Lens Spec question.... Aperture related.

GavSt

GavSt

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I will hold my hands up right now and say I AM A NEWBIE.. i'm still trying to get my head round all this photography stuff so please be gentle :)

The question:

What is good/bad about a lens that has a fixed aperture rather than a variable one? ie 70-210 f4 over a 70-210 3.5-5.6. Do they suit a different type of subject?

TIA
 
When you comment say the 70-210 F4, the F4 is the widest aperture that the lens will run to. In the case of F3.5-5.6, it means that the widest aperture is variable, dependant upon the focal length that you chose to use.
 
It is best that a lens be fastest it can (ie... a low number F2.8 for instance)
Some zooms have their widest aperture at the zoom length not as wide as when it is at the shortest focal length.

Therefore a lens with its widest aperture being 2.8 all the way through is best. It can still be varied up the the narrow end of the range... but the F2.8 means that is its widest aperture at all the focal lengths.

Well I know what I mean!! ;) :D but explained so much better by Mr Sukebe!!!
 
A fixed aperture such as F4 or f2.8 is classed as a 'faster' lens, as in, right across its zoom range, you can have a maximum wide open aperture that 'collects light quickly' - fixed aperture lenses are, as a rule of thumb, better quality than variable. Of course a fixed aperture doesn't mean you can't stop it down to other f-stops, it just means it has the same max right across its range.... I think I am making sense. :thinking:
 
right guys that makes sense to me.... but can you give me a for example as to when a wider aperture at the longest length would come in useful? what sort of things can you use that for?
 
A constant aperture throughout a lens' entire zoom range means, for any given light value, the shutterspeed will remain constant. Particularly is this important when using manual exposure.

It also dictates that the depth of field, albeit quite shallow, remains constant.

In essence, it keeps you in charge of all the variables - except the light value because of the weather. ;)
 
Wider apertures will let more light in therefore you can use a slower shutter speed and still maintain the correct exposure.

Wider apertures will also give a shallower depth of focus and help to isolate a subject from its background.

The widest aperture available is seen through the viewfinder when composing the shot....this gives a brighter image to help with your composition (or manual focus if you're using it).

Bob
 
GavSt said:
right guys that makes sense to me.... but can you give me a for example as to when a wider aperture at the longest length would come in useful? what sort of things can you use that for?
Click to expand...

Grrr... can't type fast enough, posting clash!

Depth of field mostly. Especially at the longest 'end'. You can obtain remarkable effects with a very narrow/shallow depth of field with a 'fast' telephoto lens. Puts subjects into isolation.

In a sports type environment, it enables you to use faster shutterspeeds.
 
Anything where faster shutter speeds are critical in poor light, so for instance, birds in flight on a dull day or as evening approaches where the light is less than perfect. Do bear in mind though that you will lose in the depth of field area, so accurate focusing is also important when opening that aperture right out [fixed aperture lenses are usually good enough quality to focus quickly and provide sharp images, though maybe not as sharp as a prime in some case]]. Alternatively, stationary shots of more distant objects when again, light is poor, if you can open the aperture wider before having to start lifting ISO level, you can reduce noise factor.
 
If you were using a 70-300 f4 lens set at 70mm and f4 and Aperture priority on the camera it might set a shutter speed of 1/500 if you now zoom to 300mm the shutter speed would still be 1/500.

if you did the same thing with a 70-300mm f4-f6 lens at 70mm the shutter would be 1/500 but if you zoom to 300 the shutter would be 1/30 sec
 
also, the constant aperture versions tend to be more towards the pro end of the scale with better glass elements and image quality(in general) and higher price to match.
 
thanks guys... that's cleared that up for me... :)

One of the reasons I am asking is because I have read the reviews of the Minolta "beercan" 70-210 f4 lens and it seems to be a bit of a diamond. I was just wondering what limitations or benefits there are with the f4 setting throughout the range.
 
Dogfish_magnet said:
If you were using a 70-300 f4 lens set at 70mm and f4 and Aperture priority on the camera it might set a shutter speed of 1/500 if you now zoom to 300mm the shutter speed would still be 1/500.

if you did the same thing with a 70-300mm f4-f6 lens at 70mm the shutter would be 1/500 but if you zoom to 300 the shutter would be 1/30 sec
Click to expand...

I agree with the first paragraph (which is what I said) but the second example is 'way off' I'm afraid.

The difference between the two quoted apertures is only 1.5 stops (approx) which would give a shutterspeed of 1/180 sec. You've made it a difference of 4 stops with a 1/30 sec!
 
chuckles said:
I agree with the first paragraph (which is what I said) but the second example is 'way off' I'm afraid.

The difference between the two quoted apertures is only 1.5 stops (approx) which would give a shutterspeed of 1/180 sec. You've made it a difference of 4 stops with a 1/30 sec!
Click to expand...


:bang: tooo early for this i gave the shutter speed for f16 @ 1/30 - i just (tried ) to put some numbers in as it can make it easier to understand.
 
Dogfish_magnet said:
If you were using a 70-300 f4 lens set at 70mm and f4 and Aperture priority on the camera it might set a shutter speed of 1/500 if you now zoom to 300mm the shutter speed would still be 1/500
Click to expand...

Not necessarily true, as the framing of the shot is changing, which may result in it getting darker over all (i.e. if the lighter bits get cropped out). Depending on your metering mode the camera may try to compensate by bumping the exposure up, thus changing shutter speed ;)
 
chuckles said:
It also dictates that the depth of field, albeit quite shallow, remains constant.
Click to expand...

Not sure if this has come out as you intended, as you clearly know your stuff :) but the DOF is dependent on focal length, not just apperature ratio, so a constant F-stop the DOF gets progressively smaller as focal length increases.
 
pearce_jj said:
Not sure if this has come out as you intended, as you clearly know your stuff :) but the DOF is dependent on focal length, not just apperature ratio, so a constant F-stop the DOF gets progressively smaller as focal length increases.
Click to expand...

James,

DOF is not affected by focal length unless you're referring to a fixed shooting position and using a different FL (hence, a different shot). DOF is simply aperture and magnification.

If you take a shot at f/4 with a 100mm lens and then move to frame the same shot with a 200mm lens, then the DOF will be the same in both cases.

Bob
 
yeah but we're talking about zoom lenses - why would you move back and then zoom in?
 
pearce_jj said:
yeah but we're talking about zoom lenses - why would you move back and then zoom in?
Click to expand...

Standing and zooming will not give the same perspective as walking and adjusting the FL.

If you're popping off at an aircraft against a blue sky then zooming wouldn't make a difference. If you're setting up a landscape shot or portrait then walking will result in a different image albeit with the same framing.

Bob
 
pearce_jj said:
Not sure if this has come out as you intended, as you clearly know your stuff :) but the DOF is dependent on focal length, not just apperature ratio, so a constant F-stop the DOF gets progressively smaller as focal length increases.
Click to expand...

Thanks for picking up on that...... it's a whole complicated subject.

I was implying that the pictorial effect is relatively constant(ish) - shallow(ish) blurredy backgrounds! (y)
 
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