Popping the Myth of the Focal Length Multiplier I

Wow, you obviously have way too much time on your hands.

Greg
--



http://www.pbase.com/dadas115/

nickleback said:

Henrik Andersson said:

nickleback said:

For 35mm systems diffraction limits are typically around f/22, but
it all depends on the situation. If you are shooting very fine
grain film and want maximum detail you should use a smaller CoC and
therefore choose a larger aperture. But then you are very limited
in what lenses you can use to get that detail through the glass.

Click to expand...

That was very pessimistic. Some supertele primes and a few zooms
have its sharpest setting at wide open. Very far from F22.

Click to expand...

You completely misunderstand me. For typical CoC, diffraction
limits to resolution are hit at f/22 on 35mm systems. At larger
apertures (i.e. f/16, f/11, f/8, ...) diffraction is not a factor
in resolution limit.

Click to expand...

Itsure is. Depending on the contrast between the fine detail you want to seperate there is not a definite 'limit' were resolution is lost. The diffraction 'limit' is only an recomendation. Raylegh found his limits for very contrasty objects in an experimental way. Other sample lower contrast detail and have to use a different CoC.

The same way goes for DoF, it's not an exact range. It's usually some old definitin about 8"x10", one foot and 4lp/mm. But that doesn't hold for every ones need.

Some define resolution limit when you only have 9%, 25% or 50% contrast still. Rayleighs criterion is closer to 9% and is usable for separation bright stars ...

I usually use the MTF at 25 or 50% depending on the resolution I can waste. I can loose less resolution when low resolution cameras like video is used. While HD, cine film or sensors can waste some more pixels.

I tend to have pretty low contrast detail before the light hit the lens ... so I can't use F16+ even with 35mm film/sensors.

nickleback said:

My comment on very fine grain films is that assuming you want max
detail you should choose a smaller CoC, therefore diffraction
starts becoming a bother sooner, so you've got to find a lens that
pretty darn sharp wide open (or close to it).

Click to expand...

And there are alot of nice stuff around F5.6-F8, diffraction limited.

nickleback said:

nickleback said:

nickleback said:

But typically, camera lenses at max aperture are not diffraction
limited. I hope you agree with that.

Click to expand...

Nja, See above.

Click to expand...

Resolution limits at max aperture (i.e. f/2.8, or whatever) aren't
typically due to diffraction, they are from lens aberrations.

Click to expand...

Correct and I've never stated the opposite. Aberrations limit up to around F4 and at F8 it's definitely diffractions that limit quality om 'my' best lenses for resolving details at many tens of km away. Apertures at 40+cm for 35mm film and 1"- 'video', diffraction limited.

Don't get me wrong F22 is probably the limit for some work but I don't look at bright stars( ) with pretty black/blue surounding so I have to deal with separating lower contrast detail and that is gone at F22.

( ) except for calibration.

For me atmosfere is a bigger problem then aberrations.

--
Henrik

Henrik Andersson said:

dcfaria said:

There's a lot of faint stuff out there so getting more of that
"blury" light is a good thing

Click to expand...

It sure is.

Henrik Andersson said:

Let's just agree on the fact that there are a lot of telescopes and
most are not operated close to their difraction limit, but rather
at the seeing limit.

Click to expand...

Correct so far.

Henrik Andersson said:

So the reason to use a bigger telescope rather
than a small one would be to gather more light. An often bigger
problem than resolving something is to get enough light from it.

Click to expand...

Nope, investments and research is made in resolving power, not
increase light intake.

There are 'cheap' and very simple ways of collecting more light but
they are not used. The money is put in other investments instead.

For instance it would 'only' cost twice the money to increase
light-gathering capacity 2x times by simple renting two telescopes
and letting them track the same object. You could even avoid some
atmosferic turbulance that way and have a small resolution
advantage.

Instead of doing this, reaserches tend to just use one telescope
and increase exposures/time instead.

When more then one telescope is used it's to increase resolving power.

If light-gathering was the primary problem we would have many more
small-aperture telescopes looking at the same objects at the same
time.

If we had enough resolution vs light-gather we would:

• not have single huge telescopes with hideious expensive mirrors.

They could save money if they scimped on quality if light-gathering
was the problem, instead the mirror now stand for about 90% of the
total system cost for a big aperture telscope. Including buildings,
cameras computer HW and SW develpment.

• not hazzling with adaptable mirrors or 10% efficient high speed

cameras.

• Try out baloons, planes and go into space. Space have other

benefits too. No atmosferic absorbation of certen wavelength etc.

• not invest in extremely complicated interferometer arrays on

earth... and plan on the same thing for space.

--
Henrik

Click to expand...

I think we are a bit off topic here and I'm not sure we are on the same page. Maybe we can try to convince eachother why astronomers have/use big telescopes somewhere else.

They should start an astronomy forum here...

I updated my word processor to replace "focal length multiplier" with "cropping factor" Hope this makes life better for you. If I can be of assistance in the future please hesitate to ask.

Chato said:

It seems that many otherwise intelligent and knowledgeable people
on this board still “believe” in the myth of the Focal Length
Multiplier (or FLM). It should be called the Cropping Factor and
that is ALL that it is.

Click to expand...

pavi1 said:

Chato said:

It seems that many otherwise intelligent and knowledgeable people
on this board still “believe” in the myth of the Focal Length
Multiplier (or FLM). It should be called the Cropping Factor and
that is ALL that it is.

Click to expand...

Click to expand...

Hopefully you meant "don't" hesitate to ask - And therefore I will ask.

Do you have any recipes for eating crow? My dog of course, is quite content to let the crow marinate in the sunshine, modified only by rain, and the kindly flys that deposit their children as a spice. But personally, having witnessed my dog happily eating HIS crow, I was wondering if you had a better recipe?

Thanks in advance...

Dave

I was just helping you get to 150.

Personally I can't wait until those who knew or care what a 50mm did or did not do a a 35mm are pushing up daisies so I no longer have to hear or read about it.

Chato said:

pavi1 said:

Chato said:

It seems that many otherwise intelligent and knowledgeable people
on this board still “believe” in the myth of the Focal Length
Multiplier (or FLM). It should be called the Cropping Factor and
that is ALL that it is.

Click to expand...

Click to expand...

Hopefully you meant "don't" hesitate to ask - And therefore I will
ask.

Do you have any recipes for eating crow? My dog of course, is quite
content to let the crow marinate in the sunshine, modified only by
rain, and the kindly flys that deposit their children as a spice.
But personally, having witnessed my dog happily eating HIS crow, I
was wondering if you had a better recipe?

Thanks in advance...

Dave

Click to expand...

--



Everything happens for a reason. #1 reason: poor planning

Chato said:

Thanks in advance...

Dave

Click to expand...

Hey, at least I can thank you for getting me to learn about microscopes. It's been fun reading.

Boris

I can hardly believe such a long list of posts on such a banal topic. The only thing unfortunate or imprecise is the language we use to describe the phenomenon. But I think most people understand it. The most unfortunate claim in the original post is the one about lens resolution, which is not a function whatsoever of focal length, per se.

The language we use helps us understand and gives us a frame of reference, but however we describe it, any lens put on an APS sensor sized digital camera will give the APPEARANCE through the lens of having a focal length 50% greater than the same viewed through a full frame DSLR camera, or a regular 35mm SLR. If you understand that much, then you know as much as you need to know when you put a lens on your camera.

Shullinger said:

I can hardly believe such a long list of posts on such a banal
topic. The only thing unfortunate or imprecise is the language we
use to describe the phenomenon. But I think most people understand

Click to expand...

That's why the very first lesson in any technical education is to be precise in the definition of terms. This thread is a good example of why that is so.

--mamallama

BorisK1 said:

BorisK1 said:

BorisK1 said:

I've had a long discussion with Dave (Chato), and I think I
understand his position - more or less. He read some papers
(mostly about microscopes), and learned that a microscope's
resolving power grows with the focal length of the lens.

Click to expand...

Really? That I would like to hear about!

Click to expand...

Well - from what I've been able to understand, it's correct - as
far as microscopes go.

With microscopes, we're talkng about the objective focal length.
That is, the distance from the object to the lens when the lens
is focused on the object . As opposed to regular focal length
(the distance from the focal plane to the lens, when the lens is
focused on infinity).

Click to expand...

Thanks--but from both of the pages you linked to I see that numerical aperture grows as the 'objective' focal length is decreased--that is, as you put the lens closer to the object.

So, what gives?