MY THOUGHTS ON ‘THE F-8 THING’
Have you seen articles, videos or reviews that tell you your lens (or lenses in general) might not be perfectly sharp with the aperture wide open? If you read a lot of articles or watch YouTube videos you are bound to come across this. It seems that after extensive testing, most reviewers will come to the conclusion that a lens will be at its sharpest at around f-8. In fact the f-8 conclusion is so common you could almost wonder why they need to do all that testing!
So, what does it mean for your photography and does it mean you should always be shooting at f-8? Here are my thoughts on the subject.
A smaller aperture reduces the amount of light hitting the sensor, and therefore reduces exposure. To maintain the best exposure, you can compensate by slowing down the shutter speed or increasing the ISO. Both of those options will increase the amount of light hitting the sensor.
Let’s say you can take a correctly exposed photo shooting at f-2.8, at 200 ISO with a shutter speed of 500th/sec. If you felt you needed to change the aperture to f-8 (because the experts say you should) then you reduce the light by (in the old terms) three ‘stops.’ You need to get that light back and you can do it by increasing the ISO to 1600, or reducing the shutter speed to 60th/sec. This would satisfy the f-8 ‘rule’ and allow you to maintain your correct exposure.
That’s all good so far. But when you slow down the shutter speed you increase the chances of a blurry photo – which can be caused by a moving subject or simple camera-shake when using a telephoto lens. If you increase the ISO you risk seeing increased noise in the photo, which can affect the quality of the overall image.
So strictly adhering to the f-8 thing may actually cause you to sacrifice sharpness or image quality. And what good does it do you to know that your lens is sharper at f-8, if your photo is blurry because the shutter speed is too slow?
Don’t make the mistake of thinking that because you ‘read somewhere’ that your lens is sharper at f-8 that that’s how you should always be using it. Generally speaking we like to keep the ISO low, and when shooting action (or when shooting hand-held with a large telephoto lens) we need to keep the shutter speed fast to eliminate blurring. And the best way to be sure of that is to have the aperture wide open.
The f-8 thing shouldn’t be ruled out. I use it from time to time but only when the situation is suitable. When shooting wildlife I will sometimes shoot at f-8, as long as it is a bright sunny day and I can still keep my shutter speed fast enough. I will also sometimes use it for landscapes or other stationary subjects, either in good light or when I can use a tripod to prevent camera shake (allowing me to shoot at f-8 and still keep the ISO at 200).
For the record, the waterfall example on this tip was taken at 200 ISO, f-8 and 1/6th second exposure. Even though my lens is renowned for being pretty sharp throughout the aperture range, I closed it to f-8 ‘just to be sure.’ But I was using a tripod and I was quite happy to let the slower shutter speed capture a bit of motion blur in the water.
So the f-8 thing is definitely something to consider. But use it wisely and with proper judgement about the other compromises you might have to make. It will help your photography at times, but could very well hurt it at others.