Thursday 28 March 2013

The Pitfalls of stopping down

In my early photography career if I wanted a greater depth of field or wanted to carry out a very long exposure, I would stop down and see a consequential drop in image quality.

At photographic college I learned why.

Its due to diffraction.

'Diffraction' is the 'bending power' of a medium, such as glass, on light.

But it does not just have to be glass, it can be the Aperture blades.

There is a great in-depth explanation of diffraction at Cambridgeincolour which should tell you all you need to know.

Before you dismiss this as some technobabble that will not effect you, think again.

Shooting with a Canon XF305 video camera last Summer I was caught out VERY badly indeed by stopping down (F16 I think) to way past F8.0, the truly safe 'zone' to avoid diffraction with the camera which utilises a very small sensor.

The results were dramatic, and not in a good way.

If I were to say 'soft focus' footage which was much, much closer to 360p than 1080p, you will have some kind of idea of the world of pain I went through.

I should have known better, resorting to an ND filter instead of stopping down.

I have sine burrowed down deep into the custom functions of the Canon XF305 and have discovered you can limit the aperture to F9.5 (fractionally not far enough, as beyond F8.0 diffraction well and truly kicks in, it would be great if they would change this or allow this to be user set)

Lesson (re)learned the hard, costly way.

Why am I talking about this?

Today I received an email link from Phase One software whizz Niels V. Knudsen better known as the Image Quality Professor

Niels shows some really great samples of diffraction at work and suggests using Neutral Density filters instead of stopping down to really small apertures.

I heartily recommend this blog even if you don't use the excellent Capture One software, as he shares a whole wealth of knowledge on a regular basis in an easy to understand way.

Beware too if you go down the ND filter route, particularly some brands of vario ND filters which rob so much detail that they should really be rebadged as soft focus filters.

After much pain on the rocky vario ND road (with various brands) I switched to the old school, but super sharp, Lee filters (who can only really be criticised for sometime poor availability of their products)

Another tool at your disposal is low ISO which seems really unfashionable amongst camera manufacturers, and I really can't fathom why.

Until recently you could not shoot below 200ISO on some Nikons and even now most Canon and Nikon camera's, only allow very low ISO in 'expanded' setting, as often it delivers inferior colour graduation  compared to 'standard' ISO selection and poorer dynamic range, thanks to its lower colour bit depth

I will be talking more about lower ISO in the very near future, where shooting ay the very low native 35ISO of the Phase One made a world of difference to my latest forest shoot.


Libby said...

Drew thank you loads for that link by Niels - That is a HUGE wake up call for me. I only do outdoors night shots a few times a year really. This helps explain a few things on a night shoot I had last summer.

And I am going to do some experiments with my adjustable compacts too - according to the Cambridge calculator, I should really be shooting at f/4 if it's feasible. I never thought of the sensor size coming into play.

I've been through the diffraction issue with friends who have APS C DSLRs who were trying to shoot wildlife at f/22 (because the forums told them that would be the best DOF LOL)

The whole picture with regards to the diffraction issue has lit a major light bulb in my head. Many thanks, Happy Easter -)

Unknown said...

Hi Libby,

It is an interesting factor which many shooters do not take into account.

A fine way to cause your shots to not be as sharp as they may have been.

Have a great Easter

Unknown said...

Hello Drew,
Thank you for your excellent post and links. This is certainly an eye opener and a reminder for me. I am aware of low quality image issues as I have fallen into the trap of using very small apertures before but then I'm not completely happy with my ND filters, as feel they take away detail too. Time for more shooting and analysis methinks.
Happy Easter!

Good old Clive said...

Hello Drew,
Illuminating article, I was aware of the effect of stopping down but never interested enough to delve deeper. Having spent years in an engineering environment anything slightly geeky puts me in a coma. Having said that a decent understanding of the theory improves results without necessarily turning us into pixel peeping obsessives. Looking forward to the next images from your forest series.

Unknown said...

Diffraction is a much bigger factor than most people realise.

As says 'It limits the total resolution of your photography no matter how many megapixels you have'


Good old Clive said...

Apologies for the late (additional) comment. Do I understand correctly that pixel size impacts on diffraction and if so does that have a negative effect on the D800 as opposed to the D700?

Unknown said...

Hi Clive,

Now, I do not profess to know the answer to this but I would guess it is pixel size in relation to sensor size is a factor, so a D800 would not be worse than a D700.

Read up if you have time....and the inclination?