internal focusing lens, DOF and effective f-stop

[JMB]

I did a lot of macro photography years ago with wide array of equipment and formats (MFT to 5x7 film) and also photo microscopy but have not done any recently. I would like to get back and I am trying to decide what digital format would be best. For more general photography presently I am using MF (Hasselblad 907X) and using for macro a Laowa 85mm f5.6 macro or a Rodenstock App-Rodago-D 2x with tubes but that both setups need a focusing rail (especially for bracketing) and therefore are slow, fine for indoors but not for outdoors. In order to understand if smaller sensor format cameras would make more sense for macro photography I am trying to understand and calculate DOF for different sensor formats.

For calculating DOF one needs to know the effective f-stop and for lenses like the Rodenstock which focus by shifting the entire lens that is easy to calculate with the formulas I found.

But most modern lenses nowadays have internal focusing so the standard formulas do not make sense to me as front and back lens elements are stationary and internal lens element get shifted changing the focal length at different focus distances. My Laowa lens is internal focusing, the lens stop stays constant when viewed from the object side but get smaller from the sensor side when focusing close. As my sensor is larger than the image circle I noticed vignetting is increasing as I focus closer just the opposite when one uses a conventional lens which increase image circle with lens extension.

So how to determine DOF and effective f-stop with internal focusing lenses, especially considering that focal length is also changing?

[Doppler9000]

The answer to the format question may be constrained by your magnification needs and guided by your subject goals.

The Hasselblad (and the Fujinon GFX) 120mm macro lenses top out at 0.5X magnification. Because they are floating element designs, there is significant degradation of off-axis performance if you use extension tubes to get higher magnifications.

In addition, unless you need the wider field of view for either bigger subjects or the need for more environmental context, there is little advantage to medium format at higher magnifications.

The Sigma 70mm and 105mm full frame macro lenses are excellent, optically, and will do native focus bracketing to 1X with various bodies. They are excellent values, at $570 to $800 US, versus $4,500 for the Hasselblad 120mm.

The Canon RF 100mm macro goes to 1.4X, and is $1,100 at the moment.

There are options for smaller formats, but I will let others comment on them.

[Lou Jost]

As you know (judging from your use of such a wide variety of formats), there is no "one size fits all" answer to this kind of question. What kinds of subjects do you shoot? Is working distance very important? What magnification? And what will you do with the photos?

[JMB]

I am mostly interested in objects between 10cm and 2cm so the magnification is dependent on sensor format. I used NiSi 3D closeup lens on my Hasselblad lenses with good image quality at around 0.3x and automatic focus bracketing works but it is quite slow especially after 10 exposures (buffer issues).
I had in the other extreme an OM 5MII with their 60mm lens and the new OM-1 would be much faster and now allows in camera focus stacking (maybe useful only as preliminary test as I normally like to use Zerene) but the main issue I found often was the limited dynamic range of MFT especially compared to MF, often having issues with saturation of a single color channel (especially red or yellow flowers. The new OM-1 did not improve DR but everything else like focus stabilization and speed.
Obviously a recent FF top end camera would have a DR close to MF (maybe not quite the color accuracy) but these cameras are expensive and bulky (especially as a macro devoted camera).

My main question is more about technical understanding of internal focusing lenses used for macro and their change in focal length including the fee working distance and DOF. If I am not remembering wrongly the working distance of a lets say a 100mm lens was larger than the spec sheets of modern lenses of the same nominal focal length show.
I did some calculations of DOF with existing formulas not regarding internal focusing and using equivalent f-stops and adjusting the magnification to object size I found no significant difference between sensor sizes.

[Lou Jost]

Yes, the dynamic range of MFT is a problem; everything else about it is perfect for your range of FOV.

You don't need to worry about DoF when you are doing automatic focus bracketing.

If you can find a reasonably priced FF camera, I agree with Doppler9000 about the value of the Sigma macro lenses, particularly the 105mm. To get that extra bit of magnification to get a 2cm FOV, you could consider an auto 1.4x teleconverter and some cropping, or a high-quality close-up lens (both are very lightweight and capable of good results with a fine prime lens like the Sigma). The latter option would require some manual tweaking of your step size when auto-bracketing.

[rjlittlefield]

I am trying to decide what digital format would be best.
...
In order to understand if smaller sensor format cameras would make more sense for macro photography I am trying to understand and calculate DOF for different sensor formats.

One key thing to know is that if you scale magnification and effective f-number in proportion to the linear sensor size, then all sensor sizes capture the same size subject field with the same DOF and the same amount of diffraction blur in the final image. In addition if you expose for the same time at the same illumination level, then all sensor sizes experience the same amount of shot noise due to random arrival of photons. In other words, sensor size does not matter for these aspects.

So how to determine DOF and effective f-stop with internal focusing lenses, especially considering that focal length is also changing?

Most lenses, even ones that focus internally, manage to stick pretty close to the classic formula that Feff = Fnom*(mag+1), even if close focus involves shortening the focal length. But there are exceptions, for example some macro lenses that lose only 1 stop instead of 2 stops at 1:1. And there are the modern Nikon systems in which the camera and lens cooperate so that Feff = whatever is set on the camera. In any case the general rule noted above still applies, so the ambiguity really comes down to just what nominal f-number do you need to set, to achieve a particular effective f-number. It's a user interface issue, not anything fundamental to photography.

I think your other questions have been addressed well by Lou Jost and Dopplet9000.

--Rik