New Lens Recommendations Wanted

[Daniel Browning]

the 1.4x extender on a 7D body, it is like having a 672 mm F/5.6

Minor correction: it's actually like having a 672mm f/9. The 1.6X factor applies to both focal length and f-number.

[Tom Wertman]

[b][url="/forums/members/Daniel-Browning/default.aspx]Daniel Browning[/url][/b]<span class="ForumPostTitleDate"]

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<div>[img]/forums/Themes/hawaii/images/icon-quote.gif[/img] EdN:</div>
<div>the 1.4x extender on a 7D body, it is like having a 672 mm F/5.6</div>
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Minor correction: it's actually like having a 672mm f/9. The 1.6X factor applies to both focal length and f-number.





Can anyone confirm this 1.6 crop factor applying to the f/stop? Here is a shot of my son I took using a 40D w/ 70-200 f/2.8L IS and 1.4X ext. The exif data from Photoshop shows f/l of 280 with f/4.0. I always heard that the tele ext. of 1.4X loses 1 stop and the 2X losses 2 stops, but never heard of the 1.6 body crop effecting anything but the focal length.


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[Daniel Browning]

The exif data from Photoshop shows f/l of 280 with f/4.0.

Yes. The exif displays the actual focal length, not the full-frame equivalent (450mm).

...never heard of the 1.6 body crop effecting anything but the focal length.

That is expected. Very few people are aware of the relationships between format size and f-number. And by the way, the body crop doesn't really affect the focal length. It affects the angle of view. So 450mm on full frame has the same angle of view as 280mm on crop. In the same way, body crop doesn't affect f-number, but it does affect depth of field, diffraction, and noise. So f/5.6 on full frame has the same DOF, diffraction, and noise level as f/3.5 on crop (all else equal).

[neuroanatomist]

Can anyone confirm this 1.6 crop factor applying to the f/stop? Here is a shot of my son I took using a 40D w/ 70-200 f/2.8L IS and 1.4X ext. The exif data from Photoshop shows f/l of 280 with f/4.0. I always heard that the tele ext. of 1.4X loses 1 stop and the 2X losses 2 stops, but never heard of the 1.6 body crop effecting anything but the focal length.

Yes, the 1.6x FOVCF applies to the effective aperture as well, in terms of depth of field and total light gathering ability.


Presumably the lens was set to 200mm f/2.8 for the shot, with the teleconverter. The EXIF data shows the correct focal length (280mm, which is 200mm x 1.4), and the correct aperture (f/4, which is one stop slower than f/2.8). Those data are correct for the lens + TC. You'll note that the EXIF data don't show the focal length as 448mm (200mm x 1.4 x 1.6); likewise, the correct aperture (for the lens) is shown, not the aperture adjusted for your sensor's crop factor.


The effective focal length for your shot is 448mm, but that's because of the effect of the crop factor on the field of view. Put another way, the magnification of the subject is the same on 1.6x and FF sensors, but the FoV is restricted by the crop factor. So, to get the same framing on a crop sensor as you'd have on a FF sensor, you need to move further away from the subject. Moving further away from a subject (but keeping the same real focal length and aperture) increases the depth of field for that shot. Thus, a 1.6x crop sensor, compared to FF for the same subject framing, has a deeper DoF - equivalent to an aperture of about 1.6 times narrower.


You can read more in the DoFMaster tutorial on the subject.