?

stulid · 16 Oct 2015 at 17:52

If I have a 50mm f1.8 lens and took the same photo as with say a 55mm 5.6.

I then got the item/subject filling exactly the same amount of the picture on both lenses and compared images.

Would the 1.8 be image be brighter due to the larger aperture?

Andy90 · 16 Oct 2015 at 18:05

The focal length and the same angle of view / frame filling aren't important.

What is important is the amount of light hitting the sensor.

If you expose the sensor for exactly the same amount of time for both lenses, say 1/250th of a second and the brightness of the scene does not alter, then as the 50mm f/1.8 lens has a larger aperture than the 55mm f/5.6 it will let more light in for the duration of the 1/250th of a second. As a result the image will be 'brighter'.

D.P. · 16 Oct 2015 at 20:35

As Andy() says, the only things that are going to effect image brightness are Aperture, exposure time and ISO.

If you shot both at f/5.6 and had automatic expure time and/or ISO you will get identical image exposures (in theory, in practice there might be subtle differences due to things like vignetting).

Russinating · 16 Oct 2015 at 21:01

Is that entirely true? I can't remember which lenses I thought I spotted it on, but I'm pretty sure there's a difference between our 24-70mm 2.8 @ 70mm and 70-200mm 2.8 @ 70mm.

The quality and thickness of the glass surely changes how much light reaches the sensor.

Kei · 16 Oct 2015 at 21:22

That would be T stops rather than F stops. i.e light transmission stops. Most f2.8 lenses are around T3.1-3.3. No lens that I know of has both the same F & T stop, at best they are within 0.1 of each other.

stulid · 16 Oct 2015 at 21:27

Cool, yeah keeping manual iso/shutter the same, the lower f lens is going to be brighter in dull lighting

D.P. · 17 Oct 2015 at 12:46

Russinating said:
Is that entirely true? I can't remember which lenses I thought I spotted it on, but I'm pretty sure there's a difference between our 24-70mm 2.8 @ 70mm and 70-200mm 2.8 @ 70mm.

The quality and thickness of the glass surely changes how much light reaches the sensor.

There is a difference in transmission, measured in T-stops. I alluded to this by saying in practice there cane be subtle difference, vignetting was a simpler example). But if you are using auto ISO or auto exposure then the camera will automatically make the adjustments because it measures exposure after the light is transmitted through the lens.

Cameras actually do some other sneaky things to hide potential exposure differences. As lens aperture get wider then around f2.2 there is a fall off in the amount of light that is actually absorbed by the sensor compared to the theoretical value, basically light rays with acute angles reflect off the sensor compared to perpendicular rays. The micro lens array on the sensor aims to mitigate this. It can't control it entirely, so an f/14 lens is actually going to exposure a lot less than you would imagine compared to f/2.8 (less than the 2 stops). Cameras hide this fact by increasing the sensor gain, I.e. ISO, but without reporting the actual ISO used. So if you shot ISO 100 f/1.4 the camera might have bumped that up closed to ISO 200 and you have no way of knowing that.

Another things is the actual apertures are typically rounded to reasonable number. An f/2.8 lens might actually be closer to f/3.0 for example. But again, if you use any automatic exposure or ISO control these differences largely disappear.

D.P. · 17 Oct 2015 at 12:47

stulid said:
Cool, yeah keeping manual iso/shutter the same, the lower f lens is going to be brighter in dull lighting

Only if you increase the aperture to key more light in, if you keep the same f-stop try there is basically no difference.