This snake-in-the-grass camera setting is secretly ruining your manual photos

If you’re shooting in manual mode but you still find yourself wrestling with your camera automatically changing the exposure, you need to check your ISO setting. If it’s still on auto, you’ve found the problem.

Auto ISO is a fabulous invention – if you are using auto exposure modes. What it does is adjust the ISO, shot by shot, to maintain a minimum ‘safe’ shutter speed to prevent camera shake even in really low light. You can specify the maximum ISO you want the camera to go to, and the minimum shutter speed you want it to use. It’s brilliant.

But if you don’t switch auto ISO off when you swap to manual exposure mode, something else will happen. The camera will keep adjusting the ISO to give you what it thinks is the correct exposure with the manual settings you’ve chosen. You’re in manual mode, but you’re still getting auto exposure, this time via the ISO setting. What the heck?!

This is especially annoying if you're doing any night photography in the city. You want manual exposure control and a low ISO for quality and long-exposure times to capture light trails. You don't want the camera to bump the ISO through the roof in search of a fast shutter speed.

ISO and the exposure triangle

These days, people talk about the exposure triangle, which consists of shutter speed, aperture, and ISO. Manual mode puts shutter speed and aperture directly under your control, but you also need to select a manual ISO setting to finish the job.

Really? It’s true. It’s easy to switch your camera to manual mode, but that doesn’t automatically set the ISO to manual mode at the same time. You have to do that with a separate button, dial, or menu setting. If you don’t, manual mode will only do half a job. It will let you set the shutter speed and the lens aperture manually, but it will still use automatic ISO adjustments to get the ‘correct’ exposure. You might think manual mode will give you full control over the exposure, but it won’t, as the camera is simply making decisions behind your back.

Some cameras even make a feature of this. Pentax’s TAv mode (Time and Aperture Value Priority) is found on the best Pentax cameras and is designed so that you can set whatever shutter speed and aperture you like, and the camera will use ISO to adjust the exposure (it’s more effective in low light, where the ISO setting comes into play, not so much in bright light).

For most people, the whole point about manual exposure is not just that you can change the shutter speed and aperture settings directly, but that you can control the brightness of the image at the same time. And for this to work, you need to set the ISO manually.

Personally, I don’t much like using ISO for shot-by-shot exposure control in manual mode. I think it just confuses things. I think it’s much better to choose an ISO setting appropriate to the conditions and then adjust the exposure using shutter speed and aperture alone. That already gives you plenty to think about, whereas one more exposure variable – ISO – just tips things over the edge. If you realise your ISO setting is wrong for the conditions, then sure, change it, but don’t treat it as a third exposure adjustment for every single photo.

So remember, true manual control requires choosing your ISO manually, not just your shutter speed and lens aperture. Otherwise, if you leave auto ISO enabled, you're just using a different version of auto-exposure.

You might also like...

For more of my thought-provoking photography tips, "Limitations" boost photographic creativity – here's why I ditched RAWs and zooms, and embraced JPEGs and prime lenses. If you're looking for your first serious camera, check out the best camera for beginners and the best cheap cameras.