Macro Magnifaction Factor

[jwkramer]

I have a question regarding the use of the 100mm f2.8 Macro Lens on a 10D. This lens normally does 1:1 magnification at 18" close focus on a 35mm film camera. With the 1.6x lens factor of the 10D, will the 1:1 magnification factor increase to larger than life at 18" close focus, or will the maximum magnification of the lens remain 1:1, but the close focus distance increase to something like 27"?

I guess this lens actually becomes more like the much more expensive 180mm f3.5L lens... (at least in focal length)

Thanks!

-Jim

[Belmondo]

A guess:

The image size at the sensor is the same as on a 35mm frame. However, since the image is cropped to about 60% of the 35mm frame size due to the smaller area of the sensor, you record less of it. The magnification remains the same.

Focusing distance will be the same and magnification will be the same. When a 35mm image is blown up to a certain size print, and the cropped DSLR image is enlarged to the same size, the magnification will only appear to be greater.

In other words, all the dynamics should be the same as with a non-macro lens.

Someone please confirm.

[jwkramer]

belmondo wrote:
A guess:

The image size at the sensor is the same as on a 35mm frame. However, since the image is cropped to about 60% of the 35mm frame size due to the smaller area of the sensor, you record less of it. The magnification remains the same.

Focusing distance will be the same and magnification will be the same. When a 35mm image is blown up to a certain size print, and the cropped DSLR image is enlarged to the same size, the magnification will only appear to be greater.

In other words, all the dynamics should be the same as with a non-macro lens.

Someone please confirm.

hmmmm..... although most of what you are saying makes sense, some of it does not to me. They routinely refer to the 1.6x "magnification factor" of the D30, D60 & 10D. I follow the logic of the smaller image sensor.... but there is still a mental block there for me {somewhere}. When I put my 17-35mm Sigma on the 10D, it doesn't appear to be anywhere near as wide as when it is on my EOS-3. It seems like more of an issue than just the size of the image... it seems like it is also "less wide" if that makes sense. Maybe it's all just an illusion. Do you know for a fact that the distance from the lens to the film plane (EOS-3) is the same as the distance from the lens to the CMOS sensor (10D)?

It's probably just some kind of a mental block for me...

Thanks for your input!

-Jim

[Belmondo]

jwkramer wrote:When I put my 17-35mm Sigma on the 10D, it doesn't appear to be anywhere near as wide as when it is on my EOS-3.

That's because the sensor is only seeing roughly 60% of the scene that the lens is recording. The viewfinder on the 10D and Rebel is designed to show only the cropped image. The 17-35 lens is faithfully capturing the full image and projecting it onto the focal plane just like it does on your 35mm body. The sensor, because of it's size, is only recording a portion of that image however. In essence, we're not maginifying the image by 60% --- we're wasting 40% of the image.

Tom

[Belmondo]

jwkramer wrote:
Do you know for a fact that the distance from the lens to the film plane (EOS-3) is the same as the distance from the lens to the CMOS sensor (10D)?



Jim:
The one constant between your EOS-3 and a 10D is the lens. Any given lens is going to function precisely the same on either body, thus the focusing point will of necessity be the same distance behind the rear element of the lens.

Tom

[phidong]

jwkramer wrote:
it doesn't appear

Exactly. It just appears that way, but it really is the same maginifcation.

[EXA1a]

belmondo wrote:
jwkramer wrote:When I put my 17-35mm Sigma on the 10D, it doesn't appear to be anywhere near as wide as when it is on my EOS-3.

That's because the sensor is only seeing roughly 60% of the scene that the lens is recording. The viewfinder on the 10D and Rebel is designed to show only the cropped image. The 17-35 lens is faithfully capturing the full image and projecting it onto the focal plane just like it does on your 35mm body. The sensor, because of it's size, is only recording a portion of that image however. In essence, we're not maginifying the image by 60% --- we're wasting 40% of the image.

Tom




Tom, the 10D sensor wasts more than 60% of the image area compared to 24x36mm (343 squaremillimeters vs. 864 squaremillimeters is 39.7%)
--Jens--

[Belmondo]

You're right, Jens. I was referring only to linear dimensions which, for my purposes, are more relevant to the discussion of magnification/crop factor. Your point is well-taken, however. Wasted area is in excess of 60%