Nikon lens compatibility...

(John Francis) writes:

In article ,
Bruce Murphy wrote:

There is a lot more to the optics of a lens than just an arbitrarily
chosen area to focus it onto. Depth of field being the main difference -
it will not be the same if you put a 300mm on an F5 and a 200mm on a D100
(same apertures of course).

Yes, obviously. Do you know what it /will/ be the same with?

Actually, it *will* be the same, if the final print is the same size.
And it will be the same if you put that 200mm, or a 50mm, on the F5,
take a shot *from the same place*, and make a print from the cropped
portion of the negative.

The extra enlargement you need to do to fill the same area of the
print from a smaller portion of the negative decreases depth-of-field
(i.e. increases the size of the circle of confusion) by exactly the
same amount as switching to the longer focal-length lens.

But what it /doesn't/ do is overcome the difference in depth of field
you get because your aperture is physically 1.5 times smaller. (in the
200/300 case)

Now, if you keep the physical aperture size (in mm or any other length
unit) the same, then that's fine. In which case you're going to end up
with different exposures.

Informally, I'm sure everyone is aware of this effect; an image that
looks sharp at 200 pixels wide can look soft when viewed at 400 pixels.

This is a red herring.

B

John Francis writes:
Peter Hirons wrote:
John Francis wrote:

There is a lot more to the optics of a lens than just an
arbitrarily chosen area to focus it onto. Depth of field being
the main difference - it will not be the same if you put a 300mm
on an F5 and a 200mm on a D100 (same apertures of course).

Are you sure about that?

Absolutely! Search the web for "depth of field calculation" and do
some reading.

Well, when you do find the time, I suggest that you might try to
explain it - to yourself, for a start. Don't forget to take the
multiplication factor during printing into account - that affects
the size of the circle of confusion. you can't use the same size
circle on the two cameras.

That is correct - but it doesn't give the result you seem to think it
does. Todd Walker has a very good illustrations of what really hapens
on this page:

http://www.toddwalker.net/doftest/

It shows two near identical shots taken with the same FOV and same
aperture with a Canon G1 (4.8c crop) and a Canon 10D (1.6x crop).
It is pretty obvious that the DOF in the tow images is not the same,
but that the FOV are.

Check your calculations, and be prepared for a surprise.

Been there, done that (my calculations are based on the DOF formula
in Allen R. Greenleaf: Photographic Optics, MacMillan, New York, 1950).

The results are plotted in two graphs on this page:

http://heim.ifi.uio.no/~gisle/photo/dof.html

Graph 1 just confirms what is illustrated by Todd Walkers pair of
images. This is actually a well known fact - and one of the
reason people that want shallow DOF don't like "cropped" sensors.

Now, graph 2 on the same page show that if we keep the focal length
constant, DOF actually decreases when we move to smaller sensor
diameters. That is caused by the effect of the magification factor
you are talking about - but it has less effect than you think.
--
- gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
================================================== ======================
When you say you live in the real world, which one are you referring to?

John Francis writes:
Peter Hirons wrote:
John Francis wrote:

There is a lot more to the optics of a lens than just an
arbitrarily chosen area to focus it onto. Depth of field being
the main difference - it will not be the same if you put a 300mm
on an F5 and a 200mm on a D100 (same apertures of course).

Are you sure about that?

Absolutely! Search the web for "depth of field calculation" and do
some reading.

Well, when you do find the time, I suggest that you might try to
explain it - to yourself, for a start. Don't forget to take the
multiplication factor during printing into account - that affects
the size of the circle of confusion. you can't use the same size
circle on the two cameras.

That is correct - but it doesn't give the result you seem to think it
does. Todd Walker has a very good illustrations of what really
happens on this page:

http://www.toddwalker.net/doftest/

It shows two near identical shots taken with the same FOV and same
aperture with a Canon G1 (4.8x crop) and a Canon 10D (1.6x crop).
It is pretty obvious that the DOF in the two images is not the same,
but that the FOV is.

Check your calculations, and be prepared for a surprise.

Been there, done that. The results are plotted in two graphs on
this webpage:

http://heim.ifi.uio.no/~gisle/photo/dof.html

Graph 1 just confirms what is illustrated by Todd Walkers pair of
images. (This is actually a well known fact - and one of the
reason people that want a shallow DOF for focus control don't
like "cropped" sensors.)

Now, graph 2 on the same page show that if we keep the focal length
constant, DOF decreases when we move to smaller sensor diameters.
That is caused by the effect of the magification factor you are
talking about - but it has less effect than you think.
--
- gisle hannemyr [ gisle{at}hannemyr.no - http://folk.uio.no/gisle/ ]
================================================== ======================
When you say you live in the real world, which one are you referring to?