F numbers explained

This is F1.2 and as you can see the background is very out of focus leaving the subject completely isolated. This also gives a 1/1000th shutter speed which is a good advantage of using a low F number - you'll get much higher shutter speeds so if you want a photo with good subject isolation you will able to do so at a low ISO which will also give you better image quality.

It's important to note you have to be VERY precise with your focus at F1.2 to F2 as the depth of field is very thin so anything that isn't the exact same distance as your focal point will be very out of focus, especially at F1.2 !

This is F5.6 and notice now that although the subject is still isolated the background is much more recognisable.

You'll also see the huge difference using F5.6 has made to the shutter speed. It has dropped from 1/1000th to 1/50th which at 85mm handheld you may have to increase the ISO to get a faster shutter speed to reduce the risk of blurred images. This will always vary depending on how still you can keep your hands!

At F5.6 you can will also have a less shallow depth of field so your chance of portraits being completely in focus will be much higher than at F1.2 - F2.5.

This is F11 and now the background is much more in focus and everything is much more easy to identify than at the lower F numbers. The ISO has had to be vastly increased from ISO 200 all the way up to ISO 1000 meaning the image noise on some camera bodies will be more noticeable. Again the shutter speed is still only 1/60th so you may find yourself having to increase the ISO even more to get the speeds you need to reduce the risk of a blurred image.

At F11 you will have a much deeper depth of field to play with at the cost of not having as much subject isolation as you would at lower F numbers.

I would recommend F13 - F16 when you want to take a photograph of a person with a landmark or a background you dont want to miss ! Also note that at higher F numbers if you have dust on your camera's sensor they will be visible where as at lower F numbers sensor dust may not be so evident.

It was a relatively overcast gloomy day when these photo's were taken. On a bright sunny day you'll be able to get much better shutter speeds at higher F numbers. We will cover that now as the difference a sunny day ( or a place with good lighting ) can make. Also in this example we can look at the difference between isolating a single object ( above ) and isolating a part of something.


A nice bright sunny day ! And you'll see the huge difference it's made to my shutter speed and ISO ! At F1.2 I've had to turn off my highlight priority as the shutter speed was going way beyond 1/8000th and over-exposing too much. This is the only downside to a very fast lens - sometimes they can let in too much light ! So to fix this problem you can decrease the ISO to allow you to shoot at F1.2 and in this example you can better see how thin F1.2 really is ! The bunch of flowers are close together but at F1.2 I can now easily isolate the front flower from the one just behind it. The background bokeh is very evident and if you scroll down and click on the F16 example you'll see just how much the difference in isolation is. If you want to see it gradually then go through each image in order to see the changes in F number, shutter speed, ISO and bokeh.























































Now we're getting to the point where the shutter speed is getting too low to handhold at this ISO so if I'm happy with the way the isolation and bokeh looks I can shoot with these settings until the light changes. If I want even more detail in the background then I'm going to have to increase the ISO to get the shutter speeds I'll need. You might also notice the image itself is getting sharper - this is common in many lenses F8 usually being the sharpest but it varies from lens to lens.











































Here at F16 even on a bright sunny day you can see just how much I've needed to change the ISO to get the shutter speeds I need to capture all the extra detail from the front of the image all the way to the back. The ISO needed to be increased from 50 to 400 and this is still giving me a barely usable shutter speed of 1/160th - luckily I was kneeling down so I had more stability. If I was shooting standing up and handheld I'd have needed to go to ISO 640 just to be safe. The reason the background is still slightly out of focus is because I was very close to the flowers in the foreground and am still using the front flower as my focal point. If I wanted everything in focus, I'd take a few steps back and use all focal points, not a single focal point ( or if using a zoom lens I'd zoom out ).

So to wrap things up :
The lower your F number is, the faster shutter speeds you'll get and more subject isolation and vice versa higher F numbers will give you slower shutter speeds and less subject isolation.