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I'm with Sy: there's no simple "use it or don't" guidance I can think of for cropping vs TC. In the OP's example, the 1.3x exposure increase could be due to 2 factors:
- T/ number of the lens+TC (the TC adds elements that reduce the light arriving at the sensor more than just the 1-stop you'd expect from adding the TC).
- Cropping changes what the light meter sees; I often see disproportionate exposure changes when I use a TC.
In addition, for us who have to add a dedicated TC as opposed to flipping a built-in TC in/out, there are considerations like:
- Can an acceptable image be gotten with the exposure changes needed to accommodate the TC?
- Which TC is being used (Canon? I, II, or III? Tamron? Another brand? 1.4x, 2x?)
- Which lens is it being used on?
- Which camera? (How does the camera's AF do at lower light levels?)
- Is the camera supported adequately for the lens + TC?
For cropping:
- Will the resulting crop have enough pixels covering the subject to enable you to use the image as intended? (Severely cropped 1Dx images have a LOT fewer pixels than 5DS images cropped the same amount, for example).
- If you needed to increase ISO to accommodate the smaller f/stop, would the resulting noise be acceptable?
And for both, as Sy Sez, are you using cropping/TC to compensate for an inability to get closer to the subject? In which case the TC will magnify atmospheric effects at full-sensor magnification, whereas cropping probably won't show more atmospheric stuff because of the effectively lower resolution of the crop.
I often use my 500 f/4L II, and sometimes the 100-400 (II), with a TC1.4 (II) or III) and rarely with a TC2 (III). My older 100-400L (I) fared much worse with a TC1.4 (II) and wasn't very useful with a TC2. The 180L macro does pretty well with a TC1.4. I've read wildly inconsistent experiences with various third party extenders used on third party lenses. In the end, I've tested each of my lenses that can take a Canon TC with/without the TC and made my own call about whether/when I can use a TC and when not.
Bottom line is that if you want to use a TC, consider trying it under noncritical conditions before taking it into the field. Also, be sure to check MFA: lens + TC usually requires (for me) different MFA correction than bare lens. Also, Liquidstone's point is well taken: it depends on the image—sometimes you need both TC AND cropping to get the composition you're after.
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