Driving a Big Lens – James Allan

When I was at Mannum, a kite swooped down low and grabbed a fish.  I snapped the scene with my Nikkor 300mm lens and 1.6 teleconvertor.  Chris and Richard were standing right behind me and took the same shot with their Sigma 120-400mm zoom lens.  Why was mine less sharp than their shots?  This question is the reason I am writing this article.  Actually Heather Connolly asked a similar question in a different way at the recent critique night.  “What settings should I use when I’m doing bird photography?”  I think she was concerned about what shutter speed to select.

In my minds eye I should be able to pick up my camera, swing it around 90 degrees to focus on an overhead flying bird, have the camera focus automatically and take a salvo of 5 to 10 crisp and sharp images as it flies away.  Why doesn’t it always happen like that?  Have I got a dud lens?  Have I got the wrong settings?  Am I a bad photographer?

As I researched this question I have come to the following understandings.

Back Focus.

Can my lens take a sharp photo?  Sometimes the lens may be mounted a little too far forward or behind the ideal position.  This is called back focus.  Interestingly my camera (and many brands) have a feature that will allow the camera to adjust for this.  What you have to do is take a photograph of a special target designed to detect back focus.  It consists of a bullseye on one side and a sloping ruler angled towards the camera on the other.  When the camera auto focuses on the target, you will expect the ruler to be sharpest at the mid point.  If the sharpest point occurs behind or in front of the target, then you will have to make an adjustment for the lens.  My lens was perfectly positioned.

Picture11

Is the camera stable enough?

Frances always told me that hand shake would blur photos if you used a shutter speed less than 1/60th of a second.  I got a bit cocky with my little Nikon / Panasonic cameras taking hand held photos down to 1/4 a second with minimal blurring.  I guess this was possible because I would brace myself, hold my breath and turn on the image stabilisation.  With the big lens it is not so easy.  I am trying to hold a much larger weight.  The big lens tends to tip forward.   Furthermore the long focal length tends to amplify the effect of my hand shake.  A little bit of shake will cause more blurring in the long lens than my wide angle lens.  It’s like trying to sign your name while holding the pencil from the eraser end.  You have less control over the tip and it wanders all over the place.  I have read that for a 35mm camera you should select a shutter speed equivalent to 1/focal length of the lens.  In my case this is not 1/300.  It is more like 1/600 because of the 1.7 teleconverter and the 1.5 cropped sensor.  Unlike Chris and Richard my old Nikkor lens has no image stabilisation (IS) capabilities.  IS has the effect of  improving the acceptable hand held shutter speed by around two stops.  ( back to 1/150)  I think this is the chief reason why their photos were sharper than mine.  So what should I do?  Turn up the shutter speed?  This means I may loose out in other areas – like f stop and ISO.  There is another answer.   I don’t need to take all my shots hand held.  I can use a tripod.  In fact a monopod is used by a lot of bird photographers.  I watched an interesting You-tube clip of a photographer who attained amazing flexibility on a monopod with a few cheap accessories.   I will have to explore this option.  Another recommendation was to use the flash.  (preferably a speedlite on the flash shoe)  Using a flash will improve the lighting and thus the shutter speed and reduce the impact of hand vibration.

The F-stop.

I looked at some of the photos taken with the big lens.  There is a narrow depth of field with a pleasing Bokeh (the blurred areas).  However the narrow depth of field was also causing problems.  The eye of the duck was sharp, but the bill and the feathers on the neck were both blurred.  This is more of a problem when the subject is closer to me.  I had the lens fully open, (f/4) but I would do better by shutting it down a bit.(f:5.6 up to f:8).  There is another effect to be aware of.  The central sharp area of the photo (eye of the duck) is sharper on many lenses once it is stopped down 1 or 2 stops.  An F/4 lens will not be as sharp at f:4 than a F/2.8 lens at the same aperture f:4.  In summary, for both of these reasons – it is better to shut the lens down a little bit.  Jo Tabe calls this the “sweet spot”

Picture12

Narrow Depth of Field.  The bill and the back of the duck are blurred, yet the eye is sharp.

Shooting mode.

I have heard all sorts of different recommendations for various reasons.  Should I use aperture priority / shutter priority or program mode?  Spot or area focus?  Should I allow continuous focus tracking, or focus locking?  I’m not sure I’ve got this all sorted yet.  I’ll tell you what I do.  I go straight to aperture priority, closing the lens down 1 or 2 stops, with ISO set at 400 (to allow a faster shutter speed) – but allowing the camera to increase the ISO up to 1600 if the shutter speed drops below 1/500.  I use spot focus and continuous tracking to capture birds in flight.  I have all these settings preset in the custom shooting mode, so I can flick to it quickly when I need to. Sometimes I might just switch over to shutter priority set at 1/1000 of a second.

Rosella 1Well there you have it.  I don’t know all the answers.  I suspect that my initial expectations are too high for my lens.  By experimenting I can get a feel for its capabilities.  By some small fine tuning I can get sharp photos at a good focal length (around 600mm equivalent).  Certainly they are sharper than an equivalent photo on my old Panasonic.  I hope these comments are helpful.

Post script – Filters

I later discovered that a not so cheap UV filter used to protect the lens from damage was causing a significant drop in sharpness and the picture quality improved when I removed it.

 

Advertisements