What's so great about f/11?

October 15, 2017  •  Leave a Comment

What's so great about f/11?

Someone recently noted that a lot of my images are shot at f/11. Not all of them to be sure, but enough to make this person notice and ask me why.

It's not an accident, it's a result of a combination of research and experience with my particular camera and lenses.

When I first started out I read some books on landscape photography and got some very basic advice about aperture and depth of field.

The advice was, for a wide depth of field, to have sharpness all the way from the foreground to the background, use a small aperture (a high f number) and for a small depth of field, to blur the foreground or background or both, use a large aperture (a low f number).

Of course it's not quite as simple as that.

For a start the only part of the image that is truly in focus is the point that is focused on. Then there are areas in front of that point and behind that point that are considered 'acceptably sharp'. The size of these areas is dependant on the focal length used, the aperture, and how far away the focus point is.

Then there is the issue that smaller apertures can result in diffraction that can cause a general softness in the image.

All of this then leads on to the concept of Hyperfocal distance. Hyperfocal distance has some mathematical formula behind it but what it means is that, for a given sensor size, focal length and aperture, there is a point that you can focus on that will mean that everything from halfway to that point all the way to infinity will be 'acceptably sharp'.

So, for example. A 20mm focal length on an APS-C (1.5 x crop) sensor at f/8 has a hyperfocal distance of 8.2 feet (2.5 metres). If you focus on a point that far away then everything from 4.1 feet (1.25) metres to infinity will be acceptably sharp.

This means that you can calculate exactly what aperture is necessary to get the depth of field required and exactly how far away to focus.

There are plenty of charts that you can download from the net and, for a while, I carefully calculated the hyperfocal distance for my shots so that I could use the optimum aperture to get what I needed in terms of depth of field without having to worry about diffraction.

Of course the challenge is that the distances were all estimated. I didn't go out with a tape measure and check exactly how far away object were and exactly what the distance was that I was focusing on.

Then there's the problem of defining what is acceptably sharp. I'm not even going to get into that definition and the 'circle of confusion' in this post, maybe in a later one if I'm feeling like a challenge.

I also found that by using the hyperfocal distance charts I could often achieve the required depth of field using much larger apertures like f/5.6 or even f/4.

And this is another challenge because most lenses are not at their sharpest when they are wide open either. In fact most lenses have a 'sweet spot', an aperture when they are the sharpest they can possibly be.

I did research on the lenses that I use most often in my landscape photography. According to most information I could find they should all be at their sharpest at around f/8. But each lens, and each lens and camera combination can be different.

So I did some tests.

I took images at a variety of apertures. I made sure that the subjects were within the depth of field range for the focal length and aperture chosen and I shot them in the same light and used a tripod, cable release and mirror up mode to make sure they were all the best they could be.

I then reviewed them at 100% in Lightroom to see what differences I could make out.

I found that the lenses I was testing were sharpest at between f/8 and f/11. There was no noticeable difference on either of my main lenses between these two apertures.

Larger and smaller apertures than this did show some, very very minor, softness so both f/5.6 and f/16 would produce slightly 'softer' images.

So, for optimum sharpness I should shoot at either f/8 or f/11.

So why f/11?

Given that there is no noticeable difference between the two apertures I choose to shoot at f/11 to give me some extra leeway on depth of field, just in case I misjudge how far away the closest point in the frame is or how far away I am focusing.

Shot at f/11 for maximum sharpness and depth of field

That doesn't mean that I won't shoot at other apertures.

If I feel I need a little extra depth of field I will shoot at f/16, I may also use f/16 to get a sun star effect or to achieve a slower shutter speed.

Shot at f/16 to get the 10 second exposure I wanted

Likewise I may shoot with a larger aperture if I feel that I need to increase shutter speed (and can still get the depth of field I need).

Shot at f/8 to increase shutter speed because I was shooting hand held and with a long focal length

The difference in overall sharpness is so minor that I don't worry about it.

Having f/11 has my default starting point allows me to focus my attention on the creative aspects and making sure that I get the composition right.

Once I have the composition how I feel it should be then I will consider if my vision for the image would be better served by changing the aperture. If it is then I change aperture. If it isn't then I leave it at f/11 and shoot away.

Of course this is with my lenses and my camera. Other equipment combinations may have very different results and that's why I would also do the tests again if I changed any of the equipment. Even if I was changing to the same make and model I would still want to check for that specific set of equipment. Plus, there are still some occasions where I really want to keep my aperture from getting too small and where I know I will be pushing the depth of field limits.

For those occasions I still carry and refer to a Hyperfocal distance chart (or use the Photo Pills app) in order to be more precise.

So, what's the tip here?

Put in a bit of work up front to understand how your gear performs best. Then you don't have to worry about it so much in the field and you can get on with just taking pictures        

I hope you've enjoyed this short tip. 

If you haven't already, head over to Youtube and check out my Landscape Photography videos 


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