Originally posted by Na Horuk
Wait, I don't get those ISO ideas at all. If we have two lenses that are f2.8, but one is is t2.8 and the other is t4 (for sake of argument), the camera would just choose a higher ISO or longer shutter to match the same EV, the same histogram curve. So you would end up with the same image, just different metering/settings. I mean, if the lens is darker, than the metering wouldn't know whether its lens or light conditions, so it would choose the settings that would give the desired overall EV. One lens and another would only expose differently if you were in M mode and kept the same shutter, aperture, and ISO. Then t4 would produce a darker image. But it would only bring more noise if you chose to set a higher ISO, or use PP to brighten it. Neither of these is affected by lens. The only way a lens could affect noise directly is if one lens would be radioactive to the point that it would affect the sensor (unlikely, I don't think thats how physics work) or if it would block incoming noise from outside of the camera. These two are both based on the premise that outside things can affect the noise performance. Now, maybe heat can affect noise, but this is usually ambient heat and sensor-generated heat, so again the lens would have next to no effect.
I am curious about this, maybe someone can link to research or an article. I did in the past think that some lenses are a little noisier than others, but I think that was just before I understood EV and didn't know that the f4.5-5.6 lens would use a higher ISO than the f2.8 lens in the same ambient light
Wonderful! Looks like the lens is a winner, at least in good hands!
High isos wash out small details in noise... Lenses do the same to a different extent. When we see sharpness results for example we usually keep look at the number for 50% sharpness. That you have some high contrast black and white scene and the lense transform them into ligh gray, dark gray scene due to loss of contrast. If you accept this level of degradation you'll still be able to see say 50 lines per mm. But if you don't accept to loose 50% contrast but only 10% you'll get only 35 lines per mm.
If you take a higher end, better performing lense, it will keep more of the initial signal strength than a lower end lens... At least, that's one factor to get a better lense. In both case the signal will go through the high iso setting but if the signal is clean enough there still be some signal visible in the noise. If the signal is already heavily degraded with the addition of iso noise it might be completely lost.
Say you look at some feathers with 10% constrast on the original image. There is just a few tones differences. This is the very fine detail you only see at @100 crop but that's a setting birders use all the time because they really need more a 2000mm than a 300mm! One lens keep 50% of this small detail, the other 20%, this give us 5% contrast on the final image for one lense, 2% for the other lense. The 5% contrast will wash out completely at some iso setting while the 2% contrast will disapear at a lower iso setting.
When you add good light, you can lower the iso (maybe even to iso 100) and you also increase the overall contrast of the image. On the initial scene with bad light the initial contrast might be only a mere 2-3% while the same feather will appear much more contrasty at maybe 10% in good light.
That why routinely a medium quality lense will still perform much better in good light at low iso than a great lense in bad light at high iso. But if you have the same conditions, the better lense will give better results. The conclusion through is that in some circonstances it might be better to think of getting better gear (if it too difficult to improve shooting conditions) or that in other cases it much better to improve the shooting conditions...
All the blury, low constrast heavy crop with cromatic aberation shoots we see with very expensive gear, especially from birders show that you can have the best gear in the world, it will simply not replace working in good conditions. It get you a bit more at the same settings, no more.