FX versus DX

It amazes me that people still don’t understand the relationship between FX and DX format sensors.

Millions of people across the planet think still that when they put a DX body on an FX lens and turn the camera on, something magic happens and the lens somehow becomes a different beast.

NO…it doesn’t!

There is so much crap out there on the web, resulting in the blind being led by the stupid – and that is a hardcore fact.  Some of the ‘stuff’ I get sent links to on the likes of ‘diaper review’ (to coin Ken W.’s name for it) and others, leaves me totally aghast at the number of fallacies that are being promoted and perpetuated within the content of these high-traffic websites.

FFS – this has GOT to STOP.

Fallacy 1.  Using a DX crop sensor gives me more magnification.

Oh no it doesn’t!

If we arm an FX and a DX body with identical lenses, let’s say 500mm f4’s, and go and take the same picture, at the same time and from the same subject distance with both setups, we get the following images:

FX versus DX: FX + 500m f4 image – 36mm x 24mm frame area FoV

FX versus DX: DX + 500mm f4 image – 24mm x 16mm frame area FoV

FX versus DX: With both cameras at the same distance from the subject, the Field of View of the DX body+500mm f4 combo is SMALLER – but the subject is EXACTLY the SAME SIZE.

Let’s overlay the two images:

FX versus DX: The DX field of view (FoV) is indicated by the black line. HOWEVER, this line only denotes the FoV area. It should NOT be taken as indicative of pixel dimensions.

The subject APPEARS larger in the DX frame because the frame FoV is SMALLER than that of the FX frame.

But I will say it again – the subject is THE SAME DAMN SIZE.  Any FX lens projects an image onto the focal plane that is THE SAME SIZE irrespective of whether the sensor is FX or DX – end of story.

Note: If such a thing existed, a 333mm prime on a DX crop body would give us the same COMPOSITION, at the same subject distance, as our 500mm prime on the FX body.  But at the same aperture and distance, this fictitious 333mm lens would give us MORE DoF due to it being a shorter focal length.

Fallacy 2.  Using a DX crop sensor gives me more Depth of Field for any given aperture.

The other common variant of this fallacy is:

Using a DX crop sensor gives me less Depth of Field for any given aperture.

Oh no it doesn’t – not in either case!

Understand this people – depth of field is, as we all know, governed by the aperture diaphragm – in other words the f number.  Now everyone understands this, surely to God.

But here’s the thing – where’s the damn aperture diaphragm?  Inside the LENS – not the camera!

Depth of field is REAL or TRUE focal length, aperture and subject distance dependent, so our two identical 500mm f4 lenses at say 30 meters subject distance and f8 are going to yield the same DoF.  That’s irrespective of the physical dimensions of the sensor – be they 36mm x 24mm, or 24mm x 16mm.

But, in order for the FX setup to obtain the same COMPOSITION as that of the DX, the FX setup will need to be CLOSER to the subject – and so using the same f number/aperture value will yield an image with LESS DoF than that of the DX, because DoF decreases with decreased distance, for any given f number.

To obtain the same COMPOSITION with the DX as that of the FX, then said DX camera would need to move further away from the subject.  Therefore the same aperture value would yield MORE DoF, because DoF increases with increased distance, for any given f number.

The DX format does NOT change DoF, it’s the pixel pitch/CoC that alters the total DoF in the final image.  In other words it’s total megapixels the alters DoF, and that applies evenly across FX and DX.

Fallacy 3.  An FX format sensor sees more light, or lets more light in, giving me more exposure because it’s a bigger ‘eye’ on the scene.

Oh no it doesn’t!

Now this crap really annoys the hell out of me.

Exposure has nothing to do with sensor size WHAT SO EVER.  The intensity of light falling onto the focal plane is THE SAME, irrespective of sensor size.  Exposure is a function of Intensity x Time, and so for the same intensity (aperture) and time (shutter speed) the resulting exposure will be the SAME.  Total exposure is per unit area, NOT volume.

It’s the buckets full of rain water again:

The level of water in each bucket is the same, and represents total exposure.  There is no difference in exposure between sensor sizes.

There is a huge difference in volume, but your sensor does not work on total volume – it works per unit area.  Each and every square millimeter, or square micron, of the focal plane sees the same exposure from the image projected into it by the lens, irrespective of the dimensions of the sensor.

The smallest unit area of the sensor is a photosite. And each photosite recieves the same said exposure value, no matter how big the sensor they are embedded in is.

HOWEVER, it is how those individual photosites COPE with that exposure that makes the difference. And that leads us neatly on to the next fallacy.

Fallacy 4.  FX format sensors have better image quality because they are bigger.

Oh no they don’t – well, not because they are just bigger !

It’s all to do with pixel pitch, and pixel pitch governs VOLUME.

FX format sensors usually give a better image because their photosites have a larger diameter, or pitch. You should read HERE  and HERE for more detail.

Larger photosites don’t really ‘see’ more light during an exposure than small ones, but because they are larger, each one has a better potential signal to noise ratio.  This can, turn, allow for greater subtle variation in recorded light values amongst other things, such as low light response.  Think of a photosite as an eyeball, then think of all the animals that mess around in the dark – they all have big eyes!

That’s not the most technological statement I’ve ever made, but it’s fact, and makes for a good analogy at this point.

Everyone wants a camera sensor that sees in the dark, generates zero noise at ISO 1 Million, has zero diffraction at f22, and has twice the resolution of £35Ks worth medium format back.

Well kids, I hate to break it to you, but such a beast does not exist, and nor will it for many a year to come.

The whole FX versus DX format  ‘thing’ is really a meaningless argument, and the DX format has no advantage over the FX format apart from less weight and lower price (perhaps).

Yes, if we shoot a DX format camera using an FX lens we get the ‘illusion’ of a magnified subject – but that’s all it is – an illusion.

Yes, if we shoot the same shot on a 20Mp FX and crop it to look like the shot from a 20Mp DX, then the subject in the DX shot will have twice as many pixels in it, because of the higher translational density – but at what cost.

Cramming more mega pixels into either a 36mm x 24mm or 24mm x 16mm area results in one thing only – smaller photosites.  Smaller photosites come with one single benefit – greater detail resolution.  Every other attribute that comes with smaller photosites is a negative one:

• Greater susceptibility to subject motion blur – the bane of landscape and astro photographers.
• Greater susceptibility to diffraction due to lower CoC.
• Lower CoC also reduces DoF.
• Lower signal to noise ratio and poorer high ISO performance.

Note: Quite timely this! With the new leaked info about the D850, we see it’s supposed to have a BSI sensor.  This makes it impossible to make a comparison between it and the D500, even though the photosites are nearly pretty much the same size/pitch.  Any comparison is made even more impossible with the different micro-lens tech sported by the D850.  Also, the functionality of the ADC/SNR firmware is bound to be different from the D500 too.

Variations in: AA filter type/properties and micro lens design, wiring substrate thickness, AF system algorithms and performance, ADC/SNR and other things, all go towards making FX versus DX comparisons difficult, because we use our final output images to draw our conclusions; and they are effected by all of the above.

But facts are facts – DX does not generate either greater magnification or greater/less depth of field than FX when used with identical FX lenses at the same distance and aperture.

Sensor format effects nothing other than FoV,  everything else is purely down to pixel pitch.

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