AND REW JAMES
Your tricky photography questions answered!
Our pro demystifies key terminology, offers inspirational tips and more
Perplexed by a photographic problem? caught up in camera confusion? Send your question to firstname.lastname@example.org and let us provide you with a solution
What does the term ‘35mm equivalent’ actually mean?
This is a common question for newcomers, Josie, so you are not alone with this
confusion. Basically, it is all linked to the fact that digital cameras have different-size sensors. If all sensors were the same size, we wouldn’t really need the term ‘35mm equivalent’, which essentially gives us a standard measurement to work from. 35mm dates from pre-digital days: it was the physical width of the most commonly used film, and was therefore adopted as the standard field of view.
No doubt you’ve seen the term used in a camera specification. For example, with a camera with a built-in zoom lens such as the Nikon Coolpix B500, it will be sold as having a 40x zoom with an equivalent focal length in 35mm terms of 22.5 to 900mm. The 35mm equivalent measurement just gives us a sense of its range from wide to very long! It’s a useful guide to those of us ‘old’ enough to still think in 35mm terms.
When it comes to buying a lens for a DSLR, understanding how a camera’s sensor affects focal length is really important. A full-frame camera has a sensor that matches this standard size, and therefore the term 35mm equivalent isn’t needed when considering a lens: if you put a 50mm lens on a full-frame camera, the focal length remains 50mm.
However, many DSLRs and CSCs with different sensors have a crop factor, such as 1.5, 1.6 or even 2. Therefore when you put on a lens, its stated focal length has to be multiplied by that crop factor to give you a real-world focal length. Therefore, with a crop factor of 1.6, a 50mm lens is in fact 80mm. This is what we call its effective focal length. You might not always see the term 35mm equivalent being used in this context, but you might see other terms: for example, with a Nikon lens, the term DX equivalent might be used.
Extending this example, a Nikon 14-24mm f/2.8 lens would be 21-36mm in DX terms; DX signifies that if used on a crop-sensor Nikon camera, the focal length has to be multiplied by 1.5 to know its effective focal length.
What does the term selective focus mean?
As far as I’m concerned, Dave, it simply means ensuring that you are focusing on the point within the frame that
you want the sharpest. You do this by carefully considering which autofocus point to use, rather than allowing the camera to make that decision for you and by your choice of aperture.
Typically this means you will choose a large aperture, such as f/2.8 or f/4, which will ensure that while the important area of your image is sharp, other areas will be out of focus. By doing this you are showing the people who view your image what the most important part of the frame is, and therefore directing where they look. Generally this is best done with a medium-to-long telephoto lens, although you can also do it with a fast standard lens such as a 50mm – or, of course, a macro lens.
Shallow-focus portraiture is an obvious example of selective focus. The photographer chooses a wide aperture and focuses carefully on an eye, with the result being the face is the sharpest point in the frame but that sharpness drops off significantly behind the subject. At an aperture of f/2.8 and with a medium telephoto lens, the eyes may be sharp but that sharpness will have significantly diminished from the subject’s ears, and the whole background should be a pleasing blur.
You can use selective focus to great effect for almost any subject – wildlife, floral, sport and even landscapes. It’s just a question of thinking about where to put that main point of focus and how a wide aperture will blur the rest of the image.
Full-frame cameras like Nikon’s D5 use focal lengths equivalent to old 35mm cameras, but other models require a crop factor conversion.