08-07-2020 11:03 AM - edited 08-07-2020 11:12 AM
"Honestly I only half understood it."
No problem let's try another approach. Do you have a binocular at home that you use? It is just a lens. Just like your camera lens. It has a constant f-ratio of, let's say f4. However they do not have a means of reducing the amount of light coming into the binoculars and the amount that comes out for your eye to see. Now if your camera lens did not have that extra system, the aperture blades, it would work the same way.
"But it raised another question to me. The lens says F4 constant aperture and I paid a lot for that ... I am expecting it to stay constant at f4 :-)"
There are two different points here. Did you mention what lens you are talking about? It really doesn't matter but let's assume it was a ef 24-105mm f4L (constant aperture) zoom. When you zoom any lens, say, from 24mm to 105mm you change the focal length and therefore you change the f-ratio. Some lenses do not have special internal systems to cope with this so their f-ratio changes. An example might be the ef-s 55-250mm f4-5.6. However, a lens like the ef 24-105mm f4L has internal lens groups to correct for this and its f-ratio remains constant.
Now where you seem to jump the track is when we have to add exposure to this equation. All the pictures you take can not be shot at the constant f4. So, how do we solve this dilemma? We add an aperture system that has the ability to reduce the amount of light that hits the sensor. These are called f-stop numbers and although they sound like the same number as your constant aperture f4 lens they are not. Bottom line here is your constant f4 lens shot the scene at f4 just like Canon claimed but the camera told the lens aperture system to reduce the amount of light to f9 because that is what made a perfect exposure. If you removed the aperture system from your constant f4 lens, it would shoot all shots at f4 and some would be over exposed.
Consider this, as we wrap this up and I hope I made it more clear for you, all prime lenses are constant aperture lenses. Zoom lenses can be either constant or variable aperture but both still have the aperture system to adjust light coming in and going out. Two, there is nothing inherently better in a constant aperture zoom lens over a variable aperture zoom lens. And, the down side is cost and weight because they need that extra lens grouping inside to keep the f-ratio constant. Plus they generally are faster lenses and that costs more.
08-07-2020 11:06 AM
"I am sorry that I am a bit thick here...."
Nah, I don't think so. It's just those one line snips don't offer or explain enough. Nobody would understand them.
08-07-2020 11:25 PM
Arrrrrggghhhhh..... I got it now. I was mixing the f label you see on the lens' barrel with the f-stop. The explanation on adjusting the amount of lights hitting the sensor through f-stop brings pieces together. I knew you mentioned this in your earlier post but I did not get it now I did.
In closing this thread, I would like to mention special thanks to:
- @ebiggs1 for your (i) patient in explaining this to me, and (ii) bring the technical details into the explanation, mixed with day-to-day example... made it easier to chew he material so to speak.
- @waddizzle to took the simplicity approach to explain this to me. Keeping me off from being more confused with all the explanations.
- @PLee who has no nicely took me to the beginning on understanding those numbers printed on the lens barrell, the max and min aperture as you zoom in and out. Combined with @ebiggs1 explanation on reducing the amount of light (otherwise you would have over exposure) it nailed the coffin.
Thank you folks! I owe you one.