Lens - speed difference

mkpatil

Assume I have two lenses

A) 50mm f/1.4 ($400)
50mm f/2.8 ($98)

Also assume I am shooting pictures with 2 identical cameras with same lighting, location, from the same place in quick sucession.

1) If I set/fix the aperture to 8.0 for both lenses, Lens A is no longer faster than B, right?

2) The only time A is really worth its high cost is when we/(or camera) sets the aperture to 1.4 in A, but B gets stuck at 2.8. right?

3) If you need to shoot pictures in low light or high speed situations, go for lens A or else B is just fine. right?

I am setting the focus to manual so no confusion about focusing speed. Not worried about lens aberration if any either.

Please let me know if 1,2,3 are correct.

Thanks,
Milind

Seefutlung

Yes, you are absolutely right. But, there is also a 4) Lens A at F/1.4 allows four times the amount of light to hit the sensor as lens B F/2.8. So your viewfinder will be 4x brighter ALL the time with lens A ... so add that to your value-added equation also.

Gary

PS- Full F-Stops go thusly:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, ...

Every jump between F-Stops will allow twice as much light or half as much light depending which way you go.

The difference between 5.6 and 8, is that 8 will cut the amount of light in half from 5.6 Going the other way, the dif between 8 and 5.6, is 5.6 will allow twice as much light as 8.

The dif between 1.4 and 2.8 is two full stops, soo the 1.4 is actually 4x faster than a 2.8.

pathfinder

One other thing to consider, is whether the two lenses REALLY are of the exact same level of quality. Not only optical quality, but build quality, focusing speed and accuaracy, quality of bokeh, and durability of the iris diaphragm, resistance to damage of the lens coatings .

You should not just glibly assume that the ONLY difference between a 50mm f2.8 for $98, and a 50mm f1.4 for $400 is the aperture size. There are probably several other significant differences that may be valuable to some users. (SOME 50mm f2.8 lenses may even be sharper at f11 ( possibly ) than a 50mm f1.4. - But still not always more desireable )

But generally, you tend to get what you pay for. There is a reason the more expensive gear does not disappear from the market. Durability, lack of failure with heavy use, are features that are very valuable to some professional photographers, but of lesser value to some amateurs.

Lots of things to think about. The old Nikon SLRs were highly valued because you could drive nails with them when you finished shooting 35mm films. Durability counts when you make your living with a tool.

mkpatil

Pathfinder:
Thank you for the information - I do agree that the cost in part may because of the better quality of the lens and other features and you have given some of the very important and relevant points.


Gary:
I am glad you have added the value added question 4. I think that has really solved one of my question/problem that I really could not ask, because I did not know what to ask.

Here is the question, it is actually the auto focusing issue that I have asked to ignore in my previous email.

"In low light situations, will lens A 'auto focus' better just because it has more light (4 times more light at A=1.4 compare to A=2.8) to work with? Lens B may keep moving in circles all direction trying to focus, but does not have enough light to make the decision, until I dipress the shutter button and almost break it, trying to force an important picture to be taken. Lens A will come out a winner in this situation, right?"

Thanks again,
Milind

claudermilk

Yes, the faster lens will have an easier time focusing in low light--remember that the lens is wide-open until you actually take the picture. Of course the tradeoff is with larger pieces of glass, the AF motor may (likely will) move slower than the "slower" lens.

mkpatil

claudermilk wrote:

Yes, the faster lens will have an easier time focusing in low light--remember that the lens is wide-open until you actually take the picture. Of course the tradeoff is with larger pieces of glass, the AF motor may (likely will) move slower than the "slower" lens.

claudermilk:

Thanks, great point.

I think it is okay to assume that the 'net' gain [pluses minus tradeoff] in speed will be in favor of the faster lens.

Thanks,
Milind

Seefutlung

mkpatil-

Don't know what maker of camera you own ... but Canon's auto focusing does kick into a faster and better gear (if you will) at 2.8. So you're cover there with either lens.

Chris is absolutely correct that the 1.4 will have less trouble locking onto a subject than a 2.8 .... but I don't think the difference is all that significant (just an opinion). I shoot a lot of low light stuff (nighttime sports and such) ... fastest lens is 2.8 ... and I have minimal trouble locking onto a subject ... also I look for a contrasty "seam" on the subject and line up a focus point there in order to help the camera focus.

Gary

mkpatil

The camera I have is EOS-1D Mark IIN, I was having too many focusing problems with my 35-350mm F/3.5 L series lens focusing in low light wedding situations, even at close 35 mm range. I just ordered a 50mm F/1.4 hoping to solve the focusing problem. Hope it solves that problem.

Thanks,
Milind

mercphoto

mkpatil wrote:

The camera I have is EOS-1D Mark IIN, I was having too many focusing problems with my 35-350mm F/3.5 L series lens focusing in low light wedding situations,

I haven't really heard all that much good about that lens at all. Any lens with an optical zoom of 10X is going to have a lot of compromise in it.

As many have pointed out, there are a lot of good reasons to have fast glass, even if you never shoot below f/5.6. Mostly it has to do with the fact a lens is not stopped down until you actually fire the shutter button. So its wide open as you view (good!), as you auto or manual focus (good!), as you exposure meter (good!). These are all great advantages of fast glass.

LiquidAir

Technically the speed of a lens is its widest apeture regardless how you have it set at the moment. The reason for that is your view (and the AF system) through the viewfinder is when the lens is wide open. So the 50/1.4 will give you 2 stops more light when composing the shot than the 50/2.8 even if you are stopping down to f/8 when you acutally expose the sensor.

In the Canon world, I find the cream of the crop for low light fast focus are the 35/1.4, 85/1.8, and the 135/2. The 50/1.4 is an older design and the AF is not as snappy.

rebelxtnewbie

LiquidAir wrote:

In the Canon world, I find the cream of the crop for low light fast focus are the 35/1.4, 85/1.8, and the 135/2. The 50/1.4 is an older design and the AF is not as snappy.

In the canon world you can also get a 50mm f/1.8 for $80!! Great image quality but poor build.