F ratio

Faster is better, right? The f -ratio of a lens determines how bright the image will be, and hence how much you can photograph in a short exposure. Lower f -numbers give a brighter image.

But there's a catch: speed is expensive. Nowadays, fast lenses are quite sharp but cost a lot of money. In earlier years, the cost was in performance, not just

Table 7.1 Field of view of various lenses with APS-C sensor.

Field of view

Focal _

Field of view

Focal _

Table 7.1 Field of view of various lenses with APS-C sensor.

length

Height

Width

Diagonal

28 mm

30°

43°

50°

35 mm

24°

35°

42°

50 mm

17°

25°

30°

100 mm

8.5°

12.7°

15°

135 mm

6.3°

9.4°

11°

180 mm

4.7°

7.1°

8.5°

200 mm

4.2°

6.3°

7.6°

300 mm

2.8°

4.2°

5.1°

400 mm

2.1°

3.2°

3.8°

price, and lenses faster than f /2.8 were not completely sharp wide open. Until about 1990, it was a rule of thumb that every lens performed best if you stopped it down a couple of stops from maximum aperture, but in this modern era of ED glass and aspheric designs, some lenses are actually sharpest wide open.

Because DSLRs do not suffer reciprocity failure, deep-sky photography is no longer a struggle to get enough light onto the film before it forgets all the photons. We no longer need fast lenses as much as we used to. Under dark skies, I find that f /4 is almost always fast enough.

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