MF lenses on focal reducer: what aperture scale you use?

[aidaho]

Hi. I have small conundrum here, which I'd like to resolve with the help of the community.


Preface:

Same aperture value for lens mounted on sensors of different size means different DOF.
With same lens FF F2 DOF is roughly equal APS-C F2.8 DOF.

Although some people do attempt to recalculate how putting their FF lens on smaller sensor will translate back to FF (e.g. 50mm FF "becomes like" 75mm FF on APS-C),
no one does this with F-stops. And personally, I find these recalculations very confusing.

If a lens is 50mm, it remains so with any sensor. Sensor owner should just know, how DOF relates to focal length on his camera.

So far so good.


Trouble starts with use of focal reducer

Let's look at DOF of 50mm lens on FF and APS-C with focal reducer and dumb adapter.
Where F stands to what lens aperture ring points to, 1.5 is a crop-factor, and 0.726 is our focal reducer.
The higher the number, the thinner DOF we will get at any given subject distance:

FF DOF: F2 50/2=25, F1.4 50/1.4=35.7, F1 50/1 = 50.
APS-C focal reduced DOF: F2 50/1.5/0.726/2=22.95, F1.4 50/1.5/0.726/1.4=32.8, F1 50/1.5/0.726/1=45.9
APS-C dumb adapter or native lens DOF: F2 50/1.5/2=16.7, F1.4 50/1.5/1.4=23.8, F1 50/1.5/1=33.3

What can we see here? Few things:
1. Thinnest DOF is on the F1 on FF.
2. Focal reduced DOFs match pretty closely to original FF DOF
3. Native APS-C or dumb adapted values lag about one F-stop behind both focal reduced and native lens.

And here is the big question: do I treat F1.4 on focal reduced lens as F1? It clearly is an F1 in DOF comparison to any native lens out there.
People on YouTube seem happy to agree, reporting getting F1, by focal reducing F1.4 lens (or even F0.95 from F1.2 lens).


What's your opinion?
Because of focal reducer, do you mentally say to yourself: "I'm shooting at F5.6", whilst your ring is sitting at F8?

[Sjak]

To be honest, I don't bother too much. In the EVF I can already see the depth of field, so I have no practical reason to do the math while shooting

[dan_]

The DOF depends on the sensor resolution/print enlarging, too. A detail in your photo inside the DOF zone that looks sharp on a small enlargement/low resolution sensor may look unsharp on a greater enlargement or greater resolution sensor. That's why the DOF scale on the lens should only be considered as an orientative scale, not an absolute one. You'll be better by seeing the DOF directly in the EVF, as Sjak said.

The only situation when I really need the DOF scale on the lens is when I shoot using the hyperfocal distance of the lens. But in this situations I make my own tests and don't take the DOF scale as granted (in most situations the DOF scale is too "permissive" for my taste).

I agree with people on youtube - a 50mm/1.4 FF lens is transformed by a focal reducer into a ~35mm/1 APS-C lens and that changes the DOF accordingly. In fact, the DOF of a 35mm/1 lens is identical with the DOF of a 50mm/1.4 one.
That's why if you print an image taken with a 50mm lens at f/1.4 on a FF sensor and print the same image taken with a 35mm lens at f/1 on an APS-C sensor the 2 prints will look almost identical (at least in theory).

[visualopsins]

[luisalegria]

Interesting question.

The short answer I think is that the effective aperture doesn't change for purposes of DOF.

You do have to consider that the distance to the subject WILL change quite often because your framing will change - that is, you will be able to come closer for a headshot, etc. and will get shallower DOF just due to shorter distance. Same effect as putting the lens on a full frame camera. But for same subject at the same physical distance it should be the same.

I have to test this just to confirm.

Light gathering effect of aperture will change, because it will put more light on the sensor (with some percentage loss), at any aperture as it concentrates the image circle.

[Lightshow]

[aidaho]

[aidaho]

I thought more about this, and my conclusion is: if my 50mm has F1.4 dialled, I have to mark it as 36.3mm/F1 in EXIF.

My reasoning is as follows:
Full Frame 50mm F1.4 lens will always remain 50mm F1.4, regardless of any mounting circumstances.
It's a physical lens property, and unless someone is to redefine metric system tomorrow, this will not change.

Therefore, EXIF field LensInfo should continue indicate "50mm f/1.4" and LensModel must remain "Canon FDn 50mm F1.4".
However, the actual EXIF F-stop and focal length bear no relation to the lens info, I can put there anything I want.
I choose to treat LTII + Canon as a single system, and dial in values that make sense on APS-C.


This lens+reducer system gives me 36.3mm/F1, and that's what I'm going to put in EXIF.
There are two pieces of supporting evidence to back me up. The first is DOF relation consistency explained above.

The second is even more compelling. Any panorama stitching or some other projection manipulation software works a lot better, if program can derive exact FOV.
It does so, by taking sensor type and focal length from EXIF. And in this case focal reduced value is the only one which is correct.
Since we have no ambiguity in options here, we also cannot leave F-number unreduced.

I've finally been able to nail, what bugged me in some youtube videos.
The thumbnail for this one says "50mm F0.9" https://www.youtube.com/watch?v=p4xYA_P-bAc
But this is not correct. Nor the lens itself, nor the system as a whole conforms to these parameters.
In the video description the lens is listed as 35mm ~f/0.9, which is almost correct.

I'm not nitpicking at exact math here. Though both objective and focal reducer are lens (though FR isn't commonly called that), they could've been named lens system or something, to indicate we are no longer talking about just full frame lens strapped on top of the reducer.


And with that, I finally rest this case in peace.

P.S. In addition I've found that one of the LensInfo2 predefined choices is "78 => 'Metabones Canon EF Smart Adapter Mark III or Other Adapter', #PH/JR (also Mark IV, Fotodiox and Viltrox)".
Lens Turbo II certainly does fall into "Other Adapter", so this perhaps can be set, to indicate something has been used here.
And Lens Turbo can be put into a UserComment field.

[visualopsins]

btw, why are orange and yellow yarns swapped?

[aidaho]

Yep. Good eye!