Multiple Maximum Aperture on a Zoom Lens

[niranjan]

Hi,
I m very new to this world and will ask for some basic tech. help from the masters here.Hope you guys will help me understand the concepts in possibly lucid manner.

When talking about lens it is mentioned with range of focal length and the maximum possible aperture. I want to know what exactly that suggests ?

eg.
in 18-55mm lens f/3.5-5.6 , does this mean that at 55mm zoom, aperture cannot be opened to f/3.5 ?

regards

 

[xjoewhitex]

Absolutely correct. @18mm your aperture can be opened up to f3.5, and on the other end @55mm it can be opened up to a maximum of f5.6 Thats basically how all the lower end, kit lenses work. Higher end models have a constant through the whole zoom range, such as a 24-70 f2.8.

 

[niranjan]

Now i have this one lens Vivitar 28-70mm f/3.4-4.8.
But when i focus @70 mm i still can set my aperture to 3.4 by rotating that Aperture settings ring. How is that possible ? or do AF and manual lenses have diff settings to work with ?

 

[enzodm]

It is unlikely a manufacturer declares a lower spec. Which brand and model is?
I do not have a manual focus zoom at hand, however you may verify that aperture increases at 70mm when passing from 4.8 to 3.4, just looking through (although this is not sufficient to demonstrate max aperture is not 3.4). Perhaps is simply matter of difficulty in blocking the ring at different focal lengths.

 

[xjoewhitex]

Now i have this one lens Vivitar 28-70mm f/3.4-4.8.
But when i focus @70 mm i still can set my aperture to 3.4 by rotating that Aperture settings ring. How is that possible ? or do AF and manual lenses have diff settings to work with ?

That I am unsure of, that really shouldn't be possible since the given values are written on the side of the lens what the maximum aperture will be at given focal lengths. Is this a film camera you are using? Are you able to adjust the aperture by turning a ring on the lens? Or must it be set in camera?

 

[Patrice]

The index ring may move to 3.4 but doughnuts to dollars the actual aperture is 4.8 at the 70 mm end.

 

[cidrmakr]

The f number is the ratio of focal length to the diameter of the aperture opening. It progresses in a logarithmic scale : 1.2, 1.4, 2.0, 2.8, 3.5, 4.0, 5.6, 8, 11, 16, 22, 32. Each increase cuts the amount of light in half. So, as the focal length increases, whatever the physical diameter of the inside of the lens, the "effective" aperture changes, but the size of the hole is the same. The numbers don't mean a heck of a lot as to exactly how much light is coming in, they are just numbers so you can use aperture control in order to modify depth of field, vignetting, distortion, and shutter speed to get what you want. To further complicate it, lenses are marked with f stops and focal length corresponding to what it would be on a full size 35mm camera sensor. When the sensor is smaller - apsc is one of many sizes - the actual focal length is longer. For instance a 50mm lens on a 35mm camera is a 70mm lens (about anyway - don't remember exactly but it's about that) on an apsc size camera, and since the effective focal length is different so is the effective aperture. It's all relative and really don't matter anyway cause they are just reference numbers. The shutter speeds are usually real close to what they are marked. If you move aperture from smallest to largest your auto exposure system will change the shutter speed based on measured light. If you watch, sometimes you don't get an exact match of the two, that's cause the camera is determining exposure and compensating for the changes in effective aperture.

 

[Josh66]

The index ring may move to 3.4 but doughnuts to dollars the actual aperture is 4.8 at the 70 mm end.

I agree.

It may be that the physical size of the aperture is the same, but since the lens is longer - the f/# is not the same. Also, with a manual aperture ring, they probably felt that it would be confusing to have two f-stops listed at one position...

 

[niranjan]

Is this a film camera you are using? Are you able to adjust the aperture by turning a ring on the lens? Or must it be set in camera?

Yes..this is a film camera i m using with VIVITAR manual focus lens. (Every aspect ISO/SHUTTER SPEED and Exposure) need to be set manually.. )

I posted the query because could not get the actual reason to have diff. max aperture mentioned on zoom lens which is manually controlled and set to every f stop from 3.4 to 22 (as mentioned on the lens)

 

[enzodm]

I posted the query because could not get the actual reason to have diff. max aperture mentioned on zoom lens which is manually controlled and set to every f stop from 3.4 to 22 (as mentioned on the lens)

The reason is explained by cidrmakr, except for one mistake. If the hole is the same, aperture cannot be the same at 18 and 70 mm, being aperture a ratio. The fact you are able to select 3.4 means that there is no way of blocking the aperture ring when changing focal length. You may also check whether there is some difference in exposure taking multiple shots (or looking at exposure meter) from f/8 down to the supposed maximum, stop by stop or so (here you find 1/3 and 1/2 stop increases, in case your exposure meter allow them).
cidrmakr mistake is mentioning that a 50mm becomes 70mm focal length on APS-C: no, focal length remains the same (and so also aperture). APS-C acts just as crop. Field of view changes, but is not what focal length is meant at representing.

 

[KmH]

.....I posted the query because could not get the actual reason to have diff. max aperture mentioned on zoom lens which is manually controlled and set to every f stop from 3.4 to 22 (as mentioned on the lens)

The reason has to do with the cost to design and manufacture the lens.

Variable aperture lens, that is lens that automatically change aperture when zoomed to alonger focal length, cost less to design and manufacture than constant aperture lenses.

One of the big savings is the glass lens elements can be made smaller for a variable aperture lens.

 

[cidrmakr]

The mistake was in not adding the word "effective" in front of focal length. A 400mm f4 lens on a 35mm size camera becomes a 650mm f4 lens on an apsc size sensor - as far as field of view is concerned. The magnification does not change.

 

[niranjan]

So i will take it like this..

though i can change the aperture to f/3.4 @70mm , it will still work (bring in light) as if it is @ f/4.8 (ofcourse the diff between the two wont be clearly visible to my naked eye )