Jump to content

All the DoF calculators online seem to be incorrect, or is it just me?


Raymond Zrike

Recommended Posts

Changing sensor/film size (while distance, T-stop, and focal length stay the same) shouldn’t change the total depth of field, right? These online calculators say otherwise: https://www.dofmaster.com/dofjs.html, https://dofsimulator.net/en/. Try just changing the sensor size and you’ll see it affects their depth of field readings. Am I wrong or are they wrong?

I’m trying to figure out what super-16 lens would get an equivalent depth of field for 10mm T1.6 on a 2/3” sensor.

Link to comment
Share on other sites

  • Premium Member

If the final results are viewed on the same sized screen, smaller film formats use a more critical circle of confusion figure to compensate for the greater degree of image enlargement. So the CoC of 16mm is twice as critical as for 35mm. If you compare a 25mm lens to a 50mm lens, you find that at the same distance you’d have to stop down the 50mm by four stops to match the DOF of the 25mm, but if you’re talking about a 25mm on a 16mm camera versus a 50mm on a 35mm to match field of view, then the CoC figure that is twice as critical for 16mm means you lose 2-stops of DOF, so that 4-stop difference between a 25mm and a 50mm becomes a 2-stop difference.

Link to comment
Share on other sites

  • Premium Member

The difference you’re seeing will be because the CoC (circle of confusion) value in DOF calculators automatically changes depending on the format. It’s far from an exact science, but part of the depth of field calculation involves predicting what sort of magnification will occur when the image is viewed.  Different formats traditionally have been assigned different CoC values for this reason. Often calculators simply use the sensor or film frame diagonal divided by 1500.

This calculator (one of many, but it’s easy to use and has both film and video inputs) lets you customise the CoC value in the advanced menu:

https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#{"c":[{"f":27,"av":"1.4","fl":10,"d":1829,"cm":"0"}],"m":0,"coc":"0.01"}

With the same CoC value inputted, the same lens at the same distance and f stop will have identical DoF regardless of the format.

In your case, assuming both S16 and 2/3” images are going to be viewed at the same final viewing size/resolution, then you need to use different CoC values to compensate for the difference in final magnification. If you plan to match the field of view, then you need to change focal lengths as well.

Using that calculator and the automatic CoC values, an equivalent lens to match the field of view and DOF of a 2/3” 10mm T1.6 lens on S16 would be around 13mm at T2.1. 

Link to comment
Share on other sites

  • Premium Member

The Circle of Confusion being tied to image magnification was more obvious when you were talking about different film formats being projected on the same size theater screen. Now it’s a bit of mess to figure out what figure to use. For one thing if one digital camera has a 2K Super-35 sensor and one has a 4K Super-35 sensor and both will be shown on the same sized screen with a 4K projector… do you use the same CoC figure? 

Link to comment
Share on other sites

10 hours ago, David Mullen ASC said:

do you use the same CoC figure? 

I'd love to know the answer to that one. My guess is that you can - it's just that you shouldn't if you want to find the ideal aperture.

Leica's S 007 had the same resolution as the S2, because the engineers understood that users wanted more flexibility with apertures, and that more pixels could end up being wasted with smaller apertures. However, the demand for more resolution caused Leica to go to 64Mpx for the S3.

Link to comment
Share on other sites

Wow this was more complicated than I thought it was. Thanks for everyone's input. Lots to chew on.

On 4/29/2022 at 9:34 PM, David Mullen ASC said:

Don’t you want to first match field of view and then figure out how much to adjust to match depth of field? Otherwise you’re talking about two different image compositions / sizes.

I have a Zeiss Digizoom (made for 2/3") that I am using on super-16 with an IBE B4 to PL adapter. The adapter essentially keeps the field of view that the lens had on 2/3" by magnifying the focal length by around 1.35x. Drawback is that it takes away a stop of light and that the maximum input is T1.9.

I was considering getting a Zeiss Digiprime 10mm T1.6 that I saw on sale and wanted to see what it could take the place of in my super-16 rig (at least when it comes to depth of field and FOV, not lens speed).

On 4/29/2022 at 9:56 PM, Dom Jaeger said:

Using that calculator and the automatic CoC values, an equivalent lens to match the field of view and DOF of a 2/3” 10mm T1.6 lens on S16 would be around 13mm at T2.1.

Thanks! That's about what I was thinking. So the Zeiss Digiprime 10mm with the IBE adapter would essentially be a 13mm T2.1 when it comes to depth of field and a 13mm T2.7 when it comes to speed (given the adapter's max light input). I contacted IBE, and they said there shouldn't be any issues using the adapter with a lens faster than T1.9; the exposure just won't change at anything faster than that T-stop.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...