Luminous Landscape Forum

Equipment & Techniques => Cameras, Lenses and Shooting gear => Topic started by: Tim Gray on July 13, 2005, 09:09:16 am

Title: lens digital equivalents
Post by: Tim Gray on July 13, 2005, 09:09:16 am
Here's a thought expirement:

Imagine a slide projector with an image from a 35mm wide angle film camera on a screen a couple of feet away.

The analogy is the projector is the lens and the screen is the 35mm frame of film.  Now take a sheet of ordinary paper and put it against the screen - a bit extreme, but the difference in what is on the paper vs on the screen represents the difference in what's captured using a 1.5 or 1.6 or 1.3 sensor compared to full frame 35mm.
Title: lens digital equivalents
Post by: jani on July 13, 2005, 10:41:55 pm
Quote
do you mean 20d or 10d?
It doesn't matter, the sensor sizes are equivalent.

Quote
i understand the lens doesnt change, just the image we see. and basically, if i understand correctly, it is because the image reflected (or transmitted, i don't know the correct technical term) by the lens is bigger than the sensor that is receiving the image in the case of say, the 20d. so it's like there is an automatic crop...?!?
Yes, that's it. You have a narrower angle of view with that camera than a regular 135 film camera, which makes it easy to think of as though the image was cropped.

The image isn't cropped, though, you get the same angle of view as in what you see in the viewfinder. Except that the 20D only has 95% viewfinder coverage at 0.9x magnification, that is.
Title: lens digital equivalents
Post by: abaazov on July 14, 2005, 05:31:01 pm
well i thought i understood depth of field.....wishful thinking i guess. what started out as a circle of confusion is fast becoming circles of confusion. i appreciate you guys taking the time to answer questions that to you are probably elementary.
amnon

p.s.howard do you have a link to that article? is it on this site?
Title: lens digital equivalents
Post by: howard smith on July 14, 2005, 07:38:05 pm
"The simple fact is that the size of a piece of film or sensor size has NO bearing on DoF at all."

True.  As I suggested, check the link to the DoF calculator and look at the equations for calculating DoF.  Format is not a factor.

My only purpose in bringing this up yet again is to emphasize how DoF is calculated and then the photographer can make whatever assumption s/he desires.
Title: lens digital equivalents
Post by: RBland on June 22, 2005, 03:50:41 pm
Hello All,
I am new to the forum and digital photography.
Is there a formula one can use to get a lens' focal length when used on a digital body. I was told to multiply by 1.5 to get "in the ballpark".
Any thoughts?
Thanks,
RBland
Title: lens digital equivalents
Post by: howard smith on July 13, 2005, 09:26:08 am
Tim's example is perfect.  It also shows that an x mm lens is an x mm lens regardless of the film format.
Title: lens digital equivalents
Post by: abaazov on July 13, 2005, 10:39:55 am
do you mean 20d or 10d?

i understand the lens doesnt change, just the image we see. and basically, if i understand correctly, it is because the image reflected (or transmitted, i don't know the correct technical term) by the lens is bigger than the sensor that is receiving the image in the case of say, the 20d. so it's like there is an automatic crop...?!?
knowing all this, i guess it all really comes into play when you are choosing a wide angle lens. are there any other factors that need to be considered?
Title: lens digital equivalents
Post by: abaazov on July 14, 2005, 05:32:22 pm
yes it is..i found it, thank again
Title: lens digital equivalents
Post by: dazzajl on July 14, 2005, 07:29:17 pm
Quote
Perhaps dazzajl does understand what he's talking about
Gracious of you to say so  :D

The simple fact is that the size of a piece of film or sensor size has NO bearing on DoF at all.

You are confusing capture size and percentage of enlargement, which is different all together.
Title: lens digital equivalents
Post by: Ray on July 16, 2005, 08:43:15 am
Quote
lets go back to the reason it LOOKS like you have a longer focal length on a digiSLR.
The cropping reduces the 'field of view' so it's the same as that of a longer focal length lens, by a factor of 1.6 in the case of a D60.

A 50mm lens on a D60 produces images with the same FoV as an 80mm lens on a 1DsMkll. You would get identical images if you cropped the 1DsMkll image from a 50mm lens so it was the same size as the D60 with the same lens. Essentially no difference in any way, except the 1DsMkll would have lower noise at high ISO's (so I believe).

But you don't necessarily get identical images if you use different lenses with the same f stop; ie. 50mm on the D60 at f8 and 80mm on the 1DsMkll at f8. The D60 image will then exhibit greater DoF, at print sizes and sufficient 3-dimensionality of scene where it's possible to discern the DoF differences, of course.
Title: lens digital equivalents
Post by: Tim Gray on June 22, 2005, 04:29:44 pm
Here a link to a good resource (http://www.dphoto.us/forum/index.php?section=depthoffield)

The method is to take the ratio of the diagonal of the digital sensor to the diagonal of a 35mm frame.

Canon 1DSII  is 1
Canon 1DII is 1.3 (or maybe 1.25 if I recall the exact number)
the 20D and Rebel is 1.6

(pre edit - typo on the 1DSII - said 0 , should be 1)
Title: lens digital equivalents
Post by: Tim Gray on July 13, 2005, 09:07:27 am
Here's a thought expirement:

Imagine a slide projector with an image from a 35mm wide angle film camera on a screen a couple of feet away.

The analogy is the projector is the lens and the screen is the 35mm frame of film.  Now take a sheet of ordinary paper and put it against the screen - a bit extreme, but the difference in what is on the paper vs on the screen represents the difference in what's captured using a 1.5 or 1.6 or 1.3 sensor compared to full frame 35mm.
Title: lens digital equivalents
Post by: howard smith on July 13, 2005, 10:04:46 am
abaazov, be careful with the words.  The 70-200mm lens is still a 70-200mm lens.  When you look in the view finder of your 10D, the image will "appear" the same as the scene viewed with a 112-320mm lens on a regular 35mm camera.
Title: lens digital equivalents
Post by: abaazov on July 14, 2005, 09:08:18 am
yes it does make sense. thanks guys.

amnon
Title: lens digital equivalents
Post by: howard smith on July 14, 2005, 10:23:26 am
abaazov, yes and no.

The information per square cm is the same, so, no.

The number of square cm is less, so, yes.  Some of the information will fall outside the smaller sensor and not be recorded.
Title: lens digital equivalents
Post by: Craig Arnold on July 14, 2005, 12:32:47 pm
Quote
Quote
I was thinking of magnification where the 20D and 35mm camera have the same size view finder image.

I know, I was just trying to add on to what you were saying.  :)  

Many people talk about 200mm lenses becomming a 320 on some digi SLR and so on but as we know that's not the case. There are many times we might choose to move back from a subect and use a longer lens to shrink the depth of field to get a desired look. Changing from a full size sensor to a smaller one will of course not change the DoF at all.
Oh dear! - You've got hold of the wrong end of the stick I'm afraid.

The DOF equation has 4 variables:
1. Sensor/film size
2. Focal length of lens
3. Aperture
4. Distance to subject

If you keep 2,3,4 constant, the DOF increases with a decrease in sensor size - by precisely the value that is usually quoted as the crop factor.

So that 70-200mm lens will give 1.6 time greater DOF on the 20D as it would on the 1DsMkII.

Here's a link to a handy calculator so that you can experiment with the variables.

http://www.dofmaster.com/dofjs.html (http://www.dofmaster.com/dofjs.html)
Title: lens digital equivalents
Post by: abaazov on July 14, 2005, 03:52:40 pm
was the term circle of confusion intended as a pun???
what is circle of confusion in the photography world???
Title: lens digital equivalents
Post by: Craig Arnold on July 14, 2005, 06:31:02 pm
boku

You are of course quite correct, but as an empirical matter as you say COC is determined in the calculator by a lookup table, which in the DSLR world correlates generally very well with the sensor size, print size and viewing conditions are of course assumed to remain constant for the calculation too.

Perhaps dazzajl does understand what he's talking about, but the way he expressed himself it sounded like he was saying that if you keep 2,3,4 constant and switch from a 20D to a 1DsMkII that the DOF would remain constant, which is not correct. And he also doesn't seem to understand that when you crop an image and enlarge it to give the same print size you do in fact change the DOF.

As you say though, this subject has been done to death on these forums.

That's my last word on the matter, I have no desire to have THE last word here however. (Edit - And besides it's been weeks since we had a DOF thread  :p )
Title: lens digital equivalents
Post by: Ray on July 14, 2005, 10:41:53 pm
Quote
"The simple fact is that the size of a piece of film or sensor size has NO bearing on DoF at all."

True.  As I suggested, check the link to the DoF calculator and look at the equations for calculating DoF.  Format is not a factor.
The mathematical link between format and DoF is through the CoC. The DoF calculations are only useful in relation to a specific format and intended print size. CoC is only meaningful in relation to a print size and a degree of enlargement which is directly related to format size.

Since CoC is a required mathematical input to calculate DoF, then 'format' clearly does have a bearing on DoF. I think it might be described as a 'modifier' of input values.
Title: lens digital equivalents
Post by: drh681 on July 16, 2005, 05:48:04 am
circles of confusion  

that about covers it  

lets go back to the reason it LOOKS like you have a longer focal length on a digiSLR.

when you make your  prints, say 4x6, from a 35mm neg that is a 4x enlargement.
from a 15mmx 22.5mm sensor that same 4x6 is a 6.66x enlargement.(ackkk!!! digital is the devil's work!!!) :laugh:
Title: lens digital equivalents
Post by: abaazov on July 12, 2005, 07:23:37 pm
can anyone please explain to me, in layman's terms, the connection between the focal length of a lens and the size of the digital sensor on the camera. am i correct in assuming that the same lens, at the same length, will show two "different" pictures on, say, the 20d and the 1ds?
Title: lens digital equivalents
Post by: jani on July 13, 2005, 09:07:48 am
Quote
so a wide-angle lens on one can will not be wide angle on another. but why does the size of the sensor determine how much a camera sees?
Here's an over-simplification:

That's because a given lens projects an image of a fixed size.

If your lens is made for 35mm cameras, it's projecting an image circle of at least 43mm diameter at the plane of focus.

If your sensor doesn't cover all of that area, it isn't covering all of the image, either.

DPReview has a nice article explaining some of the basics (http://www.dpreview.com/learn/?/Glossary/Camera_System/Sensor_Sizes_01.htm), while Digital Outback Photo has a more technical article (http://www.outbackphoto.com/dp_essentials/dp_essentials_01/essay.html).
Title: lens digital equivalents
Post by: abaazov on July 13, 2005, 09:53:44 am
ok, i am beginning to understand. i appreciate the info guys. so if most of the lenses out there are made for 35mm cameras, and let's say i use a 20d, which has a diagonal sensor size of 27mm, any lens i buy (other than the ones specifically made for the 20d) i will have to use the conversion ratio of 1.6 to get the real focal lens i will effectively be using. so the 70-200 is actually a 112-320 on the 20d??
Title: lens digital equivalents
Post by: dazzajl on July 14, 2005, 07:30:01 am
Quote
When you look in the view finder of your 10D, the image will "appear" the same as the scene viewed with a 112-320mm lens on a regular 35mm camera.
This can be a really tricky subject, there are pitfalls in the statement above too. Not trying to be picky Howard as I know you were talking about the magnification factor and what appears in the frame but it goes deeper.

Yes the 70-200 when used on a 10/20D or other similar camera with fill the frame like a 112-320 would on 35mm film body BUT....  the effect of compression and the depth of field remain true to the 70-200 focal length.

One analogy I like to use is the simple window one. If you move back or forwards looking out of a window the angle of view (or the amount you can see side to side, top to bottom) will increase or decrease. This corresponds to changing a lens on your camera.

If that window was the rear window of your car, then imagine the effect of making the rear view mirror bigger or smaller. As you increase it you will see a larger view (or wider angle) if you were to make it smaller, you would see less. By seeing less you are croping into the view and this is the same as the 10/20D's are doing.

I hope that makes some sense to someone.  
Title: lens digital equivalents
Post by: abaazov on July 14, 2005, 10:09:32 am
is it fair to say that you are "losing" data when you use a lens on a camera with a smaller than 35mm camera sensor? in a way is it a waste?
Title: lens digital equivalents
Post by: boku on July 14, 2005, 12:49:18 pm
Quote
Quote
Quote
I was thinking of magnification where the 20D and 35mm camera have the same size view finder image.

I know, I was just trying to add on to what you were saying.    

Many people talk about 200mm lenses becomming a 320 on some digi SLR and so on but as we know that's not the case. There are many times we might choose to move back from a subect and use a longer lens to shrink the depth of field to get a desired look. Changing from a full size sensor to a smaller one will of course not change the DoF at all.
Oh dear! - You've got hold of the wrong end of the stick I'm afraid.

The DOF equation has 4 variables:
1. Sensor/film size
2. Focal length of lens
3. Aperture
4. Distance to subject

If you keep 2,3,4 constant, the DOF increases with a decrease in sensor size - by precisely the value that is usually quoted as the crop factor.

So that 70-200mm lens will give 1.6 time greater DOF on the 20D as it would on the 1DsMkII.

Here's a link to a handy calculator so that you can experiment with the variables.

http://www.dofmaster.com/dofjs.html (http://www.dofmaster.com/dofjs.html)
Just an observation...

Your #1 is not correct as stated. You really mean circle of confusion. That is empirically related to sensor size, but if the circle of confusion remains the same for different sensor sizes being compared, then your #1 factor is no longer relevant. Look at your link.

This has been discussed at great length here. You might want to do a search on discussions, but they are very arduous. For the practical photographer - and I am one - your #1 factor is not a strong consideration.

I just thought I'd bring this up. You are not entirely right or wrong, but you have failed to explain the rationale. I have as well since it involves final size of the image. I just need to set the record straight. #1 should correctly be stated as circle of confusion.

Over.
Title: lens digital equivalents
Post by: howard smith on July 14, 2005, 04:36:43 pm
abaazov, it is not a pun.  Check out the article "Understanding Depth of Field."  Ansel Adams described it thus:

"The image of a 'point" in the subject should be a 'point' on the film.  If the subject is not exactly in the plane of critical focus, however, this image becomes a small blurred disc, called a circle of confusion.  The size of any circle of confusion becomes smaller as the aperture is reduced, making the image appear sharper.  We define limits of the size of these circles of confusion that we consider to be acceptably sharp focus, even though they are not quite as sharp as the critical focus plane."

The depth of field is the distance between the nearest and farthest point that appear acceptably sharp (or not unacceptably out of focus).

According to Michael's article, you cannot understand depth of field until you understand circle of confusion.  That is for sure true.
Title: lens digital equivalents
Post by: boku on July 14, 2005, 06:42:45 pm
Quote
but as an empirical matter as you say COC is determined in the calculator by a lookup table, which in the DSLR world correlates generally very well with the sensor size, print size and viewing conditions are of course assumed to remain constant for the calculation too.
Yup, I noticed that.

I guess I was just side-stepping starting another DOF-fest!  :D
Title: lens digital equivalents
Post by: Ray on July 14, 2005, 09:30:49 pm
Quote
To say that DoF is reliant on sensor size is like saying you can change DoF by cropping an image.
Here we go again!  

Actually, you can change DoF by cropping an image. Shoot a 2-dimensional object, say a free standing brick wall, set against a distant background which is out of focus. The brick wall is tack sharp. The background is slightly fuzzy because you used f5.6 and were fairly close to the wall. The impression you will get from almost any size print is a sense of a rather shallow DoF.

Crop the image so that all that remains is the brick wall. How do you then describe the DoF? Is it greater or less? Would it make any difference if you used F2.8, F5.6 or f11? Is there anything in the cropped image that would give a clue?

If you think this example is absurd. then I would maintain that making DoF decisions and calculations without regard to format is equally absurd.
Title: lens digital equivalents
Post by: Ray on July 16, 2005, 02:08:27 am
Actually, I'm really surprised at the deep seated confusion on this issue. It's true that you can calculate a CoC on the negative or sensor knowing just the focal length of the lens, the f stop and the distance to subject. But without knowledge of both the format of the negative and the size of print that will result from a specific degree of enlargement from that negative or sensor, the CoC has an unknown effect on DoF. It is therefore not a DoF calculation, but a CoC calculation and incomplete as regards DoF considerations.

To calculate the true DoF that will be apparent on a given size print it is necessary to know 5 essential ingredients.

1. The format of the film or sensor, and included in that is the amended format resulting from post shooting cropping.

2. The degree of enlargement from the film format or amended format (2x, 4x 8x etc).

3. The focal length of the lens.

4.  The f stop.

5. The distance to subject.

If any one of these five ingredients is missing, then accurate DoF calculation is not possible (with regard to a complete composition, of course).

It's possible to use a couple of the variables as a constant to create a formula that only applies to a specific format and degree of enlargement, thus giving the impression that format has no bearing on the matter. But this is pure illusion.

If y = 2x so that I can substitute 2x for all instances of y in the equation, should I then pretend that y doesn't exist? That would be foolish. If y doubles in size then so does 2x.
Title: lens digital equivalents
Post by: drh681 on July 13, 2005, 04:22:14 am
you will see "more" picture from a 1Ds.

the 1Ds sensor is roughly 24mmx36mm or 864 sq.mm. with a diagonal of roughly 43mm.

the 20D sensor is 15mmx22.5mm or 337.5 sq.mm. with a diagonal of 27mm

lenses with less focal length than the diagonal of the sensor(or film) are considered wide angle.

lenses with focal lengh longer than the diagonal of the sensor are considered to be telephoto.

an "ultra" wide angle might have a focal length of half or less of the sensor diagonal.

a nice "portrait" telephoto might have a focal length double the sensor diagonal.
Title: lens digital equivalents
Post by: dazzajl on July 14, 2005, 09:51:37 am
Quote
I was thinking of magnification where the 20D and 35mm camera have the same size view finder image.

I know, I was just trying to add on to what you were saying.  :)  

Many people talk about 200mm lenses becomming a 320 on some digi SLR and so on but as we know that's not the case. There are many times we might choose to move back from a subect and use a longer lens to shrink the depth of field to get a desired look. Changing from a full size sensor to a smaller one will of course not change the DoF at all.
Title: lens digital equivalents
Post by: howard smith on July 14, 2005, 01:14:33 pm
#1 should be circle of confusion.  In addition, read the article "Understanding Depth of Field" on this site.

peripatetic, if you go to the link you provided, schroll down to "equation" (in red) and click there, you will see the way DoF is calculated and that nowhere is there a "format" term.  As Boku said, it is wraped up in the circle of confusion term along with some other things, like print size and viewing conditions.
Title: lens digital equivalents
Post by: Ray on July 15, 2005, 01:33:16 am
Thank you!  :D
Title: lens digital equivalents
Post by: abaazov on July 13, 2005, 07:16:26 am
so a wide-angle lens on one can will not be wide angle on another. but why does the size of the sensor determine how much a camera sees?
Title: lens digital equivalents
Post by: howard smith on July 14, 2005, 09:43:20 am
dazzajl,

Your comments are correct.  A 70-200mm lens is a 70-200mm lens regardless of the camera's format.  I was thinking of magnification where the 20D and 35mm camera have the same size view finder image.
Title: lens digital equivalents
Post by: dazzajl on July 14, 2005, 01:17:45 pm
Quote
Oh dear! - You've got hold of the wrong end of the stick I'm afraid

If you're failing to find me at the end of the stick then that would be because you have the wrong stick

There seems to be a common theme on this board of people ignoring common sense in order to (mis)quote from the text books.

To say that DoF is reliant on sensor size is like saying you can change DoF by cropping an image.
Title: lens digital equivalents
Post by: abaazov on July 15, 2005, 12:17:45 am
isnt it amazing they call this circle of confusion! i never knew photographers had such a well-developed, dark, ironic, sense of humor!