# How to Calculate Telephoto Effect With Digital SLR's?

Discussion in 'Digital Photography' started by Mardon, Apr 24, 2004.

1. ### MardonGuest

Can someone explain to me how to calculate the amount of telephoto effect
that will occur when using different focal length lenses, in combination
with different digital camera resolutions?

Here's my problem. I'm planning to upgrade from a Canon PowerShot G2 to one
of the new Canon EOS 1D Mark II digital SLRs but I would like to know in
advance, what focal length lens will be required on the 1D Mark II in order
to achieve the amount of telephoto effect that I require.

For example, I have photographed a distant subject using my G2, set to full
optical zoom, maximum resolution and with a 1.5x tele-extender lens
installed. The subject appears in the resulting image as an object 500
pixels wide. If I were to photograph that same subject, from the same
location, using the EOS 1D Mark II, at its maximum resolution, what focal
length lens would I have to use in order to have the subject appear as an
object 2,000 pixels wide in the resulting image?

Canon PowerShot G2 Info:
Optical Zoom: 3x
Focal length at 3x optical zoom: 21mm (35mm-equivalent: 102mm )
Tele-extender Accessory Lens: 1.5x
Maximum non-interpolated pixel count: 2,272 x 1,704

Canon EOS 1D Mark II Info:
EF Series Lens Compatibility: 35mm-equivalent = 1.3x marked focal length
Maximum non-interpolated pixel count: 3504 x 2336 pixels

Mardon, Apr 24, 2004

2. ### Mark B.Guest

Don't get pixels involved with focal length, they have nothing to do with
each other. Basically, you want the equivalent lens to 153mm focal length
on the 1D (102mm x 1.5x = 153). With the 1.3x crop factor, you need 153/1.3
= approximately 118mm. Since there's no such thing as a 118mm prime lens,
the closest you'll get is a 135mm (178mm with the crop factor). B&H lists 2
versions by Canon - f/2.0 and f/2.8. There's also the Canon 28-135mm IS
zoom lens, which wouldn't be a bad choice. With the crop factor, it would
be 36-178mm.

Mark

Mark B., Apr 24, 2004

3. ### MardonGuest

I think that I understand why you would say to
avoid confusing pixels and focal length but I am
not sure that it is true in the context of the question
that I posed. If you re-read my original post (above)
I think it will be clear that I want to wind up with an
object in the final 1D image that is 4 times as large
as the object in my current G2 image. I only mentioned
pixels as a way of trying to precisely quantify the
added telephoto effect that I want the 1D to provide.
Perhaps I only confused things.
(I realize none of this precise focal length is available) would
merely duplicate the telephoto effect that I am currently
getting with my G2. I do not think that this takes into
consideration the extra 4x telephoto effect that I was
trying to describe when I mentioned pixels. Can I just
multiply your 118mm x 4 = 472mm and conclude that
a lens in this focal length range is what I would need?

Mardon, Apr 24, 2004
4. ### dylanGuest

If I understand the question....

The lens on the G2 (equivalent to 35mm) was 102x1.5 = 153mm and covered
500/2272 = 0.22 of the picture,
on the 1DMk11 you want 2000/3504 = 0.57 of the picture, so you need
0.57/0.22 which equals 2.59x153mm =396mm (if it was 35mm), so 396/1.3 = 305
mm lens.
Nearest available 300mm or a zoom.

dylan, Apr 24, 2004

This could help http://www.sweeting.org/mark/lenses/

6. ### Dave MartindaleGuest

No, because there are two additional factors that calculation ignores.
The G2 and 1DMkII have quite different size output images, and 2000/3504
is not four times 500/2272; it's only 2.6 times as large. So you should
multiply by about 2.6, not 4. In addition, the two cameras produce
images with different aspect ratio, and the G2 is different than 35 mm
film. So the "35 mm equivalent" focal length of 102 mm is actually
based on comparing one of the horizontal, vertical, or diagonal
dimensions of the G2 with 35 film (but only one of the three is correct)
while the 1.3X crop factor for the 1DMkII applies to all dimensions.

If you *really* want the right answer to this, figure out how large the
actual image is on the G2 sensor, which is 500 times the pixel pitch.
Figure out how large you want the image on the 1DMkII, which is 2000
times its pixel pitch. The ratio between those two is the actual
image-plane magnification difference you want. Multiply that by 21 (the
actual G2 FL you used) times 1.5 (teleconverter effect) to get the
actual focal length you need on the 1DMkII. Ignore "35 equivalent"
focal lengths and "crop factors" because they are just approximations.

Dave

Dave Martindale, Apr 24, 2004
7. ### George PreddyGuest

Simply multiply the lens's rated focal length by 1.3 to calculate the
equivalent focal length when attached to the 1D MII.

This is not magnification, but rather cropping, just like digital zoom
is not magnification, but cropping. The camera's sensor sits in the
middle of the full lens projection circle and crops out some middle
section. 8.2M sensors (2.05M full color RGB sets) spread over that
area is low by todays standards, so the 1Ds MII's ability to optically
resove fine details will be very low indeed, basically the same as the
300D, but with a slightly wider FOV.
You'll have to first convert the focal lengths of the G2 to standard
35mm values in order to compare to the above, it ranges from 34mm
(wide angle) to 102mm (zoom) in the 35mm format equivalent. To buy an
equivalent lens for the 1Ds MII you'd need a 26-78mm zoom.

George Preddy, Apr 24, 2004
8. ### dylanGuest

dylan, Apr 24, 2004
9. ### Charles SchulerGuest

Please don't offer such a pathetic answer to a legitimate question when you
are unable to do so without resorting to politics and your particular
biases, which have proven to be based on a misunderstanding of the
principles of photography. George Preddy (an alias that is not appreciated,
by the way), you are so full of it!

Charles Schuler, Apr 25, 2004
10. ### JC DillGuest

Rather than use some arbitrary pixel figure (500 pixels), I think it
would be easier to use a hypothetical object that *roughly* fills the
G3 frame when at max zoom. At max zoom, you have a roughly 150mm
effective zoom, and it produces an image 1704 tall and 2272 wide in
If you use a 115 mm zoom you would have a ~150mm effective zoom, or
roughly the same zoom as the G3. But you get a larger image:
This would result in the same object being rendered in an image that
*roughly* fills the M II frame at 2336 tall and 3504 wide. But you
only need an image 72% as tall or 64% as wide to be the same image
size as your G3 image. So you only need 72% of 115mm (~82mm), or 64%
of 115mm (~75mm) as much zoom to get the same image size in pixels,
shooting at the same object from the same distance.

If my math is correct.

To double check this - take your G3 to a camera store that has a Mark
II demo unit and a zoom lens such as a 70-200 lens. Put the zoom on
the Mark II. Take your G3, go to max zoom with the 1.5 tele extender,
and find an object on the opposite side of the store that roughly
fills your G3 viewfinder. Then pick up the Mark II and zoom until the
same object roughly fills the viewfinder and see what the zoom setting
is.

HTH

jc

JC Dill, Apr 25, 2004