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Focal Length?

daverep Avatar
daverep 4 3
30 Apr 2019 9:28PM
Hi All, I have a Sigma 18-35mm ART DC lens with a Viltrox M2 adaptor (0.71x focal length) on a GX9 (crop sensor). Is the effective focal length 12.78 x 24.85 (multiply both by .71) or do I double the focal length like with a Panasonic lens (12mm actually a 24mm) then multiply by .71?
lemmy Avatar
lemmy 16 2.9k United Kingdom
1 May 2019 12:04PM
You have an actual focal length of 18-35mm. If you simply adapted this (no converter) to your GX9, the focal length remains 18-35 but since the Micro Four Thirds crops it, it gives an equivalent view to a 36-70mm lens on FF.

If you then fit your 0.71x adapter, the lens focal lengths are changed to 13mm-25mm. However, since your GX9 has the smaller sensor, it gives an equivalent view to a 26-50mm lens on FF.

So, if we must express it in FF terms the formula would be (Focal Length x Adapter) x 2.

Remember, the focal length or focal range of a lens is fixed. A 50mm lens does not change focal length when you fit it to an APS-C or Micro Four Thirds camera. It is 50mm in every case. However, it will give a horizontal Angle of View of 39° on a FF sensor, 27° on APS-C and 20° on Micro Four Thirds, due to the crop of the smaller sensors.
daverep Avatar
daverep 4 3
1 May 2019 4:28PM
Hi lemmy, thank you for your answer! metal rules!
MentorRon Avatar
MentorRon 5 102 Canada
2 May 2019 12:24AM
To save opening another topic, I have been unable to figure out why Fujifilm reduces the wide angle focal length when switching from 4:3 to 3:2, even though there are more pixels used (width) with 3:2. Most cameras I've used, the number of horizontal pixels remains the same and just the vertical pixel count changes. See camera info sheet attached.319285_1556753068.jpg
MentorRon Avatar
MentorRon 5 102 Canada
2 May 2019 12:38AM
Sorry the URL faulted just now with too many users !!!
The info says that using 4:3 it starts at 28mm, but using 3:2 it starts at 29mm, even though it is able to utilize 5.6% more pixels (+224 pix) horizontally ??? Your optics math is better than mine :-(
lemmy Avatar
lemmy 16 2.9k United Kingdom
2 May 2019 1:41PM
Off the top of my head, I'd say it would be because they are basing the focal length on the diagonal Angle of View which will vary according the the aspect ratio chosen.
LenShepherd Avatar
LenShepherd 15 4.7k United Kingdom
3 May 2019 9:21AM

Quote:Off the top of my head, I'd say it would be because they are basing the focal length on the diagonal Angle of View which will vary according the the aspect ratio chosen.

Focal length is based on a mathematical formula calculated at infinity focus distance for the intended largest format where the lens gives adequate coverage in the image corners.
The focal length remains the same as marked on the lens if used on a smaller sensor.
If you change either the format size or the format proportions then the camera viewfinder shows a different image.
It can be much easier not to get too involved with "angle of view" - and just take pictures Grin

There are good optical reasons why "angle of view" is based on the diagonal of the format (as in top left to bottom right) of the image.
In everyday life "angle of view" generally relates to the width of a scene from the shooting position.

With the vast majority of lenses angle of view is not constant at different focus distance.
The angle of view can double or half by minimum focus with some extreme wide angles, some variable aperture zooms and some AF macros.

For the related issue of depth of field the physical aperture size does not change at different focus distances despite any change in angle of view with many lenses.
This can result in changes of up to 1 stop in exposure and depth of field compared to either a hand held meter reading or a conventional depth of field table.

The OP mentioned a converter.
If it contains glass and magnifies a portion of the image (as with a tele-converter) most know some light is lost and a longer exposure time is needed to compensate.
Relative few recognise that teleconverters increase depth of field relative to using a longer focal length lens without a tele-converter.

The basics for 24x36 mm format lens is they have about half the angle of view on a DX crop sensor and about a quarter of the angle of view on a 4:3 sensor.
Beyond that by the time you have worked out equivalent angle of view and adjustments to a conventional dof table - you may have forgotten what you intended to photograph Sad
keithh Avatar
keithh 19 25.8k 33 Wallis And Futuna
3 May 2019 2:43PM
aaaaaand relax. Wink


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