Question on Lens Speed

[cpbjgc]

I am looking to get a new longer zoom lens and I was looking at the Canon 70-300 mm F4-5.6 IS (not the DO one). While poking around on various review sites, I always see comparisons of that lens being made to "L" glass, usually the 70-200 mm F4. I am primarily interested in this for certain low light situations (school play/concert settings). This had me wondering about lens speed.

Hypothetically, if I take the same picture using 2 different lenses, using the same apeture setting, and assuming the same light, will the shutter speed be the same for in both instances, or can the optical quality of a lens (using "L" glass as an example) result in a different (faster) shutter speed to get the same exposure.

Any takers on this?

 

[MarkBarbieri]

The speed of the lens has nothing to do with it's quality. It is simple physics. For any lens, the aperture controls how much light is let in. That's all that matters to the camera for the lens. The camera can control the ISO and shutter speed so that the exposure is correct. An f/4 lens is an f/4 lens as far as speed is concerned. There is essentially no difference between how much light two f/4 lenses transmit, regardless of quality. Lots of other things might be different between the two lenses, but they'll both result in the same shutter speed.

Incidentally, the f-stop number is just the ratio between the area of the lens opening and the focal length of the lens. The longer the focal length, the bigger the lens opening must be to keep the same f-stop. The f-stop number is used for apertures because it makes setting the exposure easier. You needn't worry about the size of your lens opening or you focal length, just the f-stop number.

The reason the f-stop number is the ratio of the lens opening to the focal length is that it is proportionate to the amount of light gathering area in the picture. As you move to longer focal lengths, you have a smaller field of view. That means that you have less light coming into the camera. You compensate by having a larger lens opening. That's why an f/2.8 50mm lens is quite small, an f/2.8 200mm lens is quite large, and an f/2.8 400mm lens is enormous.

 

[MICKEY88]

if you'd like a faster zoom check this one out...

Sigma 120-300 2.8

 

[cpbjgc]

I thought that was the answer, but wanted to check with the more technically minded. I thought that even if the quality fo the lens did impact on its ability to gather and pass light to the sensor, speedwise (though I think you are suggesting otherwise), that practically, at the shutter speeds we are talking about, there would be no noticeable difference. The quality of the glass seems to go more towards sharpness of the image, colour reproduction, and imperfections around the edges of the image (e.g. vingetting).

Thanks for the response Mark!

(And Mickey88, I'll check out that lens, but I have a feeling it is outside of my budget)

 

[pjacobi]

The 70-300 USM IS has a variable f/stop of f4 at 70mm and f5.6 at 300mm.

The 70-200 USM IS L has a fixed f/stop of f4 over the entire zoom range.

When zoomed out to 200mm, the maximum f/stop of the 70-300 will be about f5, while the 70-200 will be slightly faster at f4. This gives the 70-200 a slight edge is low-light situations. The 70-300 gives you an extra 100mm.

For sharpest results, you need to stop down all lenses to about f8.


-Paul

 

[cpbjgc]

Well I checked on the Sigma, and it is out of my price league.

I want to have the extra 100 mm, and I think the IS will be more beneficial for the shooting I want to do.

Thanks for the suggestions everyone!

 

[Lizziejane]

Ok, please excuse the dumb question, but when people say they want a fast lens, I'm assuming they mean one with as wide an aperture as possible, say 1.8 or 2.8. And if that is correct, is it called "fast" because wide open, you can get a faster shutter speed, than say f4?

I have the 70-300 IS, and the range is terrific, but it is not the fastest in terms of focusing. I was out the other day trying to get bird shots and missed a zillion opportunities because it was hunting to focus because of all the branches. Can anyone tell me if the 70-200 performs better from a focusing point of view? And how critical is the IS on a 200? I can't imagine using the 300 without it.

 

[AndrewWG]

Ok, please excuse the dumb question, but when people say they want a fast lens, I'm assuming they mean one with as wide an aperture as possible, say 1.8 or 2.8. And if that is correct, is it called "fast" because wide open, you can get a faster shutter speed, than say f4?

I have the 70-300 IS, and the range is terrific, but it is not the fastest in terms of focusing. I was out the other day trying to get bird shots and missed a zillion opportunities because it was hunting to focus because of all the branches. Can anyone tell me if the 70-200 performs better from a focusing point of view? And how critical is the IS on a 200? I can't imagine using the 300 without it.

Lizziejane,

For what it's worth, the 70-200 f2.8L IS focuses VERY VERY fast compared to anything else I ever owned. When shooting film, I used a Sigma 70-300 (or something like that) and it was SLOW to focus. I was in the same boat as you missing shots waiting for it to find a focus.

As far as IS is concerned, I think it is pretty darned useful on the 70-200. I also think that when shooting fully extended, most people should use a tripod. I don't find that to be very fun though. I usually just like to walk around and shoot. I know that I would get better shots if I just dragged the tripod with me, or at least the monopod. I may try this this upcoming weekend and see if my pics are improved or not.

 

[alan]

Another question:

Okay, I understand that f/4.0 means that the narrowest part of the lens is one-quarter of its focal length.

But I also feel that different qualities of glass must have different levels of light transmissiveness.

So how can we get away with something as simple as the focal length divided by the narrowest part of the lens?

Or is it really a little more complex than that (thinks: the f/ number is not really the focal length over the narrowest part, but is the amount of light that you would get if the glass in the lens were perfectly tranmissive)

regards,
/alan

 

[SharonLowe]

Well I checked on the Sigma, and it is out of my price league.

I want to have the extra 100 mm, and I think the IS will be more beneficial for the shooting I want to do.

Thanks for the suggestions everyone!

Unfortunately, I think you will learn that the lens you are planning to buy is not the right lens for low light school events/sports. You will have to up the ISO so much, all your images will be noisy. Plus, for sports, I don't think it is a very fast focusing lens so you will miss out on a lot of shots. I am not one to tell someone to rush out and buy an expensive lens but in this case, I think you might be better off saving for a while and getting a faster, more responsive lens. Believe me, I speak from experience on this one!

FWIW, I shoot my daughter's swim team photos. I shoot with 2 different f/2.8 lenses and I still have to up the ISO to 800 to get decent shots at 1/125 second. The pool area is pretty bright too.

 

[cpbjgc]

I am pretty confident it will work out for me. I've been shooting with the even lower end 75-300 over the last year, and it is meeting my requirements for freezing the action, as the sports are all outdoors (soccer, etc.). As for the School events, I've learned to become a get-there-early-to-get-a-good-shooting-point person (it helps that we live next door to the school ). I have gotten some nice photos from the 75-300 as I try to shoot where there's pauses in the action, but my throw aways in most instances are mostly from camera shake and not from motion blur (even with my monopod). That's why I'm interested in the IS.

The way I see it, it's incremental. While I would like to get the L glass, I could use something better in the meantime which I can trade in once I have the dough for the new lens. For this purchase, I'll be trading in the 75-300 and the 18-55mm kit lens that came with my camera. That'll leave me with my nifty fifty and my Sigma 17-70 F2.8-4.5 (my walk-about lens) and the new zoom.

Thanks for all the suggestions!

 

[pjacobi]

I was out the other day trying to get bird shots and missed a zillion opportunities because it was hunting to focus because of all the branches.

Try setting your camera to use only center point autofocus. This will help prevent focus searching on extraneous items.

It also works when shooting in a crowd of people with heads bobing into and out of the frame.


-Paul

 

[MICKEY88]

Ok, please excuse the dumb question, but when people say they want a fast lens, I'm assuming they mean one with as wide an aperture as possible, say 1.8 or 2.8. And if that is correct, is it called "fast" because wide open, you can get a faster shutter speed, than say f4?

I have the 70-300 IS, and the range is terrific, but it is not the fastest in terms of focusing. I was out the other day trying to get bird shots and missed a zillion opportunities because it was hunting to focus because of all the branches. Can anyone tell me if the 70-200 performs better from a focusing point of view? And how critical is the IS on a 200? I can't imagine using the 300 without it.

I personally wouldn't say IS is critical on any lens, it's nice to have, but realistically photographers managed for years without it, you learn to adapt, better ways to steady the camera, higher shutter speeds etc.


the situation you mention with bird shooting, is one of those times that manual focus might be your solution.

 

[jann1033]

i agree IS is nice but usually not imperativeimo...even with my way "less than steady" hands unless they are more shaky than normally, i have no problem using my 70-200 f4 handheld at 200.( i use that lens constantly) so i'd go for the better lens than for IS for most people especially if your normal" shakiness" can be corrected via better camera holding etc. and the 5.6 on my 28-135 at 135 stinks in even normal ie winter-ish daylight sometimes so the IS might help a little there as well( although that was with my old copy, haven't really run into that as much with this new copy so maybe that was one of #1's problems)
one thing the 70-200 f4 has for focus is a button for either1.2m-infinity or 3m-infinity so depending on what you shoot,you can set it so it doesn't have to go through the whole range for every shot

 

[seashoreCM]

...when people say they want a fast lens, I'm assuming they mean one with as wide an aperture as possible, say 1.8 or 2.8. And if that is correct, is it called "fast" because wide open, you can [use] a faster shutter speed, than say f4?.

Yes and yes.

"Fast" referring to a lens does not refer to how fast the camera can do autofocus using that lens.

But I also feel that different qualities of glass must have different levels of light transmissiveness.

So how can we get away with something as simple as the focal length divided by the narrowest part of the lens

The transmissiveness of the glass in camera lenses makes a miniscule difference. One f/stop equals a doubling or a halving of the area of the lens opening and amount of light admitted.

I want to have the extra 100 mm,!

At least be aware of the maximum aperture available at the maximum zoom setting or whatever zoom setting you wish to use. All other things being equal, a zoom lens has a larger available aperture at the minimum zoom setting.

Digital camera hints: http://members.aol.com/ajaynejr/digicam.htm

 

[Groucho]

Ok, please excuse the dumb question, but when people say they want a fast lens, I'm assuming they mean one with as wide an aperture as possible, say 1.8 or 2.8. And if that is correct, is it called "fast" because wide open, you can get a faster shutter speed, than say f4?

I have the 70-300 IS, and the range is terrific, but it is not the fastest in terms of focusing. I was out the other day trying to get bird shots and missed a zillion opportunities because it was hunting to focus because of all the branches. Can anyone tell me if the 70-200 performs better from a focusing point of view? And how critical is the IS on a 200? I can't imagine using the 300 without it.

You're correct on the definition of fast, but fast lenses also do tend to focus faster, because they allow more light in, which makes it easier on the camera to determine what to focus on.

 

[LMarge]

What is the general opinion of the 85mm f/1.8 for gymnastics shots? I've been using a friends Tamron 28-75 f/2.8 and getting acceptable results, but I'd like to get closer without going to a larger $$$ zoom lens. Thoughts on this?

 

[Anewman]

What is the general opinion of the 85mm f/1.8 for gymnastics shots? I've been using a friends Tamron 28-75 f/2.8 and getting acceptable results, but I'd like to get closer without going to a larger $$$ zoom lens. Thoughts on this?

It is a very popular lens with other indoor sports(volleyball/basketball), no zoom allows as much light in. And I think that since with gymnastics you usually have a single subject the lens would be IDEAL, with team sports the shallow DOF makes it difficult to get sharp focus on the intended target but that will not be the case when you have a single target.

Only drawback of this(or any prime) is that you will miss shots because of the lack of zoom, you must find a perfect spot or have the ability to move around for better positioning when called for.

And then you have price, fast zooms start at around $1000... for that price you can buy 2 or 3 good primes go get you better focal length coverage.