I'll second and third Piers' recommendation of "Dof32.exe" from Jonathan
Sachs. And even PW Pro as an adjunct to PS if you've got a spare $90. If
my brain hadn't been so foggy after writing the DOF piece I'd have
included the link to Dof32.exe as I've done here several times before on
DOF discussions.
One thing that you'll soon pick up in using the app and reading the help
file (go there first) is that Jonathan prefers to use "resolution" as
his sharpness measure instead of Circle of Confusion (CoC). Resolution
is just the inverse. It sounds better and it's a whole lot easier to
write 30 (lines/mm) than 0.0333333333 (1/30)
If we magnify a 35mm image 8.5X to make an 8x10 we divide the resolution
number by that amount for the actual resolution on the print. 30/8.5 is
3.5 lines/mm on the final print or, using its inverse, star images of
0.29mm diameter. Remembering that this whole business is based on the
human visual system, is that sharp enough? Depends on your own eyes,
viewing conditions, etc, etc. If not sharp enough to suit you go to a
higher resolution number like 40 lines/mm. Or, if you do a major crop
of the image, say to 1/2 its height, you'd need to double the resolution
number to 60 or 80 lines/mm to maintain the same DOF as the uncropped print.
One might need to do the same in making a 16x20 instead of an 8x10 but
maybe not. If we stand back farther than normal reading distance to
take in the larger print the normal resolution values of 30-40 lines/mm
for an 8x10 may still be perfectly adequate. But maybe not for pixel
peepers. But if you shoot with a small digital sensor, say with 1.5X or
1.6X crop factors you need to multiply the required resolution values by
the crop factor since those small digital images will be magnified more
to create the final print. If you're using the standard 30 lines/mm for
an 8x10 from 35mm but using a Nikon digital with a 1.5X crop factor you
should be using 30x1.5 = 45 lines/mm in your DOF calculator. The
Minolta A1, for example, has a 4X crop factor relative to 35mm so needs
a resolution value of 30x4 = 120 lines/mm in the calculator. The basic
rule is quite simple. If you decide that you need 3.5 lines/mm
resolution on the final print the resolution on the film or sensor needs
to be 3.5 times whatever the magnification required from the source
image to produce the print image.
For a whole lot more on DOF and discussions of the accuracy or
inaccuracy of assumptions and measures of human visual acuity see Norman
Koren's excellent treatise here:
<http://www.normankoren.com/Tutorials/MTF6.html>
Koren has excellent tutorials on lots of stuff. Recommended but it's
often pretty deep reading. Make sure your brain has been caffeinated
before visiting.
Now for a comment on Moose's excellent comments. Moose pointed out that
there are lots of factors that can enter into apparent sharpness but his
post processing tricks on the ship and chain (I would guess mostly LCE)
are probably worth 10 times more than the difference between any two
lenses or cameras. Canyon rules the roost on low noise and a Canyon 30D
image at ISO 1600 is going to have better quality pixels than the E-500
at ISO 1600. But when the E-500 is used within its ISO comfort zone I
think you'd have a very tough time telling them apart.
Another area of interest is the resolving power of old, manual focus
35mm lenses used on digital cameras, especially smaller, high pixel
density sensors like the E-500. Some older lenses may not be able to
deliver the higher resolution required for very high pixel density.
Chuck Norcutt
Piers Hemy wrote:
> DoF as a topic is one that has been explored here before, and my conclusion
> is that it is a topic unsuited to discussion in a text-only discussion forum
> - so I didn't ignore it, I did my best to avoid it! And not because it's a
> dumb question, but because the way you phrased it suggests that there is a
> simple, objective answer. I think the responses you had already received
> were as good as anybody could give - but then Chuck came up with his
> masterpiece.
>
> I won't try to cap that, but I will offer the following further references:
>
> A Depth of Field calculator from Jonathan Sachs (Picture Window Pro
> developer) http://dl-c.com/Temp/downloads/DOF%20Calculator/Default.htm which
> has the following as the first line of the help file: "Depth of field does
> not depend on the frame size (e.g. 35mm, 6x7cm, digital camera CCD,.)." The
> help file alone is worth the price (OK, it's freeware)
>
> "The Ins and Outs of Focus" a freely downloadable book by Harold Merklinger,
> which goes into some depth (oops) about the topic
> http://www.trenholm.org/hmmerk/TIAOOFe.pdf
>
> And I will also offer my personal suspicion that the best 35mm lenses can
> resolve better than the lens DoF marking convention assumes - as Jonathan
> Sachs says "a standard value of about 30 lines per mm is often used to
> compute depth of field markings on 35mm lenses". The result would be
> apparently shallower DoF than the lens markings indicate - and the DoF
> calculator will allow you to play with the numbers to see the effect.
>
> --
> Piers
>
>
> -----Original Message-----
> From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
> Of Ali Shah
> Sent: 27 April 2007 02:27
> To: olympus@xxxxxxxxxx
> Subject: [OM] Re: DOF question
>
>
> Thanks.
>
> Why is my DOF question being ignored? Is it an overly dumb question? Perhpas
> the simple answer is the smaller sensor which cant absorb as much?!?
>
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