DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
awaldram Forum Pro • Posts: 13,126
Re: DOF and Cropping take 2

moving_comfort wrote:

• Using the same lens on a APS-C crop sensor camera and a 35mm full frame body, the APS-C crop sensor camera image has 1.5x LESS depth of field than the 35mm image would have

The answer in NO it doesn't, the image created on the aps-c sensor is identical to the middle 66.6% of the FF sensor.

With an identical lens same F -stop and same registration the images painted onto the film plane is identical therefore the DoF is identical , DoF is a function of the Lens.

Perceived DoF varies because you alter the viewing distance (something that happens after your above statement)....i.e CoC is altered. i.e you will magnify the aps-c image 1.5 times therefore reducing CoC and increases apparent DoF.

Turn it all round look at  it this way

if you want maintain image size between formats you need to optically magnify the larger sensor by the crop factor of the smaller sensor.

If you use a larger magnifying lens (say for aps-c 200 ff 300) you have reduced the physical aperture by reducing the entry pupil... ergo your DoF will reduce by 1.5.

So whether you use 1.5x optical magnification or 1.5x digital enlargement the result is the same CoC reduces DoF changes.

Hence why in my first statement which you took such exception to and spawned this thread  about DoF and cropping I altered the viewing distance and crop at the same time to ensure it was correct.

But your statement above is 100% wrong became its incomplete and as it stands inferes a 300mm F2.8 lens causes a different DoF depending on how big a proportion of its projection you take and you have categorically stated that is true which it obviously isn;t and has to break the laws of physics to be so.

You may as well tell me you believe in fairies.!

The danger is your statement will lead others to believe in fairies as well.

So lets be clear a 300 F2.8 on FF is exactly the same as a 300 f2.8 on aps-c shutter iso and f-stop will be the same for any given exposure.

DoF captured will be the same but because you will veiw the ff image 1.5 time closer it will appear to have less DoF.

You can easily prove this by taking a magnifying glass to an image and note where the DoF extends vs without it.

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