Can someone explain this to me?

[OviV]

O.K. I purchased JZaun's Canon 100-300 4.5-5.6 USM lens and I have a 75-300 4.5-5.6 III. I took a couple of pictures with both to compare. Both of these pictures were taken at 300 MM. Same settings on the camera. On a tripod from the same exact distance. Why does the 75-300 give me a tighter zoom?

http://www.shareapicture.com/vsalbum/getpic.aspx?fld=\OviV\Misc&pic=75-2.jpg&OV=0
75-300 Picture.

http://www.shareapicture.com/vsalbum/getpic.aspx?fld=\OviV\Misc&pic=100-2.jpg&OV=0
100-300 Picture.

Thanks for your input,
Ovi

[Belmondo]

You might check the EXIF data to see if the 100-300 was fully 'zoomed'. I took some pictures with my 100-400, and the EXIF data reported a focal length of 375mm, yet I could swear I had the lens fully extended.

[Tom W]

I was going to say the same thing - check the EXIF data.

I will note that the 100-300 appears a bit sharper.

[OviV]

Exif shows both pictures at 300.

[slin100]

The difference has to do with the position of the focusing elements in each lens. The 100-300 is a rear-focusing zoom. The 75-300 is a front-focusing zoom. A rear-focusing zoom will increase its field-of-view (FOV) as focus distance decreases. You can only get a true 300mm FOV at infinity focus with a rear-focusing lens.

Try your test again on a distant subject. You should find the FOV for each lens much more comparable.

[Tom W]

The difference has to do with the position of the focusing elements in each lens. The 100-300 is a rear-focusing zoom. The 75-300 is a front-focusing zoom. A rear-focusing zoom will increase its field-of-view (FOV) as focus distance decreases. You can only get a true 300mm FOV at infinity focus with a rear-focusing lens.

Try your test again on a distant subject. You should find the FOV for each lens much more comparable.

Well, that's an interesting piece of information. I was stumped, but this does make some sense. I'd really like to see those long-focus pictures as well.

[OviV]

That's why I love this place. I knew someone would have the answer.

Ovi

[ron chappel]

Hey that would be right! I was going to say that the 75-300 was just cheap and crappy and couldn't give you a true 300mm :lol: :lol:
I had a similar experience when i compared them focused at about 9m

...Oh-and i also saw the same overall results :) .
The 100-300 has slightly more sharpness and contrast.
Background blur was different too-enough to play a noticable part in the character of each lens
I highly recommend the 100-300-it just gives you more and better photos
The 75-300 does give very nice pics sometimes-but makes it harder.

...Trying the 90-300 very soon (it didn't arrive in time for this weekend :cry: )