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A question for the Gurus
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PostPosted: Mon Nov 22, 2010 4:33 am    Post subject: A question for the Gurus Reply with quote
Conventional wisdom has it that the longer the focal length, the shallower the depth of field: thus on a 35mm body, the DOF for a 28mm lens (at any given aperture) is greater than for a 200mm lens (at the same aperture).

OK, how does this translate to medium format lenses?

That is, for example, does a 200mm focal length (35mm) lens have a shallower DOF than a 200mm (MF 6x7) lens, or do both have the same DOF? Question


PostPosted: Mon Nov 22, 2010 4:35 am    Post subject: Reply with quote
http://www.dofmaster.com/dofjs.html


PostPosted: Mon Nov 22, 2010 5:47 am    Post subject: Re: A question for the Gurus Reply with quote
Banjo wrote:
That is, for example, does a 200mm focal length (35mm) lens have a shallower DOF than a 200mm (MF 6x7) lens, or do both have the same DOF? Question


If you use them from exactly the same distance then you will get the EXACT same image with the same DOF.

Last edited by jjphoto on Sun Mar 10, 2013 8:55 am; edited 1 time in total


PostPosted: Mon Nov 22, 2010 7:41 am    Post subject: Reply with quote
In other words: a fullframe lens on an APS-sized dSLR gives a crop of the actual image. Parts of the actual image fall outside your sensor.
Because of the crop, the image of the APS-sized dSLR looks like it's taken with a longer focal lenght while the DOF remains the same.
To take exactly the same picture (without these crops) with an APS-sized dSLR as with the fullframe camera, you'll have to use a lens with a shorter focal length. This will result in a deeper DOF.
This explains why digital pocket camera have such a huge DOF. Because of their super small sensors, they have to use super small focal lengths, resulting in huge DOF.

Last edited by caspert79 on Mon Nov 22, 2010 5:57 pm; edited 1 time in total


PostPosted: Mon Nov 22, 2010 1:25 pm    Post subject: Re: A question for the Gurus Reply with quote
Banjo wrote:
That is, for example, does a 200mm focal length (35mm) lens have a shallower DOF than a 200mm (MF 6x7) lens, or do both have the same DOF? Question


DOF is a property of the image. It is not a property of the lens.

If you visit the link

http://www.dofmaster.com/dofjs.html

provided in the second post you will find that, in order to calculate the DOF of an image taken in a given situation, you are required to enter four parameters :

1. Lens focal length
2. Lens aperture
3. Subject distance
4. Camera format or Circle of Confusion (CoC)

The first three are easy enough and are objective measures. For the last, if you enter a particular camera, or format, the calculator will assign a particular value for the CoC. This is arbitrary and subjective.

Wikipedia will provide you with a description of CoC and also the equations used in the calculator.

Even if you don't wish to explore the detailed maths, it might be worth playing around with the calculator, keeping, in turn, three of the parameters constant and varying the fourth.


PostPosted: Mon Nov 22, 2010 4:36 pm    Post subject: Re: A question for the Gurus Reply with quote
It is actually quite a complex question as the circle of confusion depends on the final magnification factor (of he print, projection, screen view or whatever).

A 100mm 6x9 medium format lens has the same depth of field at f/2.8 as a 35mm lens provided that the MF image is printed to 60x90cm and the small format image to 24x36cm, and both are viewed from the same distance. If both are enlarged to the same print size, the small negative has much less depth of field (being in essence a factor 3 crop).

On the other hand, the relationship between focal length and negative size scales proportionally for equal angles and identical final result. So if we compare normal (42mm@24x36mm) to normal (100mm@6x9cm) for the same print size and the same CoC on the final print the former has twice the DOF.

To confuse matters (and some explanations), this also means that the DOF is exactly the same for an aperture of same absolute size (i.e. in mm radius rather than relative to focal length).


PostPosted: Mon Nov 22, 2010 6:43 pm    Post subject: Re: A question for the Gurus Reply with quote
Sevo wrote:

To confuse matters (and some explanations), this also means that the DOF is exactly the same for an aperture of same absolute size (i.e. in mm radius rather than relative to focal length).


Does this mean that a 200 mm lens at f/4 (aperture diameter 50 mm), for example, gives the same DOF as a 100 mm lens at f/2 (aperture diameter 50 mm) ?
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