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DPChallenge Forums >> Hardware and Software >> Is it 810 or 840?
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09/23/2022 10:11:03 PM · #1
Picked up a 1.4x teleconverter the other day to use with my Sony 70-400. Tested it out and it works fine. Will need good light obviously, but the images are clear and sharp, so I'm pleased so far.

One question I'm curious about; that lens, when mounted on my A77II (aps-c format) is the equivilant of a 105-600 lens in regards to focal length (at 1.5 crop factor).
Is the 1.5 factor figured in before the 1.4x of the teleconverer, or after?

When I looked at the exif from images captured with the lens and teleconverter in use it showed a focal length of 540. This makes sense (400 x 1.4),

A 400 lens at 540 (400 x 1.4 teleconverter) x 1.5 crop would be an equivalent length of 810.
If you do the math a bit differently it works out to length of 840 (400 lens x 1.5 crop = 600, then times 1.4 teleconverter = 840).

So, do I have the equivalent focal length of 810 or 840 with this combination?

09/23/2022 10:21:23 PM · #2
The factor is based on the size of the lighted area falling on the sensor, so I *think* the 1.5 factor would apply after combining all focal lengths of your lens configuration.
09/24/2022 01:22:25 AM · #3
Agreed - it must be 810. The converter only has an impact on the lens and the unchanged crop factor applies to (the totality of) the lens. (In my head at least).
09/24/2022 11:07:44 AM · #4
Agreed x2. "Crop Factor" refers to how much of the image circle is cropped out by the sensor. A lens designed for a larger sensor is "longer" on a smaller sensor because said sensor is using a smaller portion of the projected image circle to make the photo. (Focal length x teleconverter factor) x crop factor is the right formula.
09/24/2022 11:13:44 AM · #5
The answer is "it doesn't matter."
It's all multiplication, so mathematically it does not matter in what order you perform the multiplication. The answer is 840, no matter the order. Must have a math error if you are getting 810?
That said, most longer zooms actually fall a little bit short of the stated maximum focal length, so it may be that your EXIF-reported focal length may never reach quite to 840.
09/24/2022 03:32:44 PM · #6
Gah! I misread the exif and my math was invalid for scenario #1. 560 it shoul be, then 840 in the end. Sorry. Interesting conversation nonetheless. Thanks all.
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