Fast lenses, and High ISO

Started 4 months ago | Discussions thread
mosswings
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Re: Fast lenses, and High ISO
In reply to Chikoo, 4 months ago

Chikoo wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses?

Some have suggested "bright" lenses.

In the days of film, large-aperture lenses were coveted in no small part because film sensitivities were so low: ISO 400 was a very fast film stock, and the images it produced were often grainy and blah. Kodachrome 25 or 64 were great, but you had to use a tripod in anything but bright conditions.

Today, digital's insanely high ISO capabilities have made slower lenses (f4-f5.6) practical general purpose options and have opened up existing-light photographic possibilities that didn't exist previously. One must remember that every increment in ISO reduces the DR, SNR, and color rendition quality of the captured image, so that there is still a practical maximum ISO for any desired image quality. DXO uses a standard of 10 stops of DR for acceptable IQ, and this is reached in modern sensors at around ISO 800-3200 depending on sensor size and manufacturer.

Fast lenses have always brought to the table generally better imaging performance at all apertures than that provided by the f4-f5.6 general purpose kit lenses. But most casual photographers are unwilling to accept the size, weight, and price penalty for that improved imaging performance, so ISO is a godsend for the camera manufacturers, as it enables them to offer practical utility at a tolerable price and size.

Yes, fast lenses offer shallower DOF, but they offer the serious photographer signficant advantages in what counts - image quality, AF speed, and durability. I should note, however, that it's quite possible to make an extremely high quality slow lens. Consider the Nikon 70-200 f/4 and the 70-200 f/2.8. The f/4 meets and in some cases exceeds the quality of the f/2.8, long recognized as a benchmark of IQ, but in a smaller, lighter, and more affordable package. It is limited in its light gathering capability, so you won't find it at nighttime sporting events or in the most demanding situations. It is also 5 times the cost of the 55-200 f/4-5.6, but you are paying for the visibly better image quality.

For those applications where DOF needs to be deep - such as landscapes - a slower lens stopped down to f/8 will perform quite well. Landscape photogs can get away with a slower "consumer" lens...but they benefit from the IQ of the higher end lenses.

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