Focal length/focal ratio 101

[netwolf]

I need a Camera 101 class. How to the F ratio's in a camera work, and how are they adjustable. What is 200mm? Is there a good intro somewhere?
I know I can to 10s exposures on my camera, as well as adjust the Fratio, I just dont know what it means. I have worked out that increasing the exposure time gives me more light.
But If i look it at like my telescope maths, then I am guessing a maller F ratio would mean better results for shorter exposures? Is that right.

I am thinking of geting a lense hood adpater for my A610 to add a zoom lense to it.
Like this aftermarket one.
http://www.lensmateonline.com/newsite/A620A610.html
Results with various lenses.
Wide angle:
http://www.lensmateonline.com/newsite/A620A610wide.html
Telephoto:
http://www.lensmateonline.com/newsite/A620A610tele.html
Regards.

[Dennis]

Hi Netwolf

I too find this confusing, and difficult to move between telescopes and camera lenses. The use of the term aperture for photographic lenses and telescopes is potentially confusing.

For a telescope, the aperture or diameter of the objective or mirror, is (usually) fixed. That is;

A 10inch F5 dob has an aperture of 250mm and a focal length of 1250mm.
An 8inch F10 SCT has a 200mm aperture, a focal length of 2000mm.
A 6 inch F6 Mak-Newt has a 150mm aperture and a focal length of 900mm.
A 4inch F9 refractor has a 102mm diameter aperture and a focal length of 918mm.


If you place an F6.3 Reducer/Corrector on the 200mm SCT, the aperture remains the same. That is, it is still an 8 inch SCT. However, the focal length is now effectively 2000mm x 0.63 = 1260mm and it is now an 8” F6.3 system.

Now let’s look at telephoto lenses on cameras:

My 200mm F4-F32 telephoto lens for my Pentax has a focal length of 200mm.
My 135mm F2.8-F22 telephoto lens for my Pentax has a focal length of 135mm.
My 50mm F1.8-F32 standard lens for my Pentax has a focal length of 50mm
My 28mm F4-F22 wide angle lens for my Pentax has a focal length of 28mm.


The above 4 lenses all have a built in diaphragm or Iris. On the 200mm, the Iris is “wide open” at F4 meaning that the lens is operating at full aperture. When I twist the aperture ring to say, F32, this “stops” the lens down making it “slower”. The focal length does not change, it still is a 200mm telephoto lens, but now, only 1/64 of the light is getting through to the ccd chip. As we “stop down the lens” in steps of 1 stop on the aperture ring, we halve the amount of light reaching the ccd.

F4, F5.6, F8, F11, F16, F22, F32

On a telescope, there is no Iris, so it is always “wide open”, as the aperture cannot change, unless you place a sub-aperture mask over the objective.

Now here’s where it gets really confusing. In photography, a numerically large F-Ratio of say, F11 only lets in 1/16 of the amount of light compared to a numerically small F-Ratio of F2.8. How come, ‘cos the number 2.8 is smaller than the number 11? Well, in terms of light gathering, F2.8 is considered “larger” than F11 as it lets in light 16 times faster, so your camera shutter speed at F2.8 will be 16 times faster than at F11 to record the same amount of light.

So, an F2.8 lens is considered “faster” than an F4 lens and lets you shoot in lower ambient light, but you usually pay a lot more for a faster lens, as it has a larger aperture i.e. a bigger piece of expensive glass at the front end.

Hope that helps.

Cheers

Dennis

[h0ughy]

Yep Dennis that’s exactly how would have put it too!:whistle: :P

He is good with the explanations I concede that, must have been all those extra cosmic rays channelled through the scope shield one dark night…………. ;)

[netwolf]

Dennis,

Thanks so much, I am begining to understand. The Camera Lense is like a refractor with a mecahnical mask "Iris" (like the thing on the Stargate), that stops it down (as in reduces its apperture). In a Telescope we usualy use a lense (barlow) either postive or negative to increase or reduce the focal length. In a Camera lense we change the apperture size to do the same thing usin a iris. I got it.

So on my A610 when I change the F ratio I am adjusting the Iris on the lense.
I think i got it now.

So I guess if i add a telephoto lense, I would leave the F ratio on my internal Lense at the max, Iris fully open and adjust the IRIS on the external lense. The question then is at what apperture will my internal Lense itself become a limiting factor, that is act like a obstruction to the external lense. I think I will have to select the apperture and Focal length wisely.

BTW what do you guys recoomend a 52 thread or a 58 thread, which is easiert to get second hand lenses for?

Regards

[netwolf]

Am I also correct in assuming the Optical zoom in a camera is achived using adjustable barlow lense? It seems that at 0 mag I can use F2.8-F4.1 but at 4x my lowest F is 4.1-F8. From this I guess there is a second element controlling the zoom?

If this is off topic Mods could you cut this out and start a new thread. I think the information Dennis posted is very usefull. I would like to see more of this discussion. Maybe we can get Dennis to right a Camera 101.
Regards

[okiscopey]

Yep I second that! Can we make this another thread? It'll be a good one - I'd like to contribute! It's interesting that some arrive at 'Photography' via 'Astro' rather than the other way around, so a '101' could be very helpful for some folks. By the way, what's a '101'?

[netwolf]

Ah its like Maths 101 or Physics 101, its an old labeling scheme used at Universities and Colleges to highlight the course is a beginners course.

Regards

[Dennis]

You’re getting there, well done.

Yes - a Barlow will increase the focal length (aperture stays the same) and a Reducer will reduce the focal length (aperture stays the same).



However…
In a camera lens, when you change the aperture (open up or stop down the Iris), the focal length stays the same. That is, a 200mm lens wide open at F4 and the same 200mm lens stopped down to F22 still has a focal length of 200mm. The Focal Ratio changes due to the Iris effectively reducing the aperture of the lens.

Cheers

Dennis

[netwolf]

So one could use a mask on a Refractor telescope to act as a Barlow, right. Why buy a barlow for a refractor when you can just stop down the aperture. What are the pros and cons of either method?

Regards

[[1ponders]]

Putting a mask on a refractor won't act as a barlow, it will just slow the focal ration, the focal length will remain the same. To increase the focal length you need to "add length" by using lenses. Stopping down the aperture the way you are describing will reduce resolution, which is dependent on aperture size.

[netwolf]

Thanks Paul, I stuffed up there, confusing Fratio with Flength. With a barlow you can increase the Flength hence the Magnification and still retain resolution. At the cost of extended exposure time.

Regards

[okiscopey]

Netwolf, I was going to post a long screed about lenses, apertures etc. but a lot of it has already been covered by Dennis et al.

Instead, I had a quick look on the Net for additional information you might find useful. Most sites are publicising commercial photography courses, but here are three I found that have some good online information:


http://www.geofflawrence.com/photography_tutorial_exposure.htm

Look at the side bar items: Exposure / Shutter / Aperture



http://photo.net/learn/making-photographs/lens

Have a look at the ‘Lens’ and ‘Exposure’ topics. Halfway down the ‘lens’ page is a description of wide-angle- normal - telephoto lenses.

The info must be a bit old - the ‘camera’ section seems to be non-digital, but that doesn't affect the basic photo theory.


http://www.photographytips.com/

Click on ‘Equipment‘ and ‘Lenses’. You can only go so far in any topic before having to pay .. ouch!

[netwolf]

Thanks Okiscopey, will have a read through those.

Regards