Long Lens Autofocus Considerations.
If you read my previous post about the 1Dx sensor you will have seen that I mentioned my, as yet unfinished, tome about long lens autofocus for wildlife photography. It’s a frustrating project because I keep having to change various bits to make them simpler, re-order certain paragraphs etc.
But I thought I’d blog-post something here that I expand on in the project, and it’s something an awful lot of people NEVER take into consideration.
As a Nikon user I’m used to the vagaries of the Nikon AF system and I manage to work with it just fine – I have to!
But photographers who don’t shoot wildlife, and don’t use 400mm or 500mm lumps of glass as their “standard lens” might not find the vagaries I bitch about quite so apparent; indeed some might not come across them at all.
As a wildlife photographer I shoot in crappy light, I shoot with slow lenses (both in terms of f-number and focus speed), I shoot low contrast subjects on equally low contrast backgrounds, I’m constantly shooting brown-on-brown, grey on grey etc, I shoot stupidly small subjects….the list goes on!
For years, good wildlife photography has been done by pushing camera/lens capabilities beyond their performance design parameters; and this particularly applies to our “expectations” of our latest and greatest AF system – be it Canon or Nikon.
I find so many people who come to my workshops etc. are not even aware of this one simple fact – sharp focus requires more work AND increased speed of work by the lens AF motor the closer a subject is to the camera.
Just try looking at the delineations on the focusing ring of a lens:
Canon 200-400 focused at 20 meters. (Lens porn WARNING: This lens will cause movements in the front-of-trouser department).
Look at the scale and note the distance between 20m and 50m marks – that distance is indicative of the amount of work required of the autofocus controller and motor to move from 20m to 50m or vice versa.
Now look where the 10m mark is – it requires FAR MORE work from the focus controller and motor to move from 20m to 10m, than it did to move the 30 meters from 50m to 20m.
On top of that extra work, if we are tracking a subject moving at 10 meters per second the lens takes 3 seconds to move from 50m to 20m, but then has to move a lot FASTER as well to cover the extra workload moving from 20m to 10m in just 1 second.
Then you wonder why your Nikon D40 + Sigma 50-500mm is crap at doing “birds in flight”; you never realise that your autofocus system is bag of spanners and powered by a hamster on a wheel…….it’s just not fast enough kids
Autofocus accuracy is nothing without speed if you are wanting to do productive wildlife photography.
As I alluded to before, as a photographer of the old wildlife I, and YOU will always encounter problems that users in other photographic disciplines may not, or if they do then the problem has a lot less impact than it does for us.
Think of it this way – a sports photographer will use a 500mm f4 to photograph a 6 foot tall overpaid git who’s 25m to 70m away, on a sunny Saturday afternoon or under a squillion watts of flood lighting; and he’s looking for a 6×12 for the back page of the Sunday Sport. I’ll use the same lens to photograph a cute Red Squirrel at 5m to 7m in a gloomy wood in the middle of winter and I’m looking for a full size, full resolution image for stock.
Red Squirrel – this is basically the FURTHEST DISTANCE you could shoot at with a 500mm lens and still get a meaningful composition. Click for larger view.
Note the distance – 631/100 – that means 6.31 meters. Aperture is f8, so DoF is around 7 centimeters.
The image is UNCROPPED as are all the other images in this post
We don’t really want to be any further away because “his cuteness” will be too small in the frame:
The factors effecting subject distance choice are:
- lens resolving power – small, fine details need to be as close as possible.*
- sensor resolving power – we need as many pixels as possible covering the subject.*
- auto focus point placement accuracy – if the subject is too small in the frame, point placement is inaccurate.
- general “in camera” composition
*These two are inextricably intertwined
I’ve indicated the active focus point on the above image too because here’s a depth of field “point of note” – autofocus wastes DoF. Where is the plane of focus? Just between the eyes of the squirrel.
Assuming the accepted modern norm of DoF distribution – 50/50 – that’s 3.5 centimeters in front of the plane of focus, or indicted AF point, that will be sharp. Only problem there is that the squirrel’s nose is only around 1 centimeter closer to the camera than the AF point, so the remaining 2 .5 centimeters of DoF is wasted on a sharp rendition of the fresh air between its nose and the camera!!
Now let’s change camera orientation and go a bit closer to get the very TIGHTEST shot composition:
Red Squirrel – this is basically the CLOSEST DISTANCE you could shoot at with a 500mm lens and still get a meaningful composition. Click for larger view
The subject distance is 5.62 meters. Aperture is f6.3 so DoF is around 4.4 centimeters.
Now let’s change photographic hats and imagine we are a sports photographer and we are spending a Saturday afternoon photographing a bunch of over-paid 6 foot tall gits chasing a ball around a field, using the very same camera and lens:
He’s not over-paid or chasing a ball, but this is the CLOSEST distance we can shoot at with this orientation and still get a “not too tight” composition of a 6 foot git! “Shep’s” not a git really – well, not much! Click to enlarge
The distance for this shot is 29.9 meters. Aperture is f6.3 so DoF is around 1.34 meters.
And here we are at the CLOSEST distance for this horizontal camera orientation – still not too tight. Click to enlarge.
The distance here is 50.1 meters. Aperture is f6.3 so DoF is around 3.79 meters.
So with this new “sports shooter” hat on, have we got an easier job than the cold, wet squirrel photographer?
You bet your sweet life we have!
The “Shepster” can basically jump around and move about like an idiot on acid and stay in sharp focus because:
- the depth of field at those distances is large.
- more importantly, the autofocus has VERY little work to do along the lens axis, because 1 or 2 meters of subject movement closer to the camera requires very small movements of the lens focus mechanicals.
But the poor wildlife photographer with his cute squirrel has so much more of a hard time getting good sharp shots because:
- he/she has got little or no depth of field
- small subject movements along the lens axis require very large and very fast movement of the lens focus mechanicals.
So the next time you watch a video by Canon or Nikon demonstrating the effectiveness of their new AF system on some new camera body or other; or you go trawling the internet looking for what AF settings the pros use, just bear in mind that “one mans fruit may be another mans poison” just because he/she photographs bigger subjects at longer average distances”.
Equipment choice and its manner of deployment and use is just not a level playing field is it…but it’s something a lot of folk don’t realise or think about.
And how many folk would ever consider that a desired “in camera” image composition has such a massive set of implications for autofocus performance – not many – but if you put your brain in gear it’s blindingly obvious.
 | Become a patron from as little as $1 per month, and help me produce more free content. Patrons gain access to a variety of FREE rewards, discounts and bonuses. |
