Convection currents and camera lens definition. A popular topic but a recycled one

Over the past year the attention of photographers has been drawn to the problem of convection currents in the air affecting the definition in photographs taken at a long distance using long focal length lenses. The different densities and refractive indices of hot and cold air cause a shimmering effect as warm air rises through cold. The effect may be obvious or subtle. However, with a long distance between subject and lens that slight shimmer is sufficient to be picked up as a loss of definition.

All sorts of advice has been be given about avoiding less obvious pitfalls. 'Do not take photographs over a car’s hot bonnet'; 'be careful when using a lens hood since warm air may be trapped inside', etcetera, etcetera. Oh, and 'do not blame the lens'. Facebook Groups (the source though of a great deal of misinformation) and other photographic forums have seen much repetition.

Those of us in Britain and around photography since the 1950s will remember how cameras were tested for Amateur Photographer magazine. Somebody would do the short walk from the office to the Thames, point the camera at HMS Wellington, a sloop which escorted 103 Atlantic convoys in the Second World War, press the shutter, move the ship to the edge of the field and press the shutter again. Enlargements from centre and edge were published as part of each review. That went on for years until some bright spark pointed out that any differences were more likely due to changes in atmospheric conditions and at slow shutter speeds the rise and fall of the tide and the flow of the river would also be apparent.

A typical lens test from Amateur Photographer
from Photographic Memorabilia website

Incidentally, anybody wishing to test their own camera and lens 1954-style can still do so. HMS Wellington, the Thames and the Embankment are still there.

The information on Amateur Photographer can be found on the excellent website Photographic Memorabilia.

Which Long Telephoto Zoom for Video with the Nikon Z8 or Nikon Z50ii?

In previous posts I compared the performance of three Nikkor Z Zoom lenses at the same focal length of 400 mm. I looked only at sharpness. The three lenses were: 180-600 mm f/5.6-6.3 VR; 100-400 mm f/4.5-5.6 VR S; 28-400 mm f/4-f/8 VR and I used two bodies, the FX Z8 and the DX Z50ii in the comparison. As expected sharpness and the retail price of the lenses were positively related.

But what about video? A 16:9 frame cuts off the corners of the sensor, therefore edge definition is not so prominent. The video image is produced by oversampling. Therefore, I thought I would test the sharpness of the three lenses when used for video. I was not expecting the ranking of the lenses to change but was interested to see if the optical differences were less or more evident on the same two cameras. I was also keen to see if the quality of the worst performing could be improved, as I suspected from experience of one of the lenses, by simple tweaks in post-production.

I made some simple test cards in Photoshop and printed them on premium glossy 4 x 6 card. The cameras were mounted on a tripod standing on firm sandstone slabs. I recorded 10-second video clips. My normal method of video production is to shoot in 4K and then edit and output from Final Cut Pro from a timeline in 1080. From each clip I exported a freeze frame as a tiff file at 1080.

I checked first that 4k and 8k clips from the Z8 did not vary in sharpness on the 1080 timeline. Therefore I concentrated entirely on 4k clips with each lens.

As expected, and without any manipulation, the order of sharpness was 100-400 mm f/4.5-5.6 VR S > 180-600 mm f/5.6-6.3 VR > 28-400 mm f/4-f/8 VR. The greatest difference was between the latter two lenses. This result was the same with the two camera bodies.

There is also a marked in contrast between the three lenses, especially between the 28-400 and the other two. Therefore I applied two simple corrections in FCP to the 28-400 mm clip: (i) I matched the colour to that from the 100-400; (ii) I applied a little sharpening. That made an enormous difference as can be seen from the images shown below.

Essentially the same results were obtained with the DX sensor on the Z50ii. 

These tests confirmed that I have lots of options for wildlife video. For the longest focal length with a trade-off against weight and a very small decrease in sharpness then it is the 180-600. For highest optical quality with a trade-off against maximum focal length and some trade-off against weight then it is the 100-400. For carrying longer distances then the weight of the 28-400 comes out on top with a manageable trade-off against sharpness and contrast. My Z50ii with 28-400 mm will remain my set-up for many wildlife watching trips which involve walking with binoculars, a pole and other gear…although I would be tempted to carry the 100-400 mm instead.

Here are the relevant images taken from the video timeline:

Nikon Z8. FX. ISO 64. 25 fps. Shutter 1/50 sec. Tripod-mounted. All lenses at 400 mm focal length

28-400 mm Lens. Colour-matched and Sharpening added