Is 5.6 on a 4.0, the same as 5.6 on a 1.4 lens????

[SeanH]

Maybe a dumb question, but I really can't figure it out. With all the talk of fast lenses this question came to mind.....

Given the same light & same iso lets say a 50mm 1.4, stoped down to 5.6 is shoting at 125th. Now we have the 50 1.8 also at 5.6. Is it also shoting at 125th........OR is it shoting at 60th because it is 1 stop slower??? Or the question could be at 5.6 or f8 (since all lenses have this setting) is the opening exactly the same regardless of the lens?? Because if that is the case I don't understand why we would buy these fast lenses when most of them aren't that sharp wide open. But if the lens always is 1 or 2 stops faster then that makes since to buy the faster lens.

[Bruce Hamilton]

Given the same light & same iso lets say a 50mm 1.4, stoped down to 5.6 is shoting at 125th. Now we have the 50 1.8 also at 5.6. Is it also shoting at 125th...

As far as aperature goes, 5.6 on one lens is the same size as 5.6 on another, but that doesn't mean both lenses are going to be setting the shutter at 125th. The more glass elements there are inside, the less light reaches the sensor, and the slower the shutter speed. You could even have two cameras with 50/1.4 lenses at 5.6, and have different shutter speeds.

[aam1234]

For the same lens (the 50mm in your example) 5.6 is 5.6.

Not only that but a member here proved mathematically that even with different focal lengths, say 50mm and 300mm, the sensor receives the same amount of light at any given aperture. In other words, 5.6 is still 5.6 :D

As to why people buy faster lenses, well, I asked the same question long time ago, and here are a couple of answers I received. When you step down a fast lens a little you are still shooting with wide enough aperture. Second, wide open is really not that bad. I'm sure there are other reasons but can't think of any right now.

[aam1234]

As far as aperature goes, 5.6 on one lens is the same size as 5.6 on another, but that doesn't mean both lenses are going to be setting the shutter at 125th. The more glass elements there are inside, the less light reaches the sensor, and the slower the shutter speed. You could even have two cameras with 50/1.4 lenses at 5.6, and have different shutter speeds.

I'm not too sure about that. If you mean the diaphragm opening is not the same size, then you are right. But at the end, the amount of light that reaches the film/sensor is the same at any given aperture. I could be wrong though, but this is my understanding.

[DocFrankenstein]

As far as aperature goes, 5.6 on one lens is the same size as 5.6 on another, but that doesn't mean both lenses are going to be setting the shutter at 125th. The more glass elements there are inside, the less light reaches the sensor, and the slower the shutter speed. You could even have two cameras with 50/1.4 lenses at 5.6, and have different shutter speeds.
I really don't think this is true... with the coatings and what not, every optical element will let through pretty much all of the light.

I know some rifle scopes transmit 99.8% of the light that entered in them. So for optics... even if have a 7 element prime and 18 element zoom, the difference will probably be about 1% maximum... and that 1 percent won't infuence exposure.

[CyberDyneSystems]

.... Because if that is the case I don't understand why we would buy these fast lenses when most of them aren't that sharp wide open. But if the lens always is 1 or 2 stops faster then that makes since to buy the faster lens.

I own mostly "fast" lenses and I shoot them all wide open most of the time. If they weren't sharp wide open I would not keep them.

A lens set to an f/stop of f/2.8 will allways be faster than a lens set to f/5.6
Yes I can stop my 1.8 down to 5.6 of f/8.. but I can't open a 5.6 lens up any further than 5.6

Also, lenses with super fast apertures tend to be sharper across the spectrum as they are simply better made.. so a very costly fast f/1.8 lens will be sharp at f/1.8,. and sharp at f/4.5

A cheap lens that is f/4.5 at it's widest will most likely not be sharp at f/4.5

In most of the shooting situations I shoot in,. the extra speed is an absolute requirement. Whether it is because of low light indoors with no flash,. or out doors with moving subjects.

All this and we have not even gotten into the aspect of DOF control that can be found on fast lenses.

[SeanH]

Thanks guys. I thought that was the case....a 5.6 is a 5.6, just wasn't totally positive about it. Yes the DOF is an advantage of a faster lens, I agree. But I disagree with recieving the same light using a 50 or 300mm due to the angle, not to mention the reflective aspects of the shot.......maybe a photo of the sky, or the side of a wall, other than that I have a hard time believing that.

Thanks for your input!

[pcasciola]

Another big point to remember is that the faster lenses will tend to focus more accurately in lower light. An f/2.8 lens has 4 times the light available to focus with compared to an f/5.6 lens, even if both are being shot at f/5.6. Just hit the DOF preview button on an f/2.8 lens set to f/5.6 to see the difference through the lens.

Not to mention, even if an f/2.8 is a hair softer wide open than it is at f/4, that's still an advantage because most will be in their "sweet spot" when stopped down just a little, while the f/5.6 lens will be shooting wide open in it's "soft spot".

[aam1234]

But I disagree with recieving the same light using a 50 or 300mm ....... I have a hard time believing that.

It's really simple. The diaphragm opening of the 300mm is much bigger than that of the 50mm. But due to the length of the lens, so the the light has to travel further, the amount of light hitting the film/sensor is the same.

I'm sure somebody can explain it better than I did.

[aam1234]

Sean, found the link I mentioned. Check it out here (http://photography-on-the.net/forum/showthread.php?t=75237).