Why your f/1.8 background blur looks different from the tutorial

Quick answer: The f-number tells you how bright the exposure will be. Background blur depends on the physical size of the lens opening, how far you are from the subject, and how far the background sits behind it. To copy a tutorial’s look on a smaller sensor, divide the f-number by your crop factor (or buy a longer lens and step closer).

What you need and how long it takes

You need any interchangeable-lens camera with an aperture-priority mode (A on the dial for most brands, Av on Canon) and one lens, ideally a prime. A zoom works too if it holds a constant aperture or you note the focal length each time.

Plan on 30 minutes of practice in a hallway or garden. The reasoning takes five minutes to read; the habit of checking three distances before you pick an aperture takes a few sessions to stick.

Why the f-number misleads

An f-number is a ratio. It’s the focal length divided by the diameter of the opening, so a 50mm lens at f/2 has an opening about 25 mm across. The ratio keeps exposure consistent between lenses, which is its job.

Cody Mitchell, writing for Fstoppers, runs the comparison across formats. On Micro Four Thirds you’d use a 25mm lens for a similar field of view, and at f/2 its opening is only about 12.5 mm. Same f-number, half the opening, and a deeper zone of sharpness.

The same logic explains phones. Mitchell starts from an iPhone with an f/1.8 lens, which produces nothing like the shallow depth of field people expect from f/1.8 on a larger camera.

The five factors, in the order to check them

Digital Photography School lists aperture, subject distance and focal length as the three main factors. Sensor size changes how those three translate between cameras, and the distance to the background completes the set. Here’s the order that saves the most time on location.

Distance to the background. This is the cheapest lever you have. Walking your subject away from the wall costs nothing and changes the result more than a stop of aperture often does. James Tan’s examples hold the camera-to-subject distance, f/2.8 and 70mm constant, and change only the subject-to-background gap.
Distance to the subject. The shorter that distance, the smaller the depth of field. Tutsplus gives a rough illustration: at 2 metres, f/2.8 and 50mm you may get about 10 cm of depth; at 10 metres the same lens and aperture may give 100 cm.
Focal length. The longer the focal length, the shallower the depth of field, if framing distance is held constant. If you step back to keep the same framing with a longer lens, much of the advantage shifts to background compression and a larger opening.
Aperture. Set it last. In A or Av mode, rotate the control dial until you reach the f-number your sensor format needs (table below), then check the shutter speed the camera chooses.
Sensor format. Multiply or divide using the crop factor. DPS notes that a multiplier gets applied to the aperture on APS-C and Micro Four Thirds, just as it does to focal length.

Converting between formats

Crop factor is the full frame diagonal of 43.3 mm divided by the sensor’s diagonal. Multiply a focal length by it to find the full-frame lens with the same framing, and multiply the f-number by it to find the aperture with the same depth of field.

Exposure doesn’t move. f/2 is as bright on every sensor, so shutter speed and ISO stay the same. Only the look changes.

Crop factor by sensor format (full frame = 1)

Wikipedia’s worked example for the Panasonic Leica Nocticron 42.5mm f/1.2 shows the full set of equivalents, including the ISO you’d need for the same exposure time.

Format	Focal length for same framing (mm)	f-number for same depth of field	ISO for same exposure time
Micro Four Thirds	42.5	1.2	200
APS-C	57	1.6	360
Full frame	85	2.4	800

Read it as a trade. With those parameters, all three systems give the same angle of view, depth of field, diffraction limitation and motion blur. The larger sensor gets there at a higher ISO, which is where its cleaner high-ISO files come in.

Settings by situation

Treat these as starting points to adjust after a test frame. The equivalence figures come from the sources above; the rest are conventional portrait and close-up habits, not measurements.

Situation	Lens and aperture	Distance rule	Mode and notes
Head-and-shoulders portrait, full frame	85mm, f/2 to f/2.8	Subject well clear of the background	Aperture priority; adjust ISO to keep shutter speed sensible
Same look, APS-C (1.5x)	Around 57mm, f/1.6 (the equivalent of 85mm at f/2.4 on full frame)	Same as above	If you only have f/1.8, raise subject-to-background distance
Same look, Micro Four Thirds	Around 42.5mm, f/1.2 for the f/2.4 full-frame look	Same as above	Fast primes help most here; consider f/1.8 as a middle path
Phone, main camera	Fixed lens	Move very close; put the background far away	Portrait mode simulates the blur in software
Group portrait	Stop down from f/2.8 towards f/5.6	Keep everyone on the same plane	Full frame; check faces at the edges on the rear screen
Close-up of a small object	Stop down from wide open	Close range already shrinks depth of field	Aperture alone can’t rescue a near-macro distance

The “f/5.6 is the middle ground” trap

One common tip calls f/5.6 a comfortable compromise between soft background and everything sharp. Mitchell points out that it holds on full frame, but on Micro Four Thirds the same f/5.6 lands near f/11 in full-frame terms and gives a very deep depth of field. A tutorial’s “safe” aperture is calibrated to the author’s sensor.

Check the metadata of any example photo you’re copying. Camera model, focal length and aperture usually appear in the EXIF panel on the photo’s page, and from those three you can convert.

Common mistakes and fixes

Copying the f-number and ignoring the camera

The fix is the conversion above. Take the tutorial’s full-frame aperture, divide by your crop factor, and see whether any of your lenses gets there. If none does, use the other levers: longer focal length, closer subject, more distance behind them.

Expecting a phone to match a full-frame f/1.8

Photography Life states it plainly: cameras with smaller sensors have more depth of field, while larger sensors tend to offer more shallow focus. Software portrait modes exist to cover that gap, with the usual caveat of imperfect edge masks.

Shooting wide open to a flat wall behind the subject

If the subject stands near a wall, even f/1.4 leaves the background in focus-ish detail. Move the subject forward or change your angle so the backdrop sits further away.

Moving closer and wondering why the nose is soft

DPS warns that very close to a subject, depth of field shrinks regardless of aperture. At portrait distances that can mean an eye sharp and an ear soft. Stop down a stop, or step back and use a longer lens.

Assuming a lens is soft wide open when it’s the depth of field

Some lenses do lose sharpness at maximum aperture, as one photographer’s guide notes. The suggested check is to shoot the same scene at several apertures and compare. Run that test before blaming the lens for a missed focus plane.

What this means for you

If you shoot full frame and follow a tutorial made on full frame, copy the numbers directly. If you shoot APS-C, divide the tutorial’s f-number by 1.5 (or 1.6 on Canon). On Micro Four Thirds, divide by 2. Then check whether any lens you own can reach that value; if not, plan the shot around subject distance and background distance instead.

For a quick calibration session, pick one lens and one subject. Take the same portrait at a 1 m, 2 m and 3 m gap between subject and background, all at the same aperture, and compare them on a computer screen. Then take the same shot at two apertures from one spot. You’ll see which lever matters more for your usual setup, and it’s often the gap behind the subject.

Phone shooters should treat portrait mode as a stylistic tool and keep the real optical blur for the camera with the larger lens. For product and macro work, expect to stop down, and consider focus stacking if one frame can’t cover the subject.