in the identical location with all cameras, and aim at an identical subject (let's use 20' away), and use DOF calculator which assumes 20/20 perfect vision (US standard)

135 format 48mm DOF zone depth = 4.53'
645 format 86mm DOFzone depth = 2.41'
5x4 format 186mm DOFzone depth = 1.1'

Then, if we ratio the FL, and also ratio the DOF zone depth, we discover that the ratio of FL is almost inversely proportional to the DOF zone depth!

FL ratio: [645 vs. 135] = 1.79, [5x4 vs 645] = 2.16
DOF ratio: [645 vs. 135] = 1.87, [5x4 vs 645] = 2.19

IOW, if you simply ratio the 'equiv FL', you get the inverse in DOF

86mm in 645 has (1/1.87) the DOF zone of 135 format
186mm in 5x4 has (1/2.16) the DOF zone of 645 format
186mm in 5x4 has (1/4.04) the DOF zone of 135 format

IOW there is NO COMPLEX FORMULA to approximate the DOF zone depth! You simply need to ratio the 'same AOV' FL, and the inverse is the ratio of the DOF zone depth.

4 times the FL used = 1/4 the DOF zone depth

To change aperture, you simply need to understand that DOF zone depth simply gets 1.414 deeper with each aperture shallower, or 0.707 smaller with each aperture change larger. So if going from 135 format to 5x4 format, same aperture is 1/4 shallower. So one needs to alter aperture by 4 f/stops to get 'same DOF depth' on the larger format.

Sirius Glass · Mar 18, 2021

One can look at the depth of field scale on the lens once the lens is focused and the aperture is set. On the other hand use a slr and stop down the lens.

RalphLambrecht · Mar 18, 2021

Pieter12 said:
I use a FF digital camera as a Polaroid of sorts to test lighting and composition when shooting table-top still life set-ups. Because my set-ups are modest in size the camera-to-subject distance is short and a small difference in depth of field can change the nature of the shot. I use macro lenses but assume that does not factor in except for the close coccus distance.

My question is, is there a formula or direct correlation to get the same depth of field on the equivalent lenses in each format? As an example my experience is that a 50mm ("normal") lens on a 35mm/FF camera, set at f16 would have similar depth of field as an 80mm ("normal") lens on a MF camera set at f32 (2 stops difference). Both camera+lenses focused on the same subject, although the distances will be different due to the different focal lengths. Using DOF preview can be hard to judge (even with additional light) at such small apertures so I am looking for a chart, calculator or rule of thumb for this.

I suggest you prepare yourself a spreadsheet in Excel to calculate the depth of field using the variables you're interested in.
Then by using the app to 'goal seek' You can easily find the right settings to give you equivalent depth of field.

wiltw · Mar 18, 2021

Sirius Glass said:
One can look at the depth of field scale on the lens once the lens is focused and the aperture is set. On the other hand use a slr and stop down the lens.

Unfortunately, if shooting with zoom lens in one format, there are no DOF Scale marks to look at.

Nodda Duma · Mar 19, 2021

Photrio is worth searching for this info. It’s a sticky in the Exposure discussion sub-forum:

https://www.photrio.com/forum/threads/charts-of-depth-of-field-vs-focal-length-scaling-f.169727/

ChristopherCoy · Mar 19, 2021

John Will said:
These are also interesting:- https://dofsimulator.net/en/

This was an interesting experiment!

Left - 35mm format, f2.2, 63mm
Right - 6x7 format, f4.5, 127mm

pentaxuser · Mar 19, 2021

Thanks Christopher this is a great link. Always nice to see the changes as you make them.

pentaxuser

ChristopherCoy · Mar 19, 2021

I find this extremely interesting, especially because it's been something I have been very concerned about.

With a model distance of 2.5m, and a background distance of 10.3m for both portraits, the 180mm at F/8 produces nearly the same amount of background blur as the 85mm at F/2.8. I've been having to shoot at 8/11/16 during the first few days with it and was concerned that I wouldn't get the shallow DOF that I am used to. But it looks like that 3 stop ND filter I bought is going to prove to be a good choice. Now it's time to get out and test it.

Good thread!

wiltw · Mar 19, 2021

Chris,
The degree of background blur has nothing to do with DOF depth, it is purely a funciton of the DIAMETER of the aperture in use. FL divided by f/number. Bigger = 'more blurry'.
And 60mm diameter is 'twice as blurry' as 30mm aperture diameter.

And meanwhile, DOF is 'the same' regardless of FL at same aperture. DOF is only related to how the subject fills the frame...same filling = same DOF.

Pieter12 · Mar 19, 2021

wiltw said:
And meanwhile, DOF is 'the same' regardless of FL at same aperture. DOF is only related to how the subject fills the frame...same filling = same DOF.

Not sure how you came about with that. I can tell you from experience a longer focal length lens at the same aperture will not have the same depth of field as a shorter one. A 200mm lens at f5.6 focused on a subject 20 ft away will have a depth of field of maybe 1 ft, while a 21mm lens at f5.6 focused at 20 feet will have a pretty much infinite depth of field.

wiltw · Mar 19, 2021

Pieter12 said:
Not sure how you came about with that. I can tell you from experience a longer focal length lens at the same aperture will not have the same depth of field as a shorter one. A 200mm lens at f5.6 focused on a subject 20 ft away will have a depth of field of maybe 1 ft, while a 21mm lens at f5.6 focused at 20 feet will have a pretty much infinite depth of field.

If you alter camera postion in relationship to the lens FL change, the subject will appear IDENTICAL SIZE in the frame, and the DOF zone size will be virtually identical. I can prove that point, and others have proven that point for themselves. It only takes a DOF calculator program to prove the point.

I have many times stated that the DOF is identical when I shoot the following

50mm f/5.6 at shooting distance 10'
100mm f/5.6 at shooting distance 20'
200mm f/5.6 at shooting distance 40'
400mm f/5.6 at shooting distance 80'

20/20 vision DOF zone on 8x10" print viewed from 12" is about 1.43' for all four shots, as the subject is the same size in all resultant prints.

ChristopherCoy · Mar 20, 2021

Y'all are probably debating something that I termed incorrectly to begin with.

Dennis-B · Mar 20, 2021

You can also invest in a copy of the Kodak Professional Photo Guide. It has the D-O-F guides. Very handy, and you don't have to go through any calculations.

Pieter12 · Mar 20, 2021

I shoot a lot of my still lives at close distance using a macro lens. The few inches difference in depth of field between the 55mm for 35mm and 90mm for medium format show up as I use the cameras and lenses. So the chart posted earlier gives me the information I need to make sure I have critical sharpness and soft areas as I envision them for the final shot. Theory be damned.

wiltw · Mar 20, 2021

Pieter12 said:
I shoot a lot of my still lives at close distance using a macro lens. The few inches difference in depth of field between the 55mm for 35mm and 90mm for medium format show up as I use the cameras and lenses. So the chart posted earlier gives me the information I need to make sure I have critical sharpness and soft areas as I envision them for the final shot. Theory be damned.

classic DOF 'rules' are thrown out the door with macro magnification. Shooting macro 1:1 shot with 50mm FL and 50mm extension has same DOF as 100mm FL with 100mm extension has same DOF of shooting 200mm FL with 200mm extension...not much at all

Nodda Duma · Mar 20, 2021

Pieter12 said:
I shoot a lot of my still lives at close distance using a macro lens. The few inches difference in depth of field between the 55mm for 35mm and 90mm for medium format show up as I use the cameras and lenses. So the chart posted earlier gives me the information I need to make sure I have critical sharpness and soft areas as I envision them for the final shot. Theory be damned.

If you follow the link I posted above, you’ll find theory is not damned. You just have a bunch of people who don’t know the full story telling you the wrong theory.

But to your point, you can find the results in practice. Theory is more to help reduce trial and error, and to help designers anyways.

Sirius Glass · Mar 20, 2021

wiltw said:
classic DOF 'rules' are thrown out the door with macro magnification. Shooting macro 1:1 shot with 50mm FL and 50mm extension has half the DOF as 100mm FL with 100mm extension has half the DOF of shooting 200mm FL with 200mm extension.

Actually the optic laws [physics laws] apply to macro lenses too. You just do not understand optics involving the lens equations using the extension tubes. The lens focal lengths and the extensions are paired on each of you examples and are expanded equally in ever expansion by a factor of two. That is called scaling. Would you like some names of some books on optics to read and study?

Pieter12 · Mar 20, 2021

Sirius Glass said:
Actually the optic laws [physics laws] apply to macro lenses too. You just do not understand optics involving the lens equations using the extension tubes. The lens focal lengths and the extensions are paired on each of you examples and are expanded equally in ever expansion by a factor of two. That is called scaling. Would you like some names of some books on optics to read and study?

I don't give a rat's ass about the details of the theory and physics involved, just a general grasp of what's going on. The site in post #3 is really all I need. I want to make pictures.

wiltw · Mar 20, 2021

Sirius Glass said:
Actually the optic laws [physics laws] apply to macro lenses too. You just do not understand optics involving the lens equations using the extension tubes. The lens focal lengths and the extensions are paired on each of you examples and are expanded equally in ever expansion by a factor of two. That is called scaling. Would you like some names of some books on optics to read and study?

Like the other reply, I care not about the equation behind the result, I care about the RESULT.

I know conventional non-macro shot is based upon the size of the subject in the frame: same size, same DOF regardless of FL
I know in macro shooting, even with same size subject in the frame, the shooting FL does not really matters, and effectively f/stop gives more control

...that is all that matters. I don't carry Excel on tablet or smartphone to compute an otherwise difficult to solve equation. In in macro, the DOF is so darn shallow and the consideration of Working Distance may prevent change of FL to get 'better DOF'

Equivalent Depth of Field

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