light and f stop

[Claire Senft]

So Edward, perhaps you can give me an example of where it does not equal a fraction. Perhaps you could also give me an example where a ratio does not result in a fraction. How different is 1:12 to 1/12?

 

[Dan Fromm]

Did I miss something? I thought bohica asked a relatively simple question about "f/stops". Where did s/he request information about "mastering photography"?

Yes, Ed, you did. I was replying to Dave Parker, not to bohica. bohica wants to know enough and no more.

 

[Dan Fromm]

Dan,

Focal length equates manification, which in shooting the same subject at the same distance you will have less depth of field, one other way to increase magnification is macro extention tubes, or special close focus designs, which will also yield less depth of field, which of course is a different subject.

Dave

Dave, I didn't know you stood rooted to the spot and used a zoom lens ...

But seriously and all kidding aside, do the arithmetic. The results may surprise you a little. That a short lens can, in special circumstances, give more DOF than a long one in the equivalent final print was a surprise to me.

Cheers,

 

[Jonathan Brooke]

Dan, I'm afraid I've missed something too. The 'mastering photography' comment appeared before Dave's and is addressed to bohica...

 

[Dave Parker]

Dave, I didn't know you stood rooted to the spot and used a zoom lens ...

But seriously and all kidding aside, do the arithmetic. The results may surprise you a little. That a short lens can, in special circumstances, give more DOF than a long one in the equivalent final print was a surprise to me.

Cheers,

Howdy Dan,

I am a bit confused, I would expect a shorter length lens to give more DOF field..than a long lens...

I rarely shoot zoom lenses, but do use super telephotos all the time in the wildlife work I do..

Have a good one.

Dave

 

[kaiyen]

So Edward, perhaps you can give me an example of where it does not equal a fraction. Perhaps you could also give me an example where a ratio does not result in a fraction. How different is 1:12 to 1/12?

Claire,
The size of the aperture opening is a fraction/ratio, but the "f" in f-stop stands for focal length, not fraction. So it is a fraction, but not for the reason you state.

To be more accurate, it's the "/" in the more correct "f/stop" that means fraction. not the "f" itself.

allan

 

[gnashings]

I was always under the impression that it stood for focal length, since the apperture is expressed as f 1:x. But "fraction" seems to work - so whatever allows you to understand the principle, works for me. I was taught "focal length", but if I was taught "fraction" - I would get the same results, I think.
So not a good reason for WWIII, m'thinks

 

[bohica]

Didn't mean to start a war!!!!

 

[lee]

doesnt take much to start a war here Tom

lee\c

 

[Dan Fromm]

Didn't mean to start a war!!!!

You didn't start a war, you started an, um, enlightening discussion.

Cheers,

 

[moose10101]

but if I was taught "fraction" - I would get the same results, I think.

But how would you know what value to put in the numerator of the fraction?

 

[Claire Senft]

Want me to send you a PM telling you?

 

[gnashings]

Yeah, well - war or no war, I still love APUG!

 

[N O Mennescio]

I always thought that DOF increases with the distance to the subject. If you focus on infinity you get a lot more DOF than if you focus at something close, right? So if you compose a shot with a wide angle lens and stand, say 5 meters away, you'll have to stop down to f/stop X to get the DOF you want. Shot the same composition with a tele lens and you'll have to back up quite a bit to fit it all in. You'll find that you can use a wider apreture to get the same DOF. Becouse of the distance to the subject.

 

[Bob F.]

Anyone who has done macro with a LF camera and quickly run out of DoF (as you do) and then thought: "I know, I'll use a wider lens because that will give me more DoF" has soon found out that Dan is correct. Because you need to move the camera closer to the subject in order to maintain the same image size on the ground glass (read "magnification") your DoF goes back to the same as you had with the longer lens. No free lunch there unfortunately (and no arithmetic needed - just practical experience )

Cheers, Bob.

 

[benjiboy]

Better to"f" than"T"?

Claire,
The size of the aperture opening is a fraction/ratio, but the "f" in f-stop stands for focal length, not fraction. So it is a fraction, but not for the reason you state.

To be more accurate, it's the "/" in the more correct "f/stop" that means fraction. not the "f" itself.

allan

I've always thought that the T stop system used on zoom lenses in the cine industry is probably a better system as an indication of an optics actual light transmission ability rather than a geometric system, and from some of the experiences I have had with long multi-element zooms f8 is not the same on some lenses, probably due to the reflective index of the glass elements , and the type of coating used in it's manufacture.

 

[dancqu]

But seriously and all kidding aside, do the arithmetic.

Do the arithmetic correctly and there will
be no argument.

Take this case: The allowed circle of confusion is
.01 inch or .25mm. What focal length lens will allow a
near distance of 8 feet and a far distance of infinity
at an aperture of f8? Dan

 

[Dan Fromm]

Do the arithmetic correctly and there will
be no argument.

Take this case: The allowed circle of confusion is
.01 inch or .25mm. What focal length lens will allow a
near distance of 8 feet and a far distance of infinity
at an aperture of f8? Dan

Cute, but irrelevant.

According to the standard DOF formulas I copied from Cox, you can do it with an 85 mm lens focused to around 7.4 m. I'll be happy to send you the spreadsheet by e-mail.

That's nice, but you can't get an acceptably sharp contact print with a CoC of 0.25 mm, let alone an acceptably sharp enlargement. And if you're shooting MF I doubt you're contact printing.

Dave, do you contact print your 35 mm (I assume that's what you shoot wildlife with, if I'm mistaken, sorry) wildlife shots?

So why bother? Why pick an absurd CoC? It makes you look sillier than I'm sure you are.

Cheers,

 

[kaiyen]

I've always thought that the T stop system used on zoom lenses in the cine industry is probably a better system

just curious. what is the t stop system?

allan

 

[Helen B]

"just curious. what is the t stop system?"

Allan,

T stops are transmission numbers that correspond to the theoretical transmission of an f-stop of the same number - they take losses into account, which f-stops don't because they are simply the ratio of the focal length and the entrance pupil diameter. Most ciné lenses are marked in T-stops and f-stops, the difference being about a quarter or a third of a stop for primes and half to two-thirds of a stop for zooms. So you have a nice fast f/0.95 25 mm lens that is really only a slow old T 1.1, or an f/1.1 16 mm to 44 mm zoom that barely makes it to T 1.3, for example. Most of the losses occur at air-glass surfaces, so the lenses with a lot of elements lose out.

Strictly speaking, the f-stops are used for focus calculations and the T-stops for exposure.

Best,
Helen

 

[Ole]

It gets even more fun with old uncoated lenses: If each glass/air interface loses 4% of the light to reflections, an 8-surface lens like a Rodenstock Eurynar will only transmit 60% of the light. But there will be more light in the shadows, as some of the light loss ends up on the film as flare. So an Eurynar at f:8 might be said to give T 10, but the shadows might well be lighter than with a multicoated lens at f:5.6...

 

[gnashings]

So.... what this is leading to is... that... bohica WAS onto something? Or am I misunderstanding where this discussion is heading?

And also - is there a definite, fixed relationship between a T stop and an f stop? My understanding of this so far is that it strictly depends on a particular lens (or even type of glass used?) - is that correct?

I am trying to grasp this, but I am afraid my grasp of the technical aspects of this is a little bit lacking...

 

[Helen B]

The relationship between T-stop and f-stop is not fixed - it does depend on the lens so you understand correctly. Strictly speaking, this does mean that f/2.8 on one lens does not necessarily 'let in as much light' as f/2.8 on another lens, but in practice the difference is negligible, especially between two prime (ie non-zoom) lenses.

For point sources of light, like stars, the diameter of the entrance pupil (absolute aperture) is important, rather than the f-stop (relative aperture). For example, a 100 mm f/2 does let in more light than an 80 mm f/2 in this case, and there is a fixed relationship, because the 100 mm has an entrance pupil 50 mm in diameter and the 80 mm has one 40 mm in diameter. But this only applies to point sources of light, like stars.

Best,
Helen

 

[Ole]

You're right, gnashings. The T stop at a given f stop depends on the type of lens, glasses used, and sometimes even age of lens (like the radioactive APO-Lanthars - mine loses a third of a stop to yellowing).

So there is no fixed relationship. That's the great advantage of TTL measurements, and the reason noone notices that their superduper f:4 IS superzooms are really T 7.2!

 

[Dan Fromm]

You're right, gnashings. The T stop at a given f stop depends on the type of lens, glasses used, and sometimes even age of lens (like the radioactive APO-Lanthars - mine loses a third of a stop to yellowing).

So there is no fixed relationship. That's the great advantage of TTL measurements, and the reason noone notices that their superduper f:4 IS superzooms are really T 7.2!

Um, Ole, my first Beaulieu was a 4008 ZM that had an 8-64/1.9 Angenieux zoom. The lens t-stopped around t/3.3, and TTL metering or no I was painfully aware of how slow it was.

Cheers,