David Krooshof

Nerding over bokeh and aperture

I'll use 'myth' in the meaning of: Something that people tell each other, but that is not completely true.

I learned to photograph through the internet, not in a school. The nice thing about the internet, is that people can say anything and easily repeat others mindlessly. This subject is no exception.

I started shooting big cameras because I wanted something out of them, that I thought came with the sensor. This was only partly possible. I believed in a few things, listed below, that I later found out, were myths.

Myth: "The bigger the sensor, the shallower the DoF"

The reasoning is this: To get the same field of view, you need a longer focal length. It is that longer focal length that is said to get a shallower DoF, if it has the same aperture value as the shorter lens. So it would be true if you compare a 50mm f2 lens to a 80mm f2 lens. Or to a 25mm f2. But this has nothing to do with the focal length, but everything with the aperture. Read on if you are interested. This is going to get nerdy, but not too complex.

But first: Bigger sensors ask for different lenses. Here's a little list of typical, affordable normals. A "normal" lens is a lens that has a standard field of view, not wide, not tele. A bigger sensor needs a longer focal length lens to get a "normal" field of view.

I cannot easily make tables in this CMS, but imagine columns meaning:

Type Sensor height, normal focal length, typical aperture for a normal prime, some notes. I know there are faster lenses, but at a price.

m4/3 12mm 25mm f0.95 Fastest lens for the system

APSC 17mm 35mm f1.4

Full Fr 24mm 50mm f1.8

6x4.5 42mm 80mm f2.8

6x6mm 56mm 80mm f3.5 (a f2.8 Mamiya or Rollei is way more expensive than a 3.5)

6x7mm 56mm 90mm f3.5 (Lenses for large medium format cameras are generally around f3.5)

4x5inch 100mm 150mm f5.6 (faster (radioactive) lenses exist, at a price+risk))

5X7inch 125mm 210mm f5.6

8X10inch 300mm 300mm f5.6

So in short: in practice, cameras with bigger sensors do not produce a shallower DoF, because the lenses that come with bigger cameras, are typically slower than those for smaller cameras. So they come with longer focal lengths (shallower DoF) but those often have smaller aperture (deeper DoF) that mitigate the effect.

There are three exceptions:

1) There is a limit to how wide an aperture can be, and smaller cameras are close to that limit. You can't throw more money at it and get a faster lens, at some point.

2) The manual focus lenses available for APSC and M4/3 are not that different, now that the Chinese make the same lenses with different mounts for both m4/3 and APSC. Essentially, you are putting APSC lenses on M4/3 cameras. Stepping up to full frame, however, does put you in a different category of lenses, and you will notice they are on avarage a stop slower for the same price and for the same field of view.

3) Only when you move to large format, does the general aperture stay at the same F/value over the whole line of lenses. But prices of film, cameras, filmholders, lenses go up by a factor of 4 when you step up from 4x5 to 8x10 inch film. So getting a shallower DoF is still more expensive.


- A shallow DoF is expensive, because good fast lenses are expensive, regardless of the size of the camera.

- Bigger cameras only help to get you more blurry backgrounds above the 4x5 threshold. Large format sheet film cameras (analog only) are exponentially more expensive the bigger they get.

- Blur is more costly than sharpness.

A note on fast wide lenses on mirrorless cameras

You might have noticed, that for reflex cameras the shorter focal lengths are not as fast as the normals. Generally, the normals can be simpler lenses than the wider focal lenghts. The reason is that the lens needs to sit at a certain distance to the camera to make room for the mirror.

With a 50mm lens, the center of the lens is at 50mm from the sensor, leaving plenty of room for the thickness of the lens, as well as the mirror. For a 28mm lens, however, it is different. It needs to sit closer to the film or sensor, leaving no room for the mirror.

The solution is to put the lens at a larger distance, and add lens elements that guide the image to the sensor. This limits the width, and hence the speed of the lens. On mirrorless cameras, no room for mirrors needs to be spared. This means that wide angle lenses can sit closer to the sensor, and hence can be simpler and better. Leica range finder cameras are also mirrorless cameras, and one reason they were immediately popular as street and journalistic cameras, is that they could offer simple, fast, and good short focal length lenses, even befor 37mm lenses became available on SLRs.

And today, mirrorless systems like Sony Alpha, Nikon Z Canon R and M4/3 offer good wide angle lenses, faster than were ever available on SLR and TLR cameras. And lens design became better in general, because computers can now be used to solve optical challenges.

Vintage glass especially interesting for so called normal focal lengths, not so much for wide angle lenses. And for telelenses, you want image stabilizers and autofocus, and designs that aren't pumping dust into your camera like the old helicoid type lenses are.

But in reality, it is not because of the focal length that you get more blur

There is another way of looking at the amount of blur in an image. If you consider the size of the circle of confusion relative to the image a tree in the background rather than to the size of your sensor, there really is no difference between focal length. It is just that a longer focal length makes the background larger. It enlarges both the tree and the bokeh alike. I was told that mathematically, you could prove that the size of the unsharpness depends only on the distance of the subject and on the size of the apperant aperture (if you look into the front of the lens) and that the influence of the lens is only on how wide the image path to the sensor appears to be. Notice sensor size is not a factor at all.

All in all, for the bokeh lovers, it would be better to look at one specific number only: the apparent diameter of the aperture in mm, rather than as a fraction of the focal length. It is easy to calculate: F/x is actually the formula for it:

F stands for focal length (not for aperture, mind you!). So focal length divided by that number gives you the aperture. So here's a list of standard lenses again, but this time with the apertures calculated in millimeters rather than "stops":

25mm f0.95 = 26.3 (fastest M4/3 lens available)

35mm f1.4 = 25.0 (fast APSC normal)

50mm f1.8 = 27.8 (standard of standards on full frame)

80mm f2.8 = 28.6 (Like 6x6 Rolleiflex, Mamiya and Zeiss/Hasselblad

90mm f3.5 = 25.7 (Like RB67)

150mm f5.6 = 26.8 (standard on 4x5" film)

All of the above have the same absolute aperture of 25 to 28mm! Only when sizing up beyond 4x5" film, the standards lenses get relatively bigger.

210mm f5.6 = 37.5 (standard on 5x7" film)

300mm f5.6 = 53.6 (standard on 8x10" film)

So here you see the same pattern as above: Standard lenses for any sensor size, roughly produce the same DoF, because the aperture stayes the same. Only when the lens is really big, you get a shallower DoF.

Myth: Longer focal lengths have more unsharpness in the background.

Do we look at the size of the circle of confusion relative to the sensor or relative to the subject? If we look at the size compared to the sensor, then notice that a smaller sensor zooms in on the centre of the image, and also zooms in on the circle of confusion, on the bokeh balls, as they say.

So is the smaller sensor showing more unsharpness? Relative to the subject, like say a tree in the background, the circle of confusion stays the exact same size.

Real World Examples

If you shoot full frame 50mm f1.8, does going to medium format help you get a shallower DoF?

A bit, when you get an 80mm f2.8 (28.6mm) Hassy, Mamiya, PhaseOne or Rollei, but getting a faster lens like a 50mm f1.4 (35.7mm) helps more.

The normal for a digital Fujifilm camera is 63mm f2.8, and that gives a 22.5mm diameter, so that does not help te get you more blur (Very good camera, I'm not complaining).

On a Mamiya RB67, the 90mm f3.5 is similar to a 50mm 1.8 on full frame. Only when you get an RZ67 with the 110mm f2.8, you make a significant step forward, on par with 5x7 film cameras. But an RZ is still a pricey piece of gear. And shooting it easily costs 1.50 euro per picture if you scan yourself.

If you want to match the DoF of an 8x10 camera on "full frame", you will need a 50mm f0.95 lens, because 50/0.95=52.6mm diameter. That will cost you 600 euro for a not so good Chinese lens, or 15.000 for a Leica. You can buy a lot of 8x10 film for that amount of money.

Another shallow DoF king would be a Mamiya 645 series camera with a 80mm f1.9 = 42.1. Or put that lens on your digital medium format camera. It still is the fastest medium format lens. It's a manual lens, so you'd lose autofocus. As far as I know, there is no faster than f2.8 autofocus medium format lens on the market. Expect to spend 700 euro, but watch out for mold and haze.

Getting a f1.2 lens on your full frame gives similar bokeh to the 80mm f1.9, and depending on your camera, that 50mm f1,2 will be costly or very costly. The old Nikons are sweet and at 600 euro less expensive than the comparable old Canons, that may cost north of 10.000 euro. If you going to spend that much, spend it on autofocus rather than rarity value. There are adapters that sit between your mirrorless and your vintage lens, that move it in and out to focus the lens.

Conclusion: Bigger cameras do not help you to get a shallower Depth of Field or more blur, bokeh. Getting a faster lens for a full frame camera, will. There are very fast lenses for full frame, but they cost money or they are vintage manual lenses.

Myth: "Bigger sensors produce less noise" or similar: "Larger film has a higher resolution."

Probably yes, given the same ISO or same noise level in the electronics. But on smaller than 4x5 cameras, this effect is outdone by the speed of the lenses.

Consider a base line of: 35mm full frame, 100 ISO, f2 lens at 1/100th.

On a 6x4.5 or 6x6 this would be equivalent to f2.8, 1/100 at ISO200.

on a 6x7 or 6x8, this would be f4, 1/100 and thus at ISO400

and on a 4x5, your lens would typically be f5.6, 1/100 at ISO800.

Yet the 4x5 would generally be on a tripod, shooting a steady scene, so there is no problem shooting at 5.6, ISO50 at a speed as slow as 1/12.

Shooting a 8x10 keeps the shutter times and lens speed the same, so when only when sizing up from 4x5, the resolution does go up and the noise goes down, under the exact same shooting and development conditions.

In other words: There is no noise benefit from a larger film, unless you put the camera on a tripod, and compensate the slower lens with a slower shutter speed, rather than with a faster film, or use a ridicoulously big camera.

Myth: On a cropsensor, a full frame lens produces a better image


It is true that a lens produces an image disk that is limited in size. On the edges of this circle, the brightness falls of either gradually or suddenly. This is called vignetting. On a smaller sensor, vignetting might not come into play like it does on full frame.

Stopping down the lens usually reduces the vignetting, because it's own barrel is not in view as much as when the lens is fully open, but this depends in the construction of the metalwork around the lens.


In terms of sharpness, some lenses only fall off in sharpness really close to the edge of the image circle, but many gradually fall off. If you put those on a crop sensor camera, you effectively zoom into the image. This enlarges any unsharpness too. So if the lens is only really sharp in the very middle, cropping and zooming, does not give you better corner sharpness than when the lens is on a full frame camera.

So wether this is true, depends on the lens.

Light scattering through the camera

What happens to the light that is projected outsided of the sensor? If that bounces around, it might reduce contrast.

Film is white and matte, but sensors are shiny. How does the lens handle light that is reflected back at it? Where does that light go? I painted insides of several cameras and adapters black with Black 2.0, and this improved contrast, especially if the image circles of the lenses were significantly bigger than the sensor or film.

Myth: Taking a photo of a flat surface like a wall, is a good way of testing the corner sharpness

Well, no, because in such a test, you can not tell just why the corners are not sharp. The plane of focus from the lens, is hardly ever flat. So maybe the corners aren't sharp because you need to focus a bit further to get the wall in focus in the corners (giving up center focus while doing this). So you need to test if this is the matter before declaring the corners are not sharp.

There is an upshot of shooting a curved focal plane in some situations. In a forest, the foliage in the corners are often closer to the camera than the subject in the middle. With a curved plane they are more in focus.

When in a group of people, the person in the middle is usually further away than the people on the sides.

What I am saying, is that flatness of the plane of focus might not be important unless you are in the business of shooting painings for reproductions maybe, or brick walls.

If you want control over this plane, get an RB67 with a lens that has a floating lens element to correct the field curvature for the distance you have focused at.

Many RB67 users have no clue what the floating element dial is for, though, and live happily with the focus plane curved in or out, depending on where they left the dial where they last accidentally touched it.

Myth: "Coatings are for color photography"

The world is in color. If you shoot black and white, you also need all visible colors to render nicely on your sensor or film.

What coatings do, is they delay light a bit. This can be used to make two reflections of the light: on the coating and on the glass the coating is on. If the coating if of the right thickness, these two reflections can cancel each other out. Just like anti-sound in your noise cancelling headphones. This is especially helpful when light falls directly into your lens. Where an uncoated lens would be like a mirror palace, an echo chamber, a coated lens dampens it, reducing flairs.

They can also help with bending each color in the same way, repairing fringing. It helps getting blue and red light in focus at the same plane.

A black and white film benefits from coatings too, with sharper images and more contrast.

Without coatings we'd still be stuck with triplets and tessars, because coatings allowed the design of better lens formulas like planars, and the very complex lenses we can buy today. Those new Sony primes for instance, have so many elements, and correct for so many failures of the older designs, it really is step forward.

That said, I do prefer triplets over tessars, heliars over triplets, and planars over heliars. By this sentence, you can tell, that although lenses are still evolving, I am stuck in time where coatings were simpler.

Myth: "There is a Medium Format Look." If it is not resolution, lack of noise, a shallow depth of field, or flat plane of focus from a planar lens, what is it?

I can think of a few things:

Bigger lenses have less diffraction, allowing for a clearer image, especially when the lens is stopped down.

If we compare our photos to classics: Old film and photographic paper had way more silver in it, and thus a better resolution and a more refined grey scale.

Or is it the photographer that matters? You do not buy a medium format camera and burn through 120 type rolls with only 15, 12 or as little of 10 exposures per roll, if you do not have a reason to like those cameras or the results you are getting. On old cameras, you do not waste film on bad shots like do, and I do, on digital.

Assumption: People who went through the bother of handling a larger format camera, also understand the technicalities of light on the subject and scene better.

And there may be more social aspects, too:

Older cameras influence the subjects differently than new gear does, when they notice the camera.

There may be other cultural cues involved that give the photos taken with medium format cameras a different look. Some cameras are used in different locations, by different people, and are pointed at different people, if at people at all.

Or maybe it really is down to the lenses, but just not as simple as focal length / aperture.

Different lenses have different feelings to them, especially wide open.

Compare an image search on Flickr between a Flexaret, a Rolleiflex, a Hasselblad, an Arax and an RZ67. Each of them have different looks.

But they also have different subjects, different colors, different locations, different people, different moods. What aspects are the results of the lens, and what aspects are geograffically and socially interconnected to a certain type of camera?

Is the medium format look caused technical aspects of the gear, or by the time invested in shooting them?

Do I like the Mamiya look, or do I like what and how people shoot that also happen to have access to big Mamiya gear? Can we ever separate these?

I'm sure there is not a typical medium format look. But different lenses do capture moods differently. One day I will figure out a way to mount a RB67 lenses on a 4x5. Those lenses cover that size royally.