D60 Multiplier Misnomer !

[yvinu]

Hi all,

I've been a lurker here for a while... had a question/comment that I had to put forth ...

I feel that the 1.6x multiplier is a misnomer / at least is misleading because of the following reason:

A true multiplication in the effective focal length of a lens would result in a change in the depth of field from one camera to the other (from a film SLR to the D60) with the D60 having shallower depth of field than the film SLR.

Obviously this is not true because the lens is a constant and so is the iris. Only the size of the picture that is captured is altered !

I understand that the 1.6x is used to allude to the loss of wide angles - but wouldn't it be more accurate to refer to it as a FOV Crop (as is used in the DPreview comparisons) ??

I know this might be nitpicking or semantics - but for the newbies who surf through this site researching their desire to go buy a DSLR, 1.6x + shallower DOF could be seen as an attractive opportunity (for portraiture, macro, etc) - when in fact, all we get is a cropped picture !

regards
Vinu

Rebel 2000, G2, D60

[john_houghton]

So why pick on the D60? Nearly all digital cameras have a focal length multiplier. The Nikon CP995 multiplier is 4.75 (actual range 8mm - 32mm; 35mm effective range 38mm -152mm). I think most people understand what's going on.

John

[defordphoto]

He's trolling...

[Rayz]

I think the point yvinu is making is that cameras like the D30 and D60 use the same lenses as the larger format full frame 35mm and therefore depth of field at any given focal length, as marked on the lens, should be the same for both full frame and 22x15mm sensor, provided the f stop is the same.

In other words, how can mere cropping change DOF? If I take a lanscape at f5.6 with full frame 35mm and then in post-processing crop the image so that it matches a 22x15mm image, does that give me the sort of DOF which I would have got using the same lens at F4 with a D30 or D60?

It's a good question. I don't think yvinu is trolling.

[Morden]

It's a good question. I don't think yvinu is trolling.
I agree.

As I understand it, pictures obtained using the 'smaller' sensor of such as the D60 are exactly what would result from cropping a 35 mm film exposure to the same dimensions.

Or am I missing something?

[Morden]

It's a good question. I don't think yvinu is trolling.
I agree.

As I understand it, pictures obtained using the 'smaller' sensor of such as the D60 are exactly what would result from cropping a 35 mm film exposure to the same dimensions.

Or am I missing something?

[defordphoto]

That's exactly it and the subject has been beaten over and over again. The smaller sensor is basically a crop from lenses designed for 35mm size film. But, it's a very, very good crop at a full 6.5 megapixels. I like it.

Sorry to smack out but just look back (not all that far) on ANY of the forums on the Net and this has been discussed time and time and time and time again. ;)

[Roger_Cavanagh]

One point that has not been mentioned in this discussion is that DOF is also a function of the Circle of Confusion (CoC) and the degree of enlargment. For 35mm cameras, the value used is typically 0.03mm. This is supposed to be the largest circle that would appear to be a single point on an 8x10 enlargement. Or put another way, two lines with a gap of 0.03mm or less would appear to be a single line.

Since the sensor on the D30/60 is smaller than 35mm film, it is necessary to enlarge further to get the same size print. The CoC is reduced to 0.019mm and DOF is, therefore, smaller.

Bear in mind, however, that these numbers are based on the ability of the human eye to discriminate detail. Sicne people are different, what is sharp to one person may not be to another. With my eyes, everything looks blurry. :)

Regards,

[Rayz]

RFMSports wrote:

Sorry to smack out but just look back (not all that far) on ANY of the forums on the Net and this has been discussed time and time and time and time again. ;)


True! And yet this is still not understood, it seems. There's currently a discussion on this very topic at Luminous Landscape and an article there by Nick Rains attempting to clarify the matter.

In my view (or should I write IMHO), it is not often appreciated that when we use a 31mm lens designed for the 35mm format with the D60 as a standard lens, that 31mm lens throws a bigger image circle than it needs to. If the 31mm lens had been designed for the smaller 22.7x15mm format, the F stop markings on the lens would be different. There's where the 'circle of confusion' lies ..... I think .... er....not entirely sure, though.

Ray

[defordphoto]

Yeah, good point Ray. The thing that amuses me is that I read on all these forums (Here, Luminous, DPReview, Galbraith, FredMiranda, etc) and there's all this angst and worry and wringing-of-the-hands about the less-than-35mm sensor and DOF, and this and that and this and that, that I wonder if any of these fine folks actually have time to go out and use the camera rather than sitting in front of their computers arguing about it! ;)

And sure I want to know all I can about my camera but c'mon! I have much, much more fun out shooting rather than worring about 'airy disks' and all that schtuff or what the EXACT terms to use when referring to the smaller sensor and how it 'multiplies' your focal length, but not really. BLAH! I don't really care...I just wanna shoot!

Which, is where I'm headed off to now on this beautiful fall day.

[Morden]

Yeah, good point Ray. The thing that amuses me is that I read on all these forums (Here, Luminous, DPReview, Galbraith, FredMiranda, etc) and there's all this angst and worry and wringing-of-the-hands about the less-than-35mm sensor and DOF, and this and that and this and that, that I wonder if any of these fine folks actually have time to go out and use the camera rather than sitting in front of their computers arguing about it! ;)

And sure I want to know all I can about my camera but c'mon! I have much, much more fun out shooting rather than worring about 'airy disks' and all that schtuff or what the EXACT terms to use when referring to the smaller sensor and how it 'multiplies' your focal length, but not really. BLAH! I don't really care...I just wanna shoot!
Well said. :)

[Rayz]

Who's worrying about it? I'm not. I'm quite satisfied that my D60 performs well at F16.

Cheers!

[Rayz]

RFMSports wrote:
BLAH! I don't really care...I just wanna shoot!

Which, is where I'm headed off to now on this beautiful fall day.

I'm quite curious about this attitude. Do some of you guys never process the images you shoot? Last time I converted about 100 RAW D60 images to 16 bit TIFF (a couple of days ago) it took an hour or two. When I scan a batch of slides or negatives at 14 bit and 4000 dpi, it seems to take ages and a Super A3 print takes 5 to 10 minutes. This is all time one can be on the internet. There's a thing called multitasking. You don't have to make a choice between taking advantage of a fine day or sitting in front of your monitor typing postings.

Do some of you guys have trouble organising your time? (Don't know why the big smile icon is not working. Just imagine it.)

[AJSJones]

Rayz wrote:

In my view (or should I write IMHO), it is not often appreciated that when we use a 31mm lens designed for the 35mm format with the D60 as a standard lens, that 31mm lens throws a bigger image circle than it needs to. If the 31mm lens had been designed for the smaller 22.7x15mm format, the F stop markings on the lens would be different. There's where the 'circle of confusion' lies ..... I think .... er....not entirely sure, though.

Ray

Ray, You're right about the image circle being bigger than it needs to be since the sensor area is smaller. However, the F stop markings wouldn't change - they're an intrinsic property of the lens, regardless of the format being used. It's the relationship between the physical dimension (diameter) of the aperture and the focal length of the lens. In an f/1 lens they are the same; in an f/2, the aperture is 1/2 the focal length and f/4 is 1/4. That's why it's written as f/4 etc

[Morden]

.... There's a thing called multitasking. You don't have to make a choice between taking advantage of a fine day or sitting in front of your monitor typing postings.

Do some of you guys have trouble organising your time? (Don't know why the big smile icon is not working. Just imagine it.)
Whilst you are correct about having time available whilst image files are being transferred, converted, processed, printed, etc., you assume that all of that time must be spent in photography-orientated activity.

I do 'multitask' as best I can; while I await file transfers, conversions.... etc. I work (not related to photography), eat, play games and all manner of other things.

[Rayz]

AJSJones wrote:


Ray, You're right about the image circle being bigger than it needs to be since the sensor area is smaller. However, the F stop markings wouldn't change - they're an intrinsic property of the lens, regardless of the format being used. It's the relationship between the physical dimension (diameter) of the aperture and the focal length of the lens. In an f/1 lens they are the same; in an f/2, the aperture is 1/2 the focal length and f/4 is 1/4. That's why it's written as f/4 etc


I believe you're right. If the f stop markings were different, the exposure would be wrong. What has changed is not the F stop but the aperture of the lens. A 31mm lens designed for the D60 would have a smaller aperture at, say F8, than a 31mm lens designed for the 35mm format. Consequently, there would be greater depth of field with the D60 designed lens at the same F stop.

I'm making a distiction here between aperture and f stop. When I say the aperture will have changed, I mean the physical diameter of the lens opening.

Phew! Glad that's sorted.

[Rayz]

Morden wrote:


I do 'multitask' as best I can; while I await file transfers, conversions.... etc. I work (not related to photography), eat, play games and all manner of other things.

Point taken. You've got to eat.

[AJSJones]

Rayz wrote:

I believe you're right. If the f stop markings were different, the exposure would be wrong. What has changed is not the F stop but the aperture of the lens. A 31mm lens designed for the D60 would have a smaller aperture at, say F8, than a 31mm lens designed for the 35mm format. Consequently, there would be greater depth of field with the D60 designed lens at the same F stop.

I'm making a distiction here between aperture and f stop. When I say the aperture will have changed, I mean the physical diameter of the lens opening.

Phew! Glad that's sorted.

Whoa, not so fast!
Ray, A 31 mm lens is always a 31 mm lens and its aperture at f/8 will always be a shade under 4 mm. If you changed your reply to say a 31 mm lens (for the D60, which is "equivalent" to a 50 mm lens for a full frame sensor) would have a smaller physical aperture at f/8 than a "real" 50 mm lens (which would capture the same image on the full frame 35 mm camera), you'd be right. However, DOF is related to the f stop, not the physical diameter of the aperture....

Are we sorted yet? :)

Can't understand why my home computer and work computer (Mozilla and IE respectively) are both producing these "vertical" messages..

Andy

[Dans_D60]

Andy:

I checked out your "vertical" problem with FrontPage. The placement of your reply using the "Reply with Quote" is not aligned. Your reply generates a cell in a table. The other quotes are also cells. Your quote cannot “span” the table columns correctly. The PHP generator is building the following statement: “td colspan="1" b AJSJones wrote” ... it should create: “td colspan="2". Anyway, try removing some of the previous quotes or use the reply only function (the one without previous quotes).
Dan

[Rayz]

AJSJones wrote:

Ray, A 31 mm lens is always a 31 mm lens and its aperture at f/8 will always be a shade under 4 mm.

Are you sure? What about an ultra wide angle 31mm lens for MF? If its aperture is still a shade under 4mm at F8, how does it illuminate a much larger area with the same intensity so the exposure is the same? I would have thought that at any given F stop, the aperture diameter will vary in proportion to the diameter of the image circle the lens is designed to throw. Seems reasonable, doesn't it?

[AJSJones]

Dan,
Thanks for fixing the vertical problem ! Guess I should read the fine print on how to enter text in such situations...

Ray, Image circle size is determined by lens design and is sometimes called "angle of view" - which confused the heck out of me when I thought about "wide-angle" vs "normal" lenses. However, in my research for large format cameras I often come across lenses of the the SAME focal length having very different image circle diameters (or "angle of view"). For a 4x5 camera, a 47 mm lens is considered extremely "wide-angle" but if you tried to use a 47 (let's call it 50) mm lens designed for a 35 mm format on a 4x5 camera the image would not reach the edges of the film because it has a "narrow" angle of view (small image circle).

[ sidebar : If you had a tool that could take a cylindrical chunk of a 50 mm lens from the center along the optical axis, it would still be a 50 mm lens but you'd only get a small image with a narrow angle of view. As you make this cylindrical "chunk" wider in diameter, your visible image would expand out and the angle of view would be larger. i.e. you are getting a bigger sampling of the image but the image being sampled is constant. For the D60 this chunk could be narrower than for 35 mm full frame sensor size and for 4x5 it would have to be even bigger.]

So, physical apertures for f stops are DEFINED by the focal length of the lens while image circles (angles of view) are determined by lens design.

Where this all started was related to DoF and the multiplier thingy. As Roger (I think) pointed out, making an 8x12 from the D60 sensor (with your 31 mm lens) involves greater magnification than from the 24x36 from a full frame sensor (with your 50 mm lens which therefore has the same image on it) and that the CoC is correspondingly enlarged more so DoF should be smaller.

Andy

PS

This is not to be confused with the long thread started by zilch0mnd (sp?) which explained that perceived print sharpness becomes limited when you use f stops that are too small (such that diffraction occurs) and you get "Airy" disks exceeding the size of the circles of confusion. This phenomenon is separate from but still relates to the issue of DoF and CoC.

[Rayz]

AJSJones,
Not sure what you're saying. Do you agree that the actual diameter of the aperture is not a constant for a given F stop and focal length and can vary according to lens design?

Ray

[AJSJones]

No, I don't agree! The physical size of the aperture is DEFINED by specifying a given f stop and focal length, regardless of lens design. The physical size of the lens can vary with design and the physical size of the diaphragm in the shutter mechanism can vary BUT when you dial in f/8 for an 80 mm lens, the hole in the diaphragm will always be 10 mm in diameter.

Let's try another sidebar since I can't find a site with the right picture to illustrate it.
Imagine the light rays (no offense!) coming through the lens forming a cone between the lens and the film. The tip of the cone is the principal or nodal point of the lens and is where the aperture is located and the base of the cone is on the film (and is called the image circle). The height of this cone is the focal length, i.e. where the image from infinity is in focus. It is possible to keep the height of this cone the same (i.e. keep the focal length constant) but vary its diameter, by varying the lens design. A 50 mm lens for a video camera only needs a conebase which is e.g. 1/2" in diameter to cover the sensor, but a 50 mm lens for a 4x5 film needs a conebase which is over 6" in diameter to cover the film with the image. These are both 50mm lenses and at f/2 will have physical apertures of 25 mm. They are very different designs, but the image coming through is the same - it's just that the video camera only 'sees' a tiny portion in the center - has a narrower field of view. That's why the D60 factor of 1.6 is often called a FOV factor.

http://www.largeformatphotography.info/

has lots of info. See the lens comparison charts. Schneider makes (at least) 2 lenses with focal length of 210 mm. Super SymmarXL is for 8x10, has an image circle of 500 mm, has an angle of coverage of 100° and weighs a little over 4lb. The Xenar at 210 mm has an image circle of 250mm, an angle of coverage of 60° and weighs less than 1 lb. The former has a very wide cone and the latter a much narrower one. And they both focus infinity when they're 210 mm from the film (the cone-height is the same).

But at f/22 they both have an aperture of a shade under 10mm. On the 4x5 film the two lenses would produce the same image. You can use the big one on a 4x5 or 8x10, but the smaller Xenar would not produce an image that filled the 8x10 film, but the part it did produce would be the center portion of what the large one produced (circular crop)

Hope that helps...

Andy

[Rayz]

Andy,
Understood. The diameter of the aperture is given by the focal length divided by the F stop. Basic definition.

I was momentarily confused by imagining that the aperture alone governed the amount of light passing through the lens. However, as lens design changes to produce a larger image circle, proportionally more light is gathered, ie. the lens becomes a wider angle lens although the focal length remains the same.

[AJSJones]

Yes! It becomes a wider-angle lens in the sense that it gathers a wider angle of view of the same subject matter at the given focal length. (it has more "coverage")

The original confusion stems from the widespread use of "wide-angle/normal/tele" to describe FOCAL LENGTH properties of lenses - with the assumption that the whole world is based on the 35 mm format...

Fun chatting with you!
Andy

[yvinu]

Thanks for the various perspectives and the detail discussion.

I've been away for a while and just got back. Did get some photos processed and posted...

for those who're interested, the pics are at http://www.angelfire.com/or3/yvinu/pix/elp-nm/index.htm

Comments are welcome.
Regards
Vinu

[Phil Hall]

I had assumed for some time that the 1.6X factor was amultiplier of the focal, I assumed the chip surface had been moved forward. Ayhow, the cropping scenario makes more sense. I had been setting the focal length on my 550 EX to 1.6 the actual lense focal length. There was no recommended correction in the manual, which lends credibility to the cropped concept.

[hmhm]

Phil Hall wrote:
...I had been setting the focal length on my 550 EX to 1.6 the actual lense focal length. There was no recommended correction in the manual, which lends credibility to the cropped concept...


Setting the flash to a 1.6x focal length is still an appropriate thing to do (you'd think the camera would be able to automatically report the lens with the appropriate multiplier already applied, but it don't seem to). The flash just cares about the field of view of the lens, and the field of view of a lens on a D60 is the same as the field of view of a lens with a 1.6x focal length on a 35mm full frame camera.

When people say "it's not _really_ a focal length multiplier", that's true (albeit slightly pedantic), as the 1.6x multiplier is an accurate rule of thumb for some phenomena (such as field of view), but not really directly applicable for others (such as depth of field).
-harry