DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
awaldram Forum Pro • Posts: 13,246
Re: DOF and Cropping take 2

“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.

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.

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.

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.

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.

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 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.!

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

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

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.

And even considering DoF equivalence outside academia is fruitless.

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.

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

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