Lens Mathmatics for dummies

[Nick D]

I notice many people her have a good understanding of the mathematical/ scientific concept of digital photography. Now as being fairly new to slr photography, I'd like to get an understanding of 4/3 lens with reference to f stop.

As we know to maintain exposure if we step down our aperature we must increase our shutter speed, proportionally.

I also have read how with 4/3 there seems to be a greater dof at a given f stop than 2/3 or 135 sensors. the question I have is does the 35-100 f2 in effective terminology become 70-200 f2, 70-200 f4( since you double one do you halve the other?), or something in between?

Sorry if I ran on a bit. I'm not looking for all the technical aspects, just a general understanding.

 

[simpy]

I also have read how with 4/3 there seems to be a greater dof at a
given f stop than 2/3 or 135 sensors. the question I have is does the
35-100 f2 in effective terminology become 70-200 f2, 70-200 f4( since
you double one do you halve the other?), or something in between?

In terms of DOF, the 35-100 f/2 is comparable to a 70-200 f/4 on 135 format. You double both the focal length and the f-number to get the effective equivalent.

Simon

 

[Nick D]

Thank you Simon. i appreciate that

 

[ray2580]

does the 35-100 f2 in effective terminology become 70-200 f2, 70-200 f4( since
you double one do you halve the other?), or something in between?

Neither the former nor the latter, and absolutely not something in between. Simply it is still a 35-100mm f2 lens. There is no real "equivalent" value exist!

For the 4/3 system, you can regard 25mm your standard lens, longer than that are teles, shorter than that wide angles.

 

[AnandaSim]

I also have read how with 4/3 there seems to be a greater dof at a
given f stop than 2/3 or 135 sensors. the question I have is does the
35-100 f2 in effective terminology become 70-200 f2, 70-200 f4( since
you double one do you halve the other?), or something in between?

2:3 or more often called 3:2 is a ratio, not a sensor size. The Nikon and Canon APS-C sensors are not 24mm x 36mm but they are 3:2 ratio. Thus the calculation Simon gave was for a sensor that is 24mm x 36mm, not for APS-C sensors.

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[ray2580]

DOF varies with focal length of lens, focus to subject distance and aperture only, not related to what camera format!

In addition, trust your camera or the light meter for exposure.

 

[FrankyM]

http://en.wikipedia.org/wiki/Depth_of_field

Frank

 

[Louis_Dobson]

It becomes a 70-200 f2 in terms of shutter speed, and a 70-200 f4 in terms of DoF.

In practical terms, since a 35mmFF camera has a two stop noise advantage, you could also call it a 70-200 f4 (and I do), but that's when the endless arguments start.

By the way, f4 is more than fast enough in DoF terms with a lens that length. I have the Nikkor 70-200 f2.8 on 35mmFF, and while playing with that DoF is jolly amusing, it rarely works out.

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[Nick D]

Frank:

Thank you for that link. I just was browsing through it, and even though its wikipedia, its seems rather informative. I'll need to go ver it a bit slowly.

Ray:

Sorry if I irritated you. You absolute assertions with exclamation points display a measure of hostility I wasn't expecting. I do understand the relationship to dof with and focal length.

The reason I was asking is the threads that compare FF(35mm equivalent) to 4/3. So many people use (including oly on their lenses in stores) the efd of there 4/3 lenses. I was trying to stay as vague about this so it didn't get hijacked into a ff vs. 4/3 argument.

I don't get involved in the arguments with these topics, but i was wondering how the 2x crop factor effects dof when I read other threads that argue this point. Just was looking for some insight for my personal referrance. to the validity of these arguments.

When I was discussing the cost of oly (not saying economical or not) glass to similar other mfg. The argument that i get is you need this much of mfg X to get same reach as oly. I was wondering if though you get that reach do you lose light and what happens to dof.

i do thank you though and have a good day

 

[Nick D]

Ah Louis:

Thank you. that was really all i was looking for. I was actually hoping to hear from you here with this topic and wish I saw your reply before i added the other reply i did.

You seem to get a grasp for what someone is striving to understand without sounding pompous, arrogant, or condescending.

Always good to hear from you.

PS, its been a while since I've posted here and i switched isp so i had to change my user name. i couldn't log on my old name. Not that i expect anone to remember me but i was "photo_facinated" before

 

[Louis_Dobson]

of thing.

1:fn = A/fl so the maths is perfectly simple, keep A constant, double fl, the f number also doubles. Equally clear for DoF and perspective, where fl is not discounted. The only slight complication is aspect ratio, but that difference is marginal, and the only contentious bit is whether you should take into account pixel density and hence noise.

Simple maths leads me to the conclusion that:

1) In exposure terms an f2 lens is an f2 lens (because the f number is calculated with a focal length corrector).

2) In any calculation that does not include an fl corrector, the relevant number will double (or if ratio, halve), thus in perspective terms it is equivalent to a 70-200, and in DoF terms it is an f4.

Why is this misleading, and why does it need vague disparagement?

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[S N G]

does the 35-100 f2 in effective terminology become 70-200 f2, 70-200 f4( since
you double one do you halve the other?), or something in between?

Neither the former nor the latter, and absolutely not something in
between. Simply it is still a 35-100mm f2 lens. There is no real
"equivalent" value exist!

In terms of equivalence he is correct, although the actual lens for comparison may not exist. To get identical and from the same spot to 35/2 on 4/3rd shot one would need to use 70/4 on FF camera.

For the 4/3 system, you can regard 25mm your standard lens, longer
than that are teles, shorter than that wide angles.

But that does not go hand in hand with what you said above. I do agree with this though.

• sergey

 

[Nick D]

And thats about all I wanted to know and understand, before I overload my simple mind.

 

[S N G]

DOF varies with focal length of lens, focus to subject distance and
aperture only, not related to what camera format!

Focal length, distance to subject, and aperture.

And this is why (as Simon stated) 35-100 f/2 is comparable to a 70-200 f/4 on FF format. Larger sensor will give larger angle of view. For the same framing on FF camera one needs to either come closer to the subject (the DoF changes) or use longer lens (the DoF changes again). To compensate for DoF (or keep it same as on 4/3rd) the aperture needs to be stopped down to equivalent value as well.

In addition, trust your camera or the light meter for exposure.

Always.

• sergey

 

[S N G]

It becomes a 70-200 f2 in terms of shutter speed, and a 70-200 f4 in
terms of DoF.

You have to double focal length as well. Metering remains the same (as amount of light per unit).

In practical terms, since a 35mmFF camera has a two stop noise
advantage, you could also call it a 70-200 f4 (and I do), but that's
when the endless arguments start.

There should be no argument when there is one truth. In practical terms we have to also take into consideration light gathering ability of larger sensor in comparison to a smaller one. The numbers work out in exactly the same order; add one stop to APS-C 1.5x or two stops to FF. See my post here,

http://forums.dpreview.com/forums/read.asp?forum=1022&message=28422978

By the way, f4 is more than fast enough in DoF terms with a lens that
length. I have the Nikkor 70-200 f2.8 on 35mmFF, and while playing
with that DoF is jolly amusing, it rarely works out.

Always works for me. Absolutely wonderful lens!

• sergey

 

[John King]

Gidday Sergey

does the 35-100 f2 in effective terminology become 70-200 f2, 70-200 f4( since
you double one do you halve the other?), or something in between?

Neither the former nor the latter, and absolutely not something in
between. Simply it is still a 35-100mm f2 lens. There is no real
"equivalent" value exist!

In terms of equivalence he is correct, although the actual lens for
comparison may not exist. To get identical and from the same spot to
35/2 on 4/3rd shot one would need to use 70/4 on FF camera.

"Equivalence" is complete rubbish, Sergey, and you know it. This must be the 100th time you this up, and the 100th time that you have had to be corrected.

If "equivalence" as espoused by your mate, Joseph James (joe mama) were correct, no light meter would work, and no exposure could ever be calculated.

If the correct exposure is f8 @ 125th second for the smallest sensor camera around, it is also the correct exposure for a 10" x 8 " format camera at the same ISO.

Get an ultra-basic book on exposure, Sergey - AND read it ...

For the 4/3 system, you can regard 25mm your standard lens, longer
than that are teles, shorter than that wide angles.

That, at least, is correct.

Ray, f2 is always f2. "Equivalence" is just b/s spruiked by the c/n trolls to try to suggest that somehow an Olympus f2 lens is slow, just like a c/n f4 lens is ...

An f2/35~100 lens is a very fast, very high quality lens.

Regards, john from Melbourne, Australia.
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Bird Control Officers on active service.

 

[ray2580]

Nick,

I shouldn't have added exclamations, thanks for remind.

Because of working need, I use cameras of different brands and different frame sizes.

Experience tells me it is very easy to make calculation mistakes if you use "35mm equivalent" or "crop factor" to find the right lens and exposure factors you need, especially when in hurry. What I am doing is to remember just the "standard" value of a system. Say, 80 for 120, 50 for 35mmFF, 40 for APS-H, 30 for APS-Cs, 25 for 4/3. Let me repeat, longer than standard are teles, shorter than standard wides.

The only way to avoid calculation mistake is not to calculate, my token!

Happy shooting.

Ray

 

[Nick D]

Thank you too Sergy. These are the easy to understand explinations i'm looking for. BTW, the only reason i used 35-100 f2 was the easy numbers when doing math. 2, 100 i like those. I wasn't tryng to compare that lens with the others' conterpart, but wanted to understand when i read these "discussions" or arguments as they sometimes turn into, what is going on.

Again thank you to all who responded, i really appreciate it

 

[simpy]

If "equivalence" as espoused by your mate, Joseph James (joe mama)
were correct, no light meter would work, and no exposure could ever
be calculated.

You are not really getting what equivalence means, do you? It means that a 50mm f/2 on 4/3 gives similar images to a 100mm f/4 on full frame assuming that you bump the ISO two stops. Exposure works fine that way.

Get an ultra-basic book on exposure, Sergey - AND read it ...

And please read the arguments of other posters.

Ray, f2 is always f2. "Equivalence" is just b/s spruiked by the c/n
trolls to try to suggest that somehow an Olympus f2 lens is slow,
just like a c/n f4 lens is ...

It's a bit cheap to play the troll card, don't you think? I'm an Olympus user myself, and know a fair bit about physics and mathematics and it's all pretty obvious if you juggle the equations - and keep an open mind.

An f2/35~100 lens is a very fast, very high quality lens.

That is true, but it may not be much more useful than a 70-200 f/4 on full frame. [It could still be a better lens than any particular 70-200 f/4, of course]

Simon

 

[Louis_Dobson]

then it is f4 euivalent, no?

On the D3 you really have naff all DoF to play with wide open. Look at the chap on the left... (this shot would have been better at f4, or even 5.6)



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