AND REW JAMES

Your tricky pho­tog­ra­phy ques­tions an­swered!

Digital Camera World - - CONTENTS - AN­DREW JAMES Our ex­pert photo ad­vi­sor is here to help Josie Hall Pat Mal­lard

Our pro de­mys­ti­fies key ter­mi­nol­ogy, of­fers in­spi­ra­tional tips and more

Per­plexed by a pho­to­graphic prob­lem? caught up in camera con­fu­sion? Send your ques­tion to dig­i­tal­cam­era@fu­turenet.com 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.

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