Lens Lingo

[prudence_huston]

Hi! New person here. This forum seems great, and I am very optimistic

someone can help explain something basic to me.

To start easy, my basic lens says "MINOLTA MD 50mm 1:1.7 JAPAN 49mm".

That's pretty straightforward - I assume the ratio refers to the

widest aperture the lens is capable of. But what about one that

says"...1:4.5~5.6 f=80~200mm...". Spoken terms don't seem to equate

to this shorthand. (I realize this is a zoom lens).

Thanks

[tom_mcnicol]

On your zoom f4.5 is the effective max aperture at 80mm, f5.6 is max aperture at 200mm (the following "f=" refers to focal length range). Between 80mm and 200mm max aperture would be prorated between f4.5 and f5.6, at less than a stop.

[art_haykin]

Now, there ARE zoom lenses that maintain the same f/stop across the entire zooming range, and are called "Constant aperture zooms." They usually cost more.

[prudence_huston]

Excellent! Thanks very much.

Pru

[stuart_todd]

I agree with top answer and before you say "why".

The F-stop diameter size changes with the focal length of the lens.

Sure f8 on 50mm lets the same amount of light into the film, as does f8 on a 100mm.

So for example the aperture diameter (6mm) of f8 on your 50mm will be smaller than the aperture diameter (12mm) of f8 on a 100mm.

So that means for lens of 200mm focal length at f8 you're going to need an aperture diameter of 24mm. Can you imagine how big a f-stop of f2.8 if going to be on your 200mm lens?...

...71mm!

However to save cost in design and/or building zoom lenses, somebody worked out that instead of having fit a bigger diaphragm inside the lens, just use the same maximum diaphragm diameter for smaller focal length, but just change the f-stop value. That's why some long tele-photo lens (135mm and up) with maximum aperture of f2.8 seem to cost you your liver and kidneys and why many affordable zooms for us 'plebs' do the f4.5~5.6 change.

 

Wait until your lens lingo starts including alien words like- Planar, Sonnar, Distagon, Elmar, Tessar, Tele-Tessar, Summicron, Biogon and even a lens called a Pancake!

...In the meantime forget those names and go forth take many enjoyable photos with your Minolta.

 

Stu :)

[prudence_huston]

Thanks, Stu! No, I have never heard of any of those yet. I have had this camera for almost twenty years and have decided I'd better figure out why my pictures turn out good or bad before I add digital to my skills. Sounds like I have another twenty years of learning ahead of me!

 

Another question - so f-stop refers to an aperture rather than actually equating to an aperture?

[stuart_todd]

"Another question - so f-stop refers to an aperture rather than actually equating to an aperture?"

 

Can you clarify that please?...

 

...In the meantime does this help-

The f-stops on a particular lens refers to a set of fixed variable diameters, with the same ratio of size to other lenses with different focal lengths.

So basically an f-stop is an aperture opening using the formula below.

 

f-number = lens focal length / effective aperture diameter.

 

Now how the numbers we use (f2, f2.8, f4, f5.6, f8, f11, etc) came into existence is another bedtime story.

 

Stu :)

[prudence_huston]

My answer didn't post - sorry for the delay!

In spite of my badly worded question, you have more than answered it. Thanks for your help!