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Math question - sort of



 
 
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  #1  
Old September 23rd 09, 04:47 AM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
Eric Miller[_3_]
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Posts: 11
Default Math question - sort of

I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.
Now I am thinking of getting myself another birding camera and am trying
to figure out if there is a way to think of resolution as effective
focal length versus the 10D. So, for example, if I were to get a 7D at
18 megapixels how would that compare to 10D resolution wise in terms of
what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on
the 10D that it would be rendered on the 7D using the 400mm lens.

Eric Miller
www.dyesscreek.com
  #2  
Old September 23rd 09, 05:17 AM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
David J. Littleboy
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Posts: 2,618
Default Math question - sort of


"Eric Miller" wrote in message
...
I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.
Now I am thinking of getting myself another birding camera
and am trying to figure out if there is a way to think of resolution as
effective focal length versus the 10D. So, for example, if I were to get a
7D at 18 megapixels how would that compare to 10D resolution wise in terms
of what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on the
10D that it would be rendered on the 7D using the 400mm lens.


You could play square root games, but the easiest thing to do is to compare
the vertical (short) direction pixel counts.

The 7D is 3456 pixels in the short direction.
The 10D is 2048 pixels in the short direction.

So the 7D (cropped to 6MP) is like putting a (3456/2046)x (that is, a 1.6x)
TC on your 10D.

The only problem, though, is that nothing's for free. Any infelicities in
your 400mm lens will be magnified by the extremely fine pixel pitch on the
7D. To get pixel sharp images from the 7D, you need to project images onto
the sensor that are "1.6 times better" than the images you are now putting
on your 10D's sensor. That may mean a bigger tripod and not shooting wide
open.

By the way, I think you'll enjoy the 7D. The 5D2 was a noticeable
improvement over the 5D, and the 7D has a few niceties beyond the 5D2; it
should be a great camera.

--
David J. Littleboy
Tokyo, Japan


  #3  
Old September 23rd 09, 05:59 AM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
Paul Furman
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Posts: 7,367
Default Math question - sort of

Eric Miller wrote:
I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.
Now I am thinking of getting myself another birding camera and am trying
to figure out if there is a way to think of resolution as effective
focal length versus the 10D. So, for example, if I were to get a 7D at
18 megapixels how would that compare to 10D resolution wise in terms of
what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on
the 10D that it would be rendered on the 7D using the 400mm lens.


It's all relative, the noise level matters too for how much you can
enlarge but yes it would be useful to have a common terminology for
describing pixel magnification rather than 35mm equivalent FOV. Hmm, I
was thinking macro when I wrote "pixel magnification" but at infinity I
guess it would be pixels/field of view in degrees?

Set the number relative to a 'normal' lens, which coincidentally is very
close to 50 degrees for the diagonal FOV of a 50mm lens on a 35mm
camera. Then let's use 300dpi as a standard metric for enlargement, on
an 8x10 print that's 2400x3000 or 7.2MP. So, a full frame 7.2MP camera
making an acceptable 8x10 print with a normal lens of 50 degrees
diagonal field of view represents the basepoint. Damn, now I've exceeded
my mathematical skills and/or patience and cannot figure out how to set
up a formula to calculate/express this. It would look something like if
you doubled the focal length, the number would be 2 and the number is 1
with the default setup. A 500mm lens on FX would be a 10 and a 10mm FX
fisheye would be -5. I'm not certain what doubling the megapixel count
would do to the number, probably not double it. Can anyone finish my
logic? A chart would be nice g.

--
Paul Furman
www.edgehill.net
www.baynatives.com

all google groups messages filtered due to spam
  #4  
Old September 23rd 09, 07:39 AM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
John McWilliams
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Posts: 6,945
Default Math question - sort of

Eric Miller wrote:
I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.
Now I am thinking of getting myself another birding camera and am trying
to figure out if there is a way to think of resolution as effective
focal length versus the 10D. So, for example, if I were to get a 7D at
18 megapixels how would that compare to 10D resolution wise in terms of
what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on
the 10D that it would be rendered on the 7D using the 400mm lens.


My take is that while you can come up with an approximation of what the
comparables are, so what? You'll still want to maximize sharpness and
detail on each shot, and won't be telling yourself, well, I can stay
back and crop later to get an image I would have had on a previous set up.

Besides the math part, there're different curves for each ISO setting,
as well as available light and so on.

I am thinking of picking up a 7D, and still shoot with my 20 D and 5D.....

--
john mcwilliams
  #5  
Old September 23rd 09, 03:11 PM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
ray
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Posts: 2,278
Default Math question - sort of

On Tue, 22 Sep 2009 22:47:35 -0500, Eric Miller wrote:

I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.


400*1.6 = 640

Now I am thinking of getting myself another birding camera and am trying
to figure out if there is a way to think of resolution as effective
focal length versus the 10D. So, for example, if I were to get a 7D at
18 megapixels how would that compare to 10D resolution wise in terms of
what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on
the 10D that it would be rendered on the 7D using the 400mm lens.


Dunno - what's the resolution of the 10d? But your question will be
answered by taking the square root of the ratio.



Eric Miller
www.dyesscreek.com


  #6  
Old September 23rd 09, 03:29 PM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
Eric Miller
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Posts: 181
Default Math question - sort of


"ray" wrote in message
...
On Tue, 22 Sep 2009 22:47:35 -0500, Eric Miller wrote:

I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.


400*1.6 = 640



Yep, caught that after posting. I mixed up my 1.4x converter with the 1.6x
sensor.

Eric Miller
www.colibrihotsauce.com


  #7  
Old September 23rd 09, 03:33 PM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
Eric Miller
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Posts: 181
Default Math question - sort of


"David J. Littleboy" wrote in message
...

The only problem, though, is that nothing's for free. Any infelicities in
your 400mm lens will be magnified by the extremely fine pixel pitch on the
7D. To get pixel sharp images from the 7D, you need to project images onto
the sensor that are "1.6 times better" than the images you are now putting
on your 10D's sensor. That may mean a bigger tripod and not shooting wide
open.


I don't think that my 10D was pushing up against the resolution limit of my
400 f/5.6L and while I may not realize the all the resolving potential with
that lens on the 7D, I was more interested in the difference in potential
resolution, the conversion puzzle and not so much the practicality, i.e.,
don't spoil my fun!

Eric Miller
www.colibrihotsauce.com


  #8  
Old September 23rd 09, 04:42 PM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
No One[_3_]
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Posts: 20
Default Math question - sort of


"Eric Miller" wrote in message
...
I went from the 10D to the 5D. When I had my 10D, I learned to like the
1.6x way of fooling myself into thinking my 400mm lens was a 560mm lens.
Now I am thinking of getting myself another birding camera and am trying to
figure out if there is a way to think of resolution as effective focal
length versus the 10D. So, for example, if I were to get a 7D at 18
megapixels how would that compare to 10D resolution wise in terms of what
focal length lens would I have had to put on the 10D to get a 5 inch tall
bird at 20 meters (or any distance) to be rendered by the same number of
pixels (one dimension only or my head will hurt too much) on the 10D that
it would be rendered on the 7D using the 400mm lens.

Eric Miller
www.dyesscreek.com


Both the 10D and the 7D are APS-C sized sensors, so your 400mm lens will
have a 1.6 FOV crop, so both will be similar to a 640mm (not 560mm). I never
work in pixel pitch in photography.


  #9  
Old September 23rd 09, 06:22 PM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
Andrey Tarasevich
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Posts: 70
Default Math question - sort of

Eric Miller wrote:
So, for example, if I were to get a 7D at
18 megapixels how would that compare to 10D resolution wise in terms of
what focal length lens would I have had to put on the 10D to get a 5
inch tall bird at 20 meters (or any distance) to be rendered by the same
number of pixels (one dimension only or my head will hurt too much) on
the 10D that it would be rendered on the 7D using the 400mm lens.


If we assume that sensors in both cameras have the same physical size
(and they basically do), then all we need to know is the size of each
sensor in physical pixels. I don't know the numbers for 7D, but we can
just assume a square pixel (for both sensors) and use the square root of
the total physical pixel count instead

10D has a 6.5Mpix sensor. Sqrt(6.5) = 2.55
7D has a 19Mpix sensor, Sqrt(19) = 4.36

The above immediately means that given the same bird and fixed distance
to the bird (as you requested in your message), you'd have to increase
the focal length of 10D lens by a factor of 4.36/2.55 = 1.7. If you are
using a 400mm lens on 7D, the 10D would require a 680mm lens.
  #10  
Old September 23rd 09, 06:32 PM posted to rec.photo.equipment.35mm,rec.photo.digital,rec.photo.digital.slr-systems
John McWilliams
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Posts: 6,945
Default Math question - sort of

Andrey Tarasevich wrote:
Eric Miller wrote:
So, for example, if I were to get a 7D at 18 megapixels how would that
compare to 10D resolution wise in terms of what focal length lens
would I have had to put on the 10D to get a 5 inch tall bird at 20
meters (or any distance) to be rendered by the same number of pixels
(one dimension only or my head will hurt too much) on the 10D that it
would be rendered on the 7D using the 400mm lens.


If we assume that sensors in both cameras have the same physical size
(and they basically do), then all we need to know is the size of each
sensor in physical pixels. I don't know the numbers for 7D, but we can
just assume a square pixel (for both sensors) and use the square root of
the total physical pixel count instead

10D has a 6.5Mpix sensor. Sqrt(6.5) = 2.55
7D has a 19Mpix sensor, Sqrt(19) = 4.36

The above immediately means that given the same bird and fixed distance
to the bird (as you requested in your message), you'd have to increase
the focal length of 10D lens by a factor of 4.36/2.55 = 1.7. If you are
using a 400mm lens on 7D, the 10D would require a 680mm lens.


It's not just about the maths. Way too many other factors affecting IQ,
and I suspect most photographers will soon forget about how equivalent
one framing on one camera is to another body he's used. He'll go for the
highest IQ he can in the moment.

--
john mcwilliams

I know that you believe you understood what you think I said, but I'm
not sure you realize that what you heard is not what I meant.
 




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