Practical Photography - UK (2020-02)

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Know Your Stuff

What does effective

fo callength mean?

My indoor DSLR photos

look extremely orange,

unlike indoor shots taken

on my iPhone. Help!

Chris Sanders, Brighton

Kirk says: Most indoor spaces are

lit with incandescent bulbs. The

colour temperature of most bulbs

is usually significantly warmer than

daylight to create a cosy feel. This

isn’t particularly obvious with the

naked eye as our brains cleverly

adjust how we perceive colour. On

a camera, though, the difference

can be huge. To combat this, you

need to alter the white balance,

which instructs the camera to

compensate for any colour cast

created by the lighting in your

location. The idea is that a pure

white object should then look

white even under non-white light.

Your camera’s auto white balance

does a good job of this, but for

best results choose the tungsten

or incandescent setting. Then

don’t forget to switch back before

going outside or your photos will

be very blue.

Why are my

interior shots

so orange?

Are vibrance and saturation the same?

I never know whether to use

the Vibrance or the Saturation

slider in Photoshop – they

seem to do the same thing.

Lewis St. John, Acton

Tim says: Photoshop’s Vibrance and

Saturation sliders are indeed similar

in that they both increase or decrease

the strength of the colour in an image.

But there is an important difference.

Saturation increases or decreases the

intensity of colour equally across all

pixels, whereas Vibrance applies the

effect to a greater degree the less

saturated a pixel is. By doing this it

stops already strong colours from

over-saturating. It also intelligently

ignores skin tones to avoid that

‘fake tan’ look. This makes Vibrance

a smarter tool, and therefore the

go-to choice for most images.

I understand what focal length
is but I don’t get the term
‘effective focal length’. Could
you shed some light on this for me?
Kerry Easton, Sheffield

Adam says: The focal length of a lens
describes its angle-of-view. A 14mm focal
length is considered wide-angle because
it has a large angle-of-view, so is ideal for
landscapes and interiors. A 600mm lens
is considered telephoto because it has
a narrow angle-of-view, so is ideal for
wildlife and sports. The focal length of
a lens remains constant regardless of the
camera it is mounted on.
But if you use a full-frame lens on an
APS-C DSLR, you’ll notice that it appears
more ‘zoomed in’ than on a full-frame
camera, as in the image above. How can
this be? The answer is that although the
lens’ angle-of-view doesn’t change, the
smaller sensor means a smaller area of
the light that the lens is collecting is used

to form the image. In other words, the

outside edges of the light a full-frame

sensor would collect are cropped off.

So a 20mm lens appears to have a tighter

angle-of-view on an APS-C body, and

therefore a longer focal length (around

30mm). This is known as the ‘crop factor’

and non full-frame camera owners should

bear it in mind whenever they buy a lens.

To calculate the crop factor on APS-C,

multiply the focal length stated on the

side of the lens by 1.5x, or on Micro Four

Thirds by 2x. For example, a 10-20mm

lens will behave like a 15-30mm on an

APS-C body and 20-40mm on MFT.

When someone describes the

‘effective focal length’, they have already

done the maths and are telling you what

it will appear to be when mounted on

a particular camera. The word ‘effective’

is used because the lens’ actual focal

length remains the same – it’s purely

the sensor size that effectively changes

the angle-of-view.

20mmLENS/ FULL-FRAMESENSOR

20mm LENS / APS-C SENSOR