FF shallow DOF question

[HoppyUK]

Not again.

There isn't a calculation to be done. Light from the lens falls on the film plane. A larger sensor collects lots of the image, a smaller sensor collects just the middle bit of it.

Tell me how on earth the light passing through the same lens, aperture and focal distance can be affected so that it's DoF changes depending only on how much of the image you collect. (Please don't give me a link to a DoF calculation thingy.)

The key part you are forgetting is the change in magnification required by the smaller/larger image to deliver a final size print. That is the fundamental basis of all DoF calculations, based on internationally agreed standards - argue with those if you like, but nobody else does.

That's why when you change the size of the sensor, the circle of confusion changes in the calcs. Here you go www.dofmaster.com

 

[hoohaaa]

The key part you are forgetting is the change in magnification required by the smaller/larger image to deliver a final size print. That is the fundamental basis of all DoF calculations, based on internationally agreed standards - argue with those if you like, but nobody else does.

I thought the whole point of this (and the previous thread) was to make sure that anyone trying to learn about this stuff by visiting 'talk basics' doesn't get confused about just exactly what a crop sensor is.

Jelster said that if you take a shot on 2 cameras with different sensors while focal length, aperture and subject distance remains constant you will get a different FoV for each, the DoF on each will also be different. To someone who is just trying to learn and has never even heard of CoC etc etc that statement seems to me to be, at the very least, misleading. To suggest that if everything to do with the lens stays the same and yet the DoF changes just because you only look at the middle bit of it is hardly likely to aid understanding.

If you don't agree then ask yourself what DoF you'd get if you replaced the sensor with a translucent screen and viewed the image on the back of the camera with no sensor there at all. Would it make any difference if you cropped some of this image away? As you crop more of the image away would the out of focus areas magically sharpen up? If you accept what Jelster said is true you would imagine that it would.

I know about agreed standards for DoF and I know that all those DoF calculations use sensor size (although I was particularly grateful to be able check that again). If this was 'talk advanced' I might agree with you but it isn't, it's 'talk basics'.

 

[boyfalldown]

The key part you are forgetting is the change in magnification required by the smaller/larger image to deliver a final size print. That is the fundamental basis of all DoF calculations, based on internationally agreed standards - argue with those if you like, but nobody else does.

That's why when you change the size of the sensor, the circle of confusion changes in the calcs. Here you go www.dofmaster.com

I know about agreed standards for DoF and I know that all those DoF calculations use sensor size (although I was particularly grateful to be able check that again). If this was 'talk advanced' I might agree with you but it isn't, it's 'talk basics'.

get a room you two

 

[craig.walton2]

Wow. Many thanks for all the comments folks. Didn't mean to stir up debate, just clarify the concept. Given to some degree that the people with more knowledge than I can't agree then it gives little hope for myself. I'll just have to put this one to bed and revisit some other time. Thanks all the same though

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