DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Ian Stuart Forsyth
OP Ian Stuart Forsyth Veteran Member • Posts: 3,380
Re: DOF and Cropping take 2

awaldram wrote:

“As sensor size increases, the depth of field will decrease for a given aperture (when filling the frame with a subject of the same size and distance). This is because larger sensors require one to get closer to their subject, or to use a longer focal length in order to fill the frame with that subject. This means that one has to use progressively smaller aperture sizes in order to maintain the same depth of field on larger sensors. The following calculator predicts the required aperture and focal length in order to achieve the same depth of field (while maintaining perspective).

This nothing different to what I've been saying or Cambridge in color for that matter if your using the longer focal length to deliver greater magnification on larger sensors the the DoF will alter i.e you will need to shrink the aperture.

But doing this is not giving you any advantage and will come at a cost to ISO and/or shutter speed.

Of course you have to go to a higher 225 iso if you want to have the same DOF but FF has the advantage that it has a better SNR and can give it up and go to a higher iso without losing anything and hold the same SNR as a cropped sensor at iso 100

Sensor size is nothing but the expected viewing size in reverse.


If you concern is to maintain perceived DoF at he same viewing angle then of cause you need to alter focal length and aperture and take the hit in ISO and shutter.


Of course you have to go to a higher 225 iso if you want to have the same DOF but FF has the advantage that it has a better SNR and can give it up and go to a higher iso without losing anything and hold the same SNR as a cropped sensor at iso 100

(with the addition) If I take a hit on iso then my shutter speed will be the same so I can still freeze the subject as a cropped camera would under the same conditions. This is why we say FF 300 F4.2

will share the same DOF and FOV and total light ( Same noise level) as a Cropped with a FL of 200 F2.8

Most people myself included could not give to hoots about DoF as its to narrow on aps-C and even worse on FF, So I care about shutter speed and target size not DoF.

i.e Fast shutter, plenty of reach and bags of sharp area controllable noise

they are mutually exclusive and you can do very little to gain anything

I could use a 5.5 crop camera this would give me loads of reach, massive DoF (shorter focal length for same target size) but I'd still be shooting in the iso1600 range and the sensor noise can't cope.

because with the great DOF less light will falls on the 5.5 crop thus more noise. I f you where to get into the DOF shooting with your cropped 1.5 camera that a compact can achieve the same DOF then many times the compact can give you better IQ than the cropped 1.5 camera as some of those compact cameras have QE over 65%

So how about FF, this would be me issues with DoF , focal lengths requiring faster shutter than the target required leading to higher ISO than the ambient light indicated driving up ISO beyond the possible stop gained by the format.

A FF with the same FOV as a Cropped camera's FOV would require the same shutter speed on both formats to freeze the subject & iso 100 on cropped would give us the same noise as iso 225 on FF so you lose nothing

APS-C allows me to shoot at subject stopping speeds which are also-in the ball park 1/x for camera induced blur whilst maintain an ISO in 1600-3200 range giving noise that Ir can cope with.


q = 50mm (f-stop irrelevant) have to shoot >1/250 to freeze subject so in in 800-1600 range noise uncontrollable.

k3 = 300mm F4 shutter 1/250ish iso 1600-3200

FF = 450mm (have to shoot F5.6 for cost and DoF) shutter has to be >1/400 for consistent non camera blur this pushes iso into the 128,000 -256,000 range resulting in a poorer IQ than the aps-C will deliver.

As stated above if you need a SS of 1/250 to freeze motion on a cropped then the FF at 1/250 will freeze the same image movement

Cropped FOV 300 x1.5=FOV of a FF 450 mm Same FOV same shutter speed to freeze the same object as it moves the same size of FOV framing.

so with FF shooting at 450mm at F5.6 1/250 3200-7200 will give you the same noise levels to work with because they capture the same amount of light. If you don't believe me look up the SNR of a FF at iso3200 to a cropped at iso 1600

I find it kind of funny how you for your k3 state that all you need is a SS 1/250 but when moving over to FF you now included that need more shutter speed to make for camera shake, you know most lenses tele's in production have IS built into them ?

Everything is a balancing act and what works for one may not work for another, In very bright light I can see Q type sensor delivering better Images than either FF or Aps-C due to its reach.

Yes a Q with a 100mm F2.8 (5.64 crop factor) can give you pretty good IQ but how often do people with a FF shoot at 560mm F 16 or a cropped at 400mm F11 we are in the territory that diffraction starts to hamper IQ and not to mention for the Q and the 100 mm F2.8 would have to 5.64 times sharp than a FF equivalent 600 or 3.7 times sharper than cropped 400mm lens. You have to also remember that lens is wide open( at its worst for IQ) just to achieve the F16 ff equivalent .

Yes if could have FF with a 200-400 f4 or the 400 f2.8 then yes I could realize the FF Dream but I don't have £10,000 to cover that aspect of my hobby.!

A FF 400mm F5.6 is in the same ball park as for cost and weight as a 300 F4 any guesses why?

Of cause of Blurred backgrounds are all you care about then MF is the better option.

not really as I don't see anything in production that pentax can give us the DOF as a FF 24 F1.4, 28 F1.8 35mm F1.4 50mm F1.4, 50mm F 1.2 or 85mm F1.4

To achieve the same blur a portrait can deliver in MF on a FF camera if you were shooting a 300 f2.8 on MF

135mm f1.2

nothing pentax produces can do this

For a pentax 645 d with 300 F2.8 all it would take is a FF 200 F 2 lens that is in production

and if you were using a 135 f4 land camera (8x10) to get the same shots you need a 14mm f.4 on a Nikon FF body.

Hence my opinion that basing purchasing decisions on DoF equivalence is doomed to despair.

Not really when you look at what equivalent lenses between FF and crop most of the time they are at the same price point , IQ and weight

And even considering DoF equivalence outside academia is fruitless.

Not when you look at the versatility FF has to offer.

The only equivalence that makes some sense today is FoV where it can be used to roughly understand what magnification factor you might perceive based off something you know.

and what does this mean? with magnification comes light loose

Given proliferation of small sensor image taking devices even that is becoming pointless.

Not really now that FF body is becoming cheaper to afford and updated with the latest technology  as a cropped sensor.

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