Why do you buy fast lenses/cameras?

Started 3 months ago | Polls thread
OP Lee Jay Forum Pro • Posts: 54,790
Re: well

Michael Fryd wrote:

Lee Jay wrote:

tko wrote:

Michael Fryd wrote:

tko wrote:

trungtran wrote:

If i want better IQ in the low light, i use a bigger sensor

Bigger sensor don't gather more light, but bigger lenses do.

In terms of the lens, the angle of view and aperture diameter are the two main factors in how much light you gather. Generally, for a given angle of view, it's easier to find a lens with a large aperture for larger sensor cameras.

The OP implied he used a bigger sensor instead of of a bigger lens. They tend to go tougher, but you still need that faster lens.

At the same f-stop, a bigger sensor is faster but the reason it's faster is that at the same angle of view the longer focal length lens has a larger entrance pupil.

At the same aperture diameter and same angle of view, the light captured is independent of sensor size.


When we compare a crop body to a full frame, we seldom compare at the same focal length, as the result of the focal length depends on sensor size.

I do that comparison, frequently, because I'm focal-length-limited or using a prime.

Why would you want to compare at the same f/stop?

Because I can mount my crop and full-frame bodies to the same lenses.

The advantage of same f/stop is that we can maintain the same light per unit area on the sensor.


But different sensor sizes give us different noise levels at the same light per unit area. That's no more a fair comparison that comparing at the same focal length.

Lenses are a real-world limitation.  I have a 70-200/2.8, but I don't have a 320/2.8 to put on my full-frame camera to compare to.  It would win easily if I did, but I don't.

If you want to compare at the same camera settings, use the same settings; same focal length, same f/stop, same ISO.

I don't have a reason to do that.

If you want to compare when you want similar results, then adjust the focal length to yield the same angle of view,

But real-world lenses can prevent that.

use the same aperture diameter,

My telescope doesn't have an aperture actuator.  The entrance pupil diameter is fixed, regardless of angle-of-view.

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Lee Jay

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