Those dreaded f/numbers...let's go...

So the latest blog articles I published were for one of two purposes. They were either a wrap-up to consolidate one-on-one sessions with clients or an attempt by me to create order in my own thoughts. I am not sure if that second option is always successful.

OK, so we have been dealing with the "where-to-start" thoughts of exposure calculation. Basically how to understand and balance the 3 concepts in photography that we needed to get a good exposure. They are the ISO setting, the shutter speed setting and the lens aperture setting.

What we are dealing with here for now are not effects regarding the appearance of the image. We are still purely working towards capturing an image that is not overly bright and over-exposed or overly dark and under-exposed.

A quick revisiting of ISO

ISO numbers were fairly easy. They are a linear set of numbers. A low setting like 100 implies low sensitivity to light in the camera sensor and so plenty light needs to be available. Remember midday sun on the beach?

A higher number such as 800 or 1600 gives a much higher sensitivity of the camera's sensor when light is low. Think sunset or shortly afterwards.

I wish for you to spend a few minutes considering the classic ISO numbers as follows:

100 - 200 - 400 - 800 - 1600 - 3200 - 6400

ISO button on Fuji X-T3

Yes, well spotted, there is no ISO 100 option on Fuji's X-T3 ISO button. The first "proper" number is 200. We will look into that in future but not now.

Now there are higher numbers such as 6400 and beyond. These high ISO settings could help you get the shot in very low light but there is a price to pay in image quality. In a future article we will dig deeper into that. For now it might be worth knowing that ISO 800 is probably the upper limit for most cameras before image quality becomes compromised. We need this ability of the camera to increase the sensor's sensitivity to light. The problem is that a higher sensitivity, despite the benefit of a faster shutter speed, will begin to compromise image quality by introducing noise into the image.

Here follows a quick example of getting it wrong with noise.

Getting a new puppy is a joyous occasion and you want to get a million pictures, right? Looking at this random image the overall exposure is OK but it does not take long to notice that something is very wrong here.

This is an absolutely off-camera image shot at ISO 3200. Many other things are very wrong here but for now let us simply consider the noise.

This is why it is always good to keep the ISO setting as low as the situation allows. More about that in future.

A quick revisiting of Shutter Speed

Shutter speed is the controlled duration of exposure during which the camera's shutter is open. It is expressed in fractions of a second. You can expect to see numbers such as 1/60 or 1/1000 and many many more.

Shutter speed selection knob on Pentax K-1000

Looking at the selector knob on the K-1000 in the image above, the shutter speeds were rather limited back in the day.

I wish for you to spend a few minutes considering the classic shutter speed numbers as follows:

1/2 - 1/4 - 1/8 - 1/15 - 1/30 - 1/60 - 1/125 - 1/250 - 1/500 - 1/1000

Shutter speed 1/1250 - ISO 1000 - f/4

Looking at this golf image the settings speak for themselves. Reasonably good cloudy daylight and ISO 1000 but with a shutter speed at 1/1250 the motion is still not frozen.

And now for those dreadful f/numbers

The third leg of the tripod is aperture setting and I will be spending some time unlocking this headache.

The information on a 50mm f/2 Pentax lens on a K-1000 35mm SLR film camera

The reason why it is important to understand all three is actually quite simple. There is a three-way balancing act in there. At first we need to understand this three-way balance to get a good exposure. Once we have that down we can start looking at the effects we wish to get by using this three-way relationship to our advantage.

The most basic of exposure control is what is known as the f-stop. The f-stop implies a doubling or halving of the exposure either in time or quantity of light. Getting your head around it is the interesting bit. ISO settings on the classic numbers makes sense. Shutter speeds make sense quite easily too. But those aperture numbers...

We see numbers that make no sense at a glance because there is no natural progression...or so it would seem. Why does it make sense that 4 follows 2,8? Why is f/16 half the light of f/11? Why does f/4 give me double the quantity of light that f/5,6 would?

I wish for you to spend a few minutes considering the classic f/numbers as follows:

1,4 - 2 - 2,8 - 4 - 5,6 - 8 - 11 - 16 - 22

Here follows a seriously nerdy explanation for those who wish to know it...

50mm lens at f/2

The whole f/number thing makes sense once we understand that it is a mathematical formula involving the area of a circle and it is closely related to the focal length of the lens in millimeters. Looking at the above image we see a 50mm lens with a maximum f/number of 2. It will happily stop down to f/22 with a great number of intermediate stops.

Wide open at f/2 we can start to demystify the formula as follows:

50mm at f/2 means the aperture opening is a diameter of 25mm. How did we get to that? Simple really - f (focal length of 50mm) - therefore, 50mm divided by 2 = 25mm.

Let's take this one step forward. the same lens at f/8 will give you a theoretical aperture of 6,25mm in diameter and that would look like this:

50mm lens at f/8

Let's take this to the extreme and stop down to f/22. Now we have the same lens with a (THEORETICAL!) aperture diameter of around 2,273mm. That looks like this:

50mm lens at f/22

So, big f/number gives a small aperture and small f/number gives us a large aperure. This is all fascinating but how do we use this information?

Let's return to reality from planet Nerd...

If we need a double-or-half adjustment to match the ISO and shutter speed adjustments it becomes worth knowing that the size of the aperture diameter at f/ 5,6 will have half the AREA of the aperture diameter of the exact same lens adjusted to f/4.

Half the area means half the quantity of light over any constant period of time.

Similarly, the very same lens set to f/5,6 will have an aperture size double the area as opposed to the same lens at a setting of f/8.

Double the area of the aperture inside the lens means double the quantity of light over any constant period of time.

Let's look at some real life examples to bring all of this together...

ISO 200 - 70-200 lens at 190mm - f/8 - shutter at 1/125

ISO 200 was good for bright daylight conditions, f/8 was a middle-of-the-road choice and a shutter of 1/125 at focal length 190 was not the way to do it. On a personal note I find the placing of the aircraft in the image pleasing.

ISO 200 - 70-200 lens at 190mm - f/8 - shutter at 1/100

This one worked...well, sort of. For one the aircraft is way too far left in the frame. Closer scrutiny will reveal some blur but at 1/100 shutter speed that is hardly surprising. Sometimes you are lucky...

ISO 200 - 70-200 lens at 140mm - f/4,5 - shutter at 1/2000

Here the shutter was simply way too fast. The rotor is completely frozen and there is a little bit of motion blur in the tail rotor. At f/4,5 and the distance away depth of field was no problem.

ISO 640 - Shutter at 1/1250 - 70-200 at 200, f/8

There is the risk of losing the detail and clarity when freezing the motion. Personal preference will dictate.

OK, enough trawling of the archives for one evening. I trust the above images will give some ideas of where to go with all these settings.

Until next time, remember to buckle up or you might lose more than a shoe...

Happy shooting,

Anton