Image Sharpness

I spent the other afternoon in the Big Tower at Gigrin, in the very pleasant company company of Mr. Jeffrey “Jeffer-Cakes” Young.    Left arm feeling better yet Jeff?

I think I’m fairly safe in saying that once feeding time commenced at 3pm it didn’t take too long before Jeff got a firm understanding of just how damn hard bird flight photography truly is – if you are shooting for true image sharpness at 1:1 resolution.

I’d warned Jeff before-hand that his Canon 5Dmk3 would make his session somewhat more difficult than a 1Dx, due to it’s slightly less tractable autofocus adjustments.  But that with his 300mm f2.8 – even with his 1.4x converter mounted, his equipment was easily up to the job at hand.

I on the other hand was back on the Nikon gear – my 200-400 f4; but using a D4S I’d borrowed from Paul Atkins for some real head-to-head testing against the D4 (there’s a barrow load of Astbury venom headed Nikon’s way shortly I can tell you….watch this space as they say).

Amongst the many topics discussed and pondered upon, I was trying to explain to Jeff the  fundamental difference between ‘perceived’ and ‘real’ image sharpness.

Gigrin is a good place to find vast armies of ‘photographers’ who have ZERO CLUE that such an argument or difference even exists.

As a ‘teacher’ I can easily tell when I’m sharing hide space with folk like this because they develop quizzical frowns and slightly self-righteous smirks as they eavesdrop on the conversation between my client and I.

“THEY” don’t understand that my client is wanting to achieve the same goal as the one I’m always chasing after; and that that goal is as different from their goal as a fillet of oak-smoked Scottish salmon is from a tin of John West mush.

I suppose I’d better start explaining myself at this juncture; so below are two 800 pixel long edge jpeg files that you typically see posted on a nature photography forum, website or blog:

Click the images to view them properly.

“THEY” would be equally as pleased with either…..!

Both images look pretty sharp, well exposed and have pretty darn good composition from an editorial point of view too – so we’re all golden aren’t we!

Or are we?

Both images would look equally as good in terms of image sharpness at 1200 pixels on the long edge, and because I’m a smart-arse I could easily print both images to A4 – and they’d still look as good as each other.

But, one of them would also readily print to A3+ and in its digital form would get accepted at almost any stock agency on the planet, but the other one would most emphatically NOT pass muster for either purpose.

That’s because one of them has real, true image sharpness, while the other has none; all it’s image sharpness is perceptual and artificially induced through image processing.

Guessed which is which yet?

Image 1. has true sharpness because it is IN FOCUS.

And you don’t need glasses to see that image 2 is simply OUT OF FOCUS.

The next question is; which image is the cropped one – number 2 ?

Wrong…it’s number 1…

Image 1 uncropped is 4928 pixels long edge, and cropped is 3565, in other words a 28% crop, which will yield a 15+ inch print without any trouble whatsoever.

Image 2 is NOT cropped – it has just been SHRUNK to around 16% of its original size in the Lightroom export utility with standard screen output sharpening.  So you can make a ‘silk purse from a sows ear’ – and no one would be any the wiser, as long as they never saw anything approaching the full resolution image!

Given that both images were shot at 400mm focal length, it’s obvious that the bird in image 1 (now you know it’s cropped a bit) is FURTHER AWAY than the bird in image 2.

So why is one IN FOCUS and the other not?

The bird in image 1 is ‘crossing’ the frame more than it is ‘closing in’ on the camera.

The bird in image 2 is closer to the camera to begin with, and is getting closer by the millisecond.

These two scenarios impose totally different work-loads on the autofocus system.

The ability of the autofocus system to cope with ANY imposed work-load is totally dependent upon the control parameters you have set in the camera.

The ‘success’ rate of these adjustable autofocus parameter settings is effected by:

1. Changing spatial relationship between camera and subject during a burst of frames.
2. Subject-to-camera closing speed
3. Pre-shot tracking time.
4. Frame rate.

And a few more things besides…!

The autofocus workloads for images 1 & 2 are poles apart, but the control parameter settings are identical.

The Leucistic Red Kite in the shot below is chugging along at roughly the same speed as its non-leucistic cousin in image 2. It’s also at pretty much the same focus distance:

So why is image 3 IN FOCUS when, given a similar scenario, image 2 is out of focus?

Because the autofocus control parameters are set differently – that’s why.

FACT: no single combination of autofocus control parameter settings will be your ‘magic bullet’ and give you nothing but sharp images with no ‘duds’ – unless you use a 12mm fish-eye lens that is!

Problems and focus errors INCREASE in frequency in direct proportion to increasing focal length.

They will also increase in frequency THE INSTANT you switch from a prime lens to a zoom lens, especially if the ‘zoom ratio’ exceeds 3:1.

Then we have to consider the accuracy and speed of the cameras autofocus system AND the speed of the lens autofocus motor – and sadly these criteria generally become more favourable with an increased price tag.

So if you’re using a Nikon D800 with an 80-400, or a Canon 70D with a 100-400 then there are going to be more than a few bumps in your road.  And if you stick to just one set of autofocus control settings all the time then those bumps are going to turn into mountains – some of which are going to kill you off before you make their summit….metaphorically speaking of course!

And God forbid that you try this image 3 ‘head on close up’ malarkey with a Sigma 50-500 – if you want that level of shot quality then you might just as well stay at home and save yourself the hide fees and petrol money !

Things don’t get any easier if you do spend the ‘big bucks’ either.

Fast glass and a pro body ‘speed machine’ will offer you more control adjustments for sure.  But that just means more chances to ‘screw things up’ unless you know EXACTLY how your autofocus system works, exactly what all those different controls actually DO, and you know how to relate those controls to what’s happening in front of you.

Whatever lens and camera body combination any of us use, we have to first of all find, then learn to work within it’s ‘effective envelope of operation’ – and by that I mean the REAL one, which is not necessarily always on a par with what the manufacturer might lead you to believe.

Take my Nikon 200-400 for example.  If I used autofocus on a static subject, let alone a moving one, at much past 50 metres using the venerable old D3 body and 400mm focal length, things in the critical image sharpness department became somewhat sketchy to say the least.  But put it on a D4 or D4S and I can shoot tack sharp focussing targets at 80 to 100 metres all day long……not that I make a habit of this most meaningless of photographic pastimes.

That discrepancy is due to the old D3 autofocus system lacking the ability to accurately  discriminate between precise distances from infinity to much over 50 metres when that particular lens was being used. But swap the lens out for a 400 f2.8 prime and things were far better!

Using the lens on either a D4 or D4S on head-on fast moving/closing subjects such as Mr.Leucistic above, we hit another snag at 400mm – once the subject is less than 20 metres away the autofocus system can’t keep up and the image sharpness effectively drops off the proverbial cliff.  But zoom out to 200mm and that ‘cut-off’ distance will reduce to 10 metres or so. Subjects closing at slower speeds can get much closer to the camera before sharp focus begins to fail.

As far as I’m concerned this problem is more to do with the speed of the autofocus motor inside the lens than anything else.  Nikon brought out an updated version of this lens a few years back – amongst its ‘star qualities’ was a new nano-coating that stopped the lens from flaring.  But does it focus any faster – does it heck!  And my version doesn’t suffer from flare either….!

Getting to know your equipment and how it all works is critical if you want your photography to improve in terms of image sharpness.

Shameless Plug Number 1.

I keep mentioning it – my ebook on Canon & Nikon Autofocus with long glass.

Understanding Canon & Nikon Autofocus

for

Bird in Flight Photography

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