Effect of teleconverters and light level on Olympus Pro lenses for for birds in flight

Effect of teleconverters and light level on Olympus Pro lenses for for birds in flight

I have always wondered which Olympus Pro lens and teleconverter combination is best for birds in flight (BIF) in terms of focus accuracy. Last year I did some extended tests (over 11,000 shots) of EM1x wth the the 300mm f4 and the 40-150 f2.8 plus the TC 1.4 and 2.0 to see what combination came out on top. The results have established beyond doubt for me that the 1.4TC extends the reach of the 40-150 lens at almost no impact to focus accuracy and sharpness, so for this lens the 1.4TC is always staying on in future. On the other hand they also show the 300mm lens should not be used with any teleconverter for BIF if a high hit rate or good sharpness is desired. Along the way I also found the effect of low light and CAF sensitivity on the system, results which might surprise you.

Birds in flight at Bempton

Olympus EM1x with pro telephoto lenses tested to see which is best for birds in flight

Bempton cliffs on the east Yorkshire Coast is an RSPB nature reserve well known to birders and photographers. Its great feature is that the cliffs are over 300 ft high and as a result shots can be taken of the birds at eye level. The year-long residents of the cliffs are Gannetts although in winter Puffins and other seabirds congregate there. Gannetts are large birds and whilst attractive, they’re not particularly interesting in flight until they start to land, when their feet and wing movements are beautiful and comic at the same time. For shooting tests and general birds in flight photography Bempton is particularly good because there’s no shortage of flying birds and so a very high number of shots can be achieved in a relatively short period of time (11,467 shots in 171 minutes).

Shooting technique and lenses used

I used the EM1x with ProCapture L and Bird recognition AI, as used with great success by Thomas Stirr. Full details are given at this link. The lenses used were the 300mm f4 Pro and the 40-150 f2.8 pro, with and without the 1.4 and 2.0 teleconverters. My goal was to find out quantitatively how accurate each Pro lens was with and without a teleconverter for birds in flight.

Checking Focus accuracy

I checked the focus accuracy of every image using FastRawViewer at 1:1 in the original raw format, with additional sharpness applied and using focus peaking tools, and exposure adjustments where necessary. I have written about my focus accuracy checking technique extensively in this post and the method was the same.


The table below shows for each lens combination the duration of the shooting session in minutes, the light levels (qualitatively and as an EV number), the total number of shots taken, the focus hit rate, and a qualitative assessment of sharpness. All lenses were shot at their maximum aperture. There were two shooting sessions, one from around 10 am to 2 pm, and the other from 5 to 6pm, in August 2021. The latter session was still perceptually quite bright – a cloud-free sky and no haze. However the effect of the lower light levels compared to the morning was quite substantial, as we will see.

I also investigated the CAF sensitivity. In the first session, it was set to 0. In the second set second I reduced this to -2, for some tests and this is shown in the table as s-2.

DurationLensLightEVTotal shotsNo in focusHit rateSharpness
00:2740-150 + 1.4Bright16.692066873%Suprisingly sharp
00:2340-150 + 2.0Medium15-16135484662%Not sharp
00:31300 + 1.4Medium15.3148457239%Not sharp at distance
00:13300 s-2Reduced12.61639107766%excellent
00:1340-150 + 1.4 s-2Reduced12132685564%Suprisingly sharp

Lens accuracy in good light

In bright light the 300mm f4 was the most accurate lens, getting a 78% hit rate. This was much better than the 61% obtained at the Hawk Conservancy Trust, and I attribute this to using the AI bird tracking plus ProCapture simultaneously, and also the bright conditions. The 40-150 f2.8 was only just behind at 74%.

The real surprise to me was the effect of using the 1.4 TC on both these lenses. For the 40-150mm f2.8, there was almost no impact on accuracy or sharpness, a superb result, that means the 1.4TC need never be taken off this lens. On the other hand for the 300mm f4, adding the 1.4TC reduced the focus accuracy by half, making it only useful for static subjects in my opinion. In addition, birds that were some distance away, even if in focus, were not at all sharp compared to the lens without the TC. So I will always use the 300mm solo for birds in flight in future – which creates a problem if I need more reach.

The 2.0TC was a disappointment on the 40-150, mostly due to the reduced sharpness, although the hit rate was respectable. A shame because had it worked, this lens could have substituted for the 300 f4. This result means that for 600mm FFE, the 300mm f4 is the only game in town for me.

Lens accuracy in reduced light

I was not expecting for the impact of lower light on the test accuracy, and I only noticed this on returning to London. As a result, I didn’t do the full suite of tests, and this will have to wait for a further trip to Bempton. What is clear is that 5 stops lower ambient light levels light reduced the focus accuracy of the 300mm f4 and the EM1x from 78% to 60%. That’s to say the focus hit rate droped by a quarter, which is quite an impact. On this basis I might expect the hit rate of the 40-140 f2.8 plus 1.4TC in low light to drop to say 56%.

Going back to my test results in my Hawk Conservancy Trust shoot-out with the A9, on that test I got 61% in focus on the EM1x with the 300mm and the 40-150 + 1.4TC. Checking the exifs, I see that the the ambient light was 12.5-13.5 EV, which seems to confirm my experience.

Effect of reducing focus sensitivity

Because I did not expect the light levels to impact the hit rate, my plan was to compare results from the morning session at CAF=0 to the afternoon with CAF=-2. Of course the light levels effect made those comparisons invalid. Purely by luck I happened to do a test with the 300mm at 0 and -2 in the afternoon session, which enabled a valid comparison. Interestingly, the 300mm f4 focus hit rate improved from 60% to 66% when the sensitivity was set to -2, so there was a real impact. For the 40-140 f2.8 plus 1.4TC, I don’t have a direct comparison, but if the low light figures are 56% as conjectured above, a similar improvement to 66% is found from reducing the sensitivity. By the way, as can be seen from this video of 176 of the best images from these tests, the Gannets were very often flying directly towards the camera, a circumstance in which you might imagine a high focus sensitivity would get better results.


  • The best affordable Pro lens combinations for BIF on the EM1x are the 300mm f4, and the 40-150mm plus 1.4TC, which give a focus accuracy of 73-78% in good light.
  • The 300mm accuracy drops by half when used with the 1.4TC , and is for my purposes unusable for BIF.
  • The 2.0TC with either lens is also unusable for BIF if good focus accuracy or sharpness is required.
  • The focus accuracy of the 300mm f4 reduces by around a quarter (78% to 60%) in reduced light
  • A focus sensitivity setting of -2 improves focus accuracy by at least 10% for the 300mm f4 (compared to a focus sensitivity setting of 0).
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