MF lens conversion factor

[Altsir49]

So a bit confused and couldn't find an article or post that covers this clearly.

As far as focal length is concerned, multiplying by a factor of 0.79 gets us the 35mm equivilant, that's straightforward.

However, does that also apply to aperture? So an F4 lens gathers as much light as a 35mm F3.2 lens approximately? Is that why most MF lenses have what seems like slower lenses?

Finally, and this is the confusing part, a Hasselblad salesperson mentioned something about the light gathering quality of MF lenses being completely different and require a different thought process and can't be translated directly. Any truth to that or complete BS

 

[davidmknoble]

There is a good article about Leica S lenses that explains this. The field of view, ie., how wide or telephoto the lens looks is determined by the conversion to 35. So a 38mm lens is a wide angle of view at 0.79 (call it 0.8). The field of view is 38mm x 0.8 = 30mm. The f/stop is more tricky.

An f/4 lens in medium format has a dept of field that is narrower for the same f/stop in 35mm. This is because the focal length is longer (38mm vs 30mm). Think of it like a zoom lens. A 75mm lens at f/2.8 has a narrower depth of field than a 50mm lens at f/2.8. The same is true with medium format.

However, physics is physics. An f/4.0 lens lets in the same amount of light regardless of what focal length it is. You do not change your exposure for a 50mm f/4.0 aperture and a 75mm f/4.0 aperture. Light is light. So the exposure is the same regardless of the lens and regardless of the format (4x5, 35mm, medium format).

This article is the best I’ve ever read about medium format lens ‘looks’ and conversions. David Farkas is a good friend of mine and this is acurate: https://www.reddotforum.com/content/2020/04/the-definitive-guide-to-leica-s-lenses/

 

[benr]

These topics are very simple conceptually, but the results can be complicated to understand intuitively.

Focal Length​

"Equivalent focal length" is a very simple linear multiplier, complicated only by the different aspect ratios of different sensor formats. Modern "medium format" (Hasselblad, Fuji, some Phase one) is a 33x44mm sensor, a 4:3 aspect ratio. Compare this to 35mm "full frame" at 24x36mm with its 1.5:1 aspect ratio.

I like a 5:4 aspect ratio generally, so I crop my "full frame" to 24x30mm and my "medium format" to about 33x41.5mm when editing images. This means that an accurate focal length multiplier for equivalent focal lengths is more like 0.72x (30/41.5). If you took the same picture on each of those two cameras, from the exact same position, with equivalent lenses per this formula, you would get identically framed photographs.

(If you prefer to crop your 33x44mm images to a 1.5:1 aspect to match your "full frame" camera, then your multiplier would be 0.82x (36/44), and you'd crop your medium format shots to 29.5x44 or 1.5:1. If you don't care about aspect ratio, and just the general "feel" of the lens, then sensor diagonal is the accepted standard, so calculating 55/43 (the diagonals of the two sensors) gives you the 0.78x you mentioned. I find this a little imprecise.)

Aperture for Exposure​

Aperture for exposure is simple. The f-stop of a lens is the ratio of the physical focal length to the physical size of the hole allowing light through to the sensor. So an f/2.0 lens – literally meaning focal length(f) divided by two – has an aperture of 25mm diameter (optical designs may mean that the actual, physical aperture is different than this, but the overall design of the lens elements will create an effective aperture of exactly this size).

Aperture is the only thing that governs light passing through the sensor (well, coatings affect it a bit, too – up to maybe 1/3 or 1/2 a stop – which is why cinematographers use transmission, or t-stops instead of f-stops, but still photographers don't worry about matching exposure between different lenses as precisely).

This means that an f/2.0 lens on any format will require the same shutter speed and ISO for correct exposure as any other lens. The internet has invented the term, "light gathering," which is unfortunately meaningless and muddies the issue. An f/2.0 lens for a larger format, with equivalent features, will typically be larger and heavier, and depending on the flange depth of the mount, may require some tricky optical design to get good sharpness and reduce unwanted aberrations. But if the lens designers can achieve it, the illumination of a given area of the sensor of any f/2.0 lens is the same (bearing in mind that lenses for larger format sensors need to project a larger image to "cover" that larger sensor).

Aperture for depth of field​

I'll preface this by saying that discussions of depth of field should really use consistent terminology. In keeping with the "depth" part of depth of field, I have always preferred "shallower" and "deeper" in these discussions. Not only is this the correct terminology based on two-hundred years of photography, it also accurately conveys the idea that the area of acceptable focus in front of or behind the plane of actual focus is lengthening (deepening) or shortening (getting shallower) as you "stop-down" or "open up" your aperture. And to say "more depth of field" should always mean "more" in focus; a deeper depth of field. Historically photographers cared more about depth of field as a way to ensure critical parts of the image were in focus, as landscape photographers do today. Depth of field calculations (taking into account the size of the grain of the film, or the pixel size on the sensor as the limiting factor of how sharp any photographed element can be) allowed you to say that "from this distance to this distance everything will be rendered as sharp". Depth of field calculations didn't really have a way to quantify how "out of focus" the "not sharp" parts of the photograph would be.

(Because of the modern tendency to equate "depth of field" with "amount of blur in the background", people have begun using "more depth of field" in a photograph to mean, "less in focus," or actually, "less depth of field". This is confusing and inaccurate.)

How aperture relates to depth of field for a given format size, focal length, and aperture is sadly, a lot more complicated. This is because it needs to account for the focus distance, which vastly affects the depth of field (the reason macro shots typically have such shallow depth of field. The equations go over my head, but smarter people than me have plugged them into online calculators, and these will help you determine the correct answers for given combinations of sensor format, focal length, and aperture.

Since when people discuss depth of field today they are as likely to mean "amount of blur behind my subject" (as do I), I find this website http://howmuchblur.dekoning.nl to be invaluable. You can enter any combination of crop factors, focal lengths, apertures, and set subject size (from which the tool infers focus distance), and see the actual calculated size of the blur circles (bokeh, if we must) at different distances from the subject. Because, yes, distance from the subject is another factor in how "out of focus" the background is.

This is an example of four "40mm equivalent lenses" (cropped to my preferred 4:5 aspect ratio): the Nikkor Z 40mm f/2 on a Nikon Z full-frame body; the Fujinon GF50mm f/3.5; and the new Fujinon GF55mm f/1.7, both on GFX bodies; and the Fujinon XF 27mm f/2.8 on a Fujifilm APS-C X-series body, all wide open. As you can see, the new GF55mm with its remarkable f/1.7 aperture is capable of very large blur circles, whereas the GF50mm f/3.5 cannot create blur circles even as large as the Nikkor Z 40mm f/2.

If all images were correctly exposed, and viewed at the same distance and size, they would all look identical (except for the small difference in focal length: the 55mm is a bit longer than the others relative to sensor size). The only visible differences would be the amount of subject in "acceptable" focus, and the relative size of the blur of the background.

You can actually view this graph live here and experiment with adding or changing parameters: http://howmuchblur.dekoning.nl/#com....8-and-1.54x-27mm-f2.8-on-a-0.9m-wide-subject

BUT, you can actually kinda sum all of this up with the same multiplier as for focal length. So from the example above, the GF 50mm f/3.5 on a 33x44mm sensor can be considered a 40mm f/2.5 equivalent on "full frame" (24x30mm in my example) as 3.5 x 0.72x is 2.52. But this is only as regards depth of field, not aperture for exposure (where the 50mm stubbornly remains f/3.5).

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Part 2 follows…

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[benr]

And now some thoughts​

As you can see, though, this idea of the "medium format look" is not a factor of the sensor size or lens aperture alone. The different "look" of different formats and lenses has to do with the design opportunities afforded by the entire system design, lens-mount diameter, flange depth (distance from lens mount to sensor plane), the cost of the lenses (sometimes) and the talent of the lens designers. One of the big early disappointments of Fujifilm's and Hasselblad's 33x44mm "medium format" sensors was that the lenses they offered featured maximum apertures of f/2.8. It was possible to get larger blur circles – shallower depth-of-field – on Fujifilm's f/1.4 lenses for their APS-C system than it was on their GFX system.

None of this discussion accounts for the other advantages of larger sensor formats. Larger sensors historically had better noise performance (although that diminished early on as better sensor technology came to "full frame" cameras sooner than "medium format"). Larger sensors – in systems with well designed lens mounts – offer their lens designers other opportunities such as the ability to more fully correct certain optical aberrations, sometimes simply because it's easier to manufacture physically larger lenses to tight tolerances than smaller lenses – one of the reasons for Leica lenses high prices. Of course, "medium format" lenses are typically larger and heavier than their counterparts, and thus designing a good autofocus system – for example – has new challenges because it has to move larger, heavier pieces of glass.

But sadly, any discussion of "compression" or any other such "medium format" pixie dust is just that.

All lenses behave differently, even two different lenses of the same focal length on the same camera system. Lens design is enormously complicated, and is inherently a compromise between sharpness across different areas of the image, control of aberrations (both pleasing and unwanted), and size, weight, and complexity, which all affect cost. The "medium format" look we all think about is really about the older film formats like 6x7, where lenses were designed without the modern obsession with sharpness (due to the limited resolving power of film), and without autofocus, making the image-creating aspects of lens design front and center (designers could make lens elements whatever weight they liked; on the Mamiya RZ system for example, the lenses didn't even have to internally focus, the entire lens was moved back and forth on a bellows). This allowed designers to focus on genuinely pleasing characteristics such as the quality of the out-of-focus regions, and the falloff between in-focus and out of focus areas. This focus on different aspects of optical design, combined with the much larger "sensor size" of 6x7 film (as compared to 33x44mm today), and modest but decent apertures such as f/2.8 and f/3.5 led to these "medium format" images looking "3D" or other such descriptors.

Hell, you want "3D", look at Alec Soth's images shot on 8x10" camera with a 300mm lens, and weep about how little background blur you can achieve on smaller formats. I'll leave this link here for you to enjoy: http://howmuchblur.dekoning.nl/#com...8-and-0.12x-300mm-f5.6-on-a-0.9m-wide-subject

But as with all things photography, the desirability of these different image qualities is highly subjective. All you can do is be informed, learn about the strengths of your own equipment, and go out and make great images of your own. The craziest thing of all is that people covet and acquire some of these mythical lenses and cameras and then shoot them stopped down to f/8 because they were maxed out on their shutter speed. And often, they make great images. I personally believe that the historical "medium format" look is as much about how the cameras force you to slow down and consider your shots – perhaps working from a tripod; asking your subject to keep still since focus had to be obtained well before the image was taken, not during it as today – and that "medium format" images can be created on any equipment if you just think about working that way.

Hope some of that's helpful!

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Not proof read, I may return later and make small edits if needed.