Sunday, June 16, 2013

Lenses for Light, Lenses for Dark: Little-known Secrets

If you remember my previous article, describing my thought process behind getting a cheap Nikkor tele-zoom, you probably remember what I said about lenses for particular needs:
I wanted to get a very cheap tele zoom that would allow me to take reasonably decent photos in plenty of daylight. That made my life much easier, as it meant I wouldn't have to go for the big and expensive f/2.8 zooms
 Well, I realized that this is an issue important enough to merit its own separate article. There is a great deal of misunderstanding going around regarding lenses and image quality, and I want to help you! I want to help you get the lens you really need, without paying a fortune. Beginners (and not only) are under the impression that a fast, big, heavy f/2.8 tele zoom is always much better than a consumer, slow tele zoom. That is simply not true! It's all about light.

When there is plenty of light (say, taking photos in bright sunshine), it's easy to achieve great image quality - this applies to cameras as well, by the way. Only advanced users would be able to tell apart photos taken with a compact from those taken with a DSLR. Nobody, not even Nikon's head engineer probably, would be able to differentiate between cropped- and full-frame sensor.

But back to lenses: When you have plenty of daylight, you have plenty of latitude in terms of aperture and shutter speed selection. Let's see with some examples, using an exposure calculator such as this one.

Let's say you are taking photos in bright sunlight - a typical summer day.
For an ISO 200 (a typical base ISO setting for many digital cameras), you would have the option to use f/11 and 1/500, or f/8 and 1/1000. That is well above what you need for the vast majority of lenses, including non-VR ones, to use hand-held. An aperture of f/11 means you have the ability to stop down 2 full stops on the long end of a typical consumer tele-zoom - either 200mm f/5.6 or 300mm f/5.6. Stopping 2 full stops and with a shutter speed of 1/500, makes all the difference.

You see, most people think consumer zooms are inherently low quality. They think that an expensive, f/2.8 zoom is always, under all circumstances better. Actually, there are cases that the reverse is true! (We'll get to that in a moment, be patient). In reality, all other things equal, few people (and very few if any beginner/intermediate photographers) would be able to tell which lens the photo is taken with. Let's run an experiment...

Of the photos below, one is taken with a 50mm f/1.8 prime, the other with a cheap consumer lens, the Nikkor G 28-80mm. Can you tell them apart?

The left one is taken with the 50mm f/1.8, the right one with the zoom. For experienced AF-D 50mm f/1.8 users, the bokeh should give it away. But other than that, there's very little to suggest a difference - especially in favor of the prime! The photo on the right is contrasty, sharp, saturated...

I spoke earlier of the consumer lenses being sometimes even better than the fast and more expensive ones. Well, that's true! You know why? Because of diffraction. At f/11, you are 2-stops down from a consumer midrange zoom that is f/5.6 at 55mm - such as the very good AF-S 18-55mm. But you are 5+ stops down with the 50mm f/1.8! Fast aperture lenses are optimal 2-3 stops down from their maximum aperture - The 50mm f/1.8 is optimal at f/5.6, with the f/8 being practically indistinguishable. But check out the 100% crop of this photo, taken with a 50mm at f/11:

The focus was on the old lady. But the sharpness and acuity just isn't there. Compare it with a shot taken with the same lens at f/4, and you will notice that it's actually sharper at the smaller aperture!

The conclusion is inescapable:
In daylight, when there's plenty of latitude with aperture and shutter speed, not only are you getting no extra quality with an expensive f/2.8 (or faster) lens, you actually sometimes lose in quality.

So, when do you need expensive and fast lenses?
- If you shoot in difficult lighting conditions. If you expect heavy overcast and you need the speed, or if you shoot in evening light, or indoors, then the fast lenses are miles ahead of the slow consumer ones.

- If for some reason you need a lens than can take a beating. Consumer lenses are cheap plastic, they can break easily. But seriously, this is an insignificant reason for 99% of you reading this article.
- If you need some functionality not found in a cheap consumer zoom - say, full manual-focus override, or better VR. Again, these are very secondary reasons, and in my opinion they don't justify the price penalty.

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