rangefinder vs slr lens to film distance and its effect. . . .

[madkins]

hey there, folks. . . .

 

sorry for the strange subject line, i just didn't have a better way

to title it.

 

i just got back some of the film from a shoot i did with my olympus

OM system (as well as a sort of random roll i took with the

leica). . . xp2 was the film (the film base is neutral enough that i

can get decent prints out of it in my darkroom).

 

anyhow. . . and i do realize that i'm using two different

systems/different lenses, i noticed that the depth of field when

shot wide open seems to come out a bit different through the

different camera systems.

 

so, please excuse my ignorance on this, but. . . with the two

different cameras, let's say you're using a 50mm lens. . . and

they're both weighing in at about an f2 wide open. . . does the

distance between the rear lens element (which is where i figure you

measure your lens to film plane distance, right?) make a significant

difference in how the lenses show depth of field/etc? just

wondering cause i'd read something in an earlier thread about how

most rangefinders (since they don't have a mirror in the way) can

have their lenses closer to the film plane. . . didn't know what

kind of difference that would make in the final photographs. . . .

 

sorry for the long rambling (maybe nonsensical) question. . . .

thanks for any answers you can provide.

[ky2]

A focal length is a focal length is a focal length. Since we're also using the same fstop and the same emulsion size-- if your subject is at the same distance, DOF shall be the same.

 

Ofcourse, OOF-rendering may be different.

[walterh]

"...make a significant difference in how the lenses show depth of field/etc? "

 

Michael the depth of field is not affected by the difference of SLR and rangefinder cameras. That includes the larger distance of the rear element to the film plane due to the mirror in the SLR. Same focal length and same aperture will always result in the same depth of field.

 

Your "etc" may include many things :-) Some of these are the higher degree of freedom for the lens design especially for wide angle lenses if the lens can be closer to the film plane in a rangefinder camera. Typically wide angle lenses could profit a lot from the less severe restrictions in the design and some lenses actually do, especially extreme wide angle lenses, take the 15mm CV Heliar for example. A nice very moderately priced extreme wide. It is possible to get excellent wide angle lenses for SLR - it is just a bit harder to do - and a bit more expensive too.

 

Also even if the depth of field is the same the way the image appears may show a subtle difference to the viewers perception of the space and the motives in focus and this could seem to effect depth of field. Probably another factor comes into play. Users of rangefinder cameras may have different expectations as how their images should look and the lens designs of many say Leica lenses will reflect this difference in image appearance. But this may not necessarily be based on the difference of rangefinder versus SLR cameras, more related to the design goals of the manufacturer. I suppose Leica or Zeiss could produce lenses that have the same "look" as say Nikon lenses, they just prefer not to do this.

 

I better say no more and leave the discussion to the Leice experts :-P The discussion could be endless^^.

[fred_c1]

"i noticed that the depth of field when shot wide open seems to come out a bit different through the different camera systems."<P>

 

Assuming both cameras were focused at the same point, the difference may be caused by the lenses having different flatness-of-fields. The simplest example is photographing a flat surface like that of a book cover, with the film plane parrallel to it and the lens wide open. All points on the cover will be in sharp focus if the lens has a flat field (at that particular distance.) If the field is not flat, only the central portion of the cover will be in focus.<P>

 

Another possible cause is <a href=http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=002ibd&tag=>focus shift</a>. A rangefinder camera is more susceptible to this because you cannot check focus like you do with an SLR.

[deardorff8x10]

I am not an optiacl engineer, but lens to film distance can be different for the same focal

lengths. Relative to focal length with fairly short focal length LF lenses, there can be

pretty different rear element to film distances. I mostly use LF and can testify that what LF

calls "film to flange distance" really does vary across lenses. This might not be the same

as the rear element, but is where the lens mounts.

 

 

I would also say that from a subjective standpoint, different types of lenses with the same

focal length (e.g. I have two different 300mm lenses for LF) certainly seem to me to have

different DOF at the same aperture, but I have no science to back this up; perhaps it is just

different quality of the out of focus areas, inaccurate f-stops, or something like that.

 

Take a look at this website (don't know how to make a direct link):

http://tonopahpictures.0catch.com/Eidoscope_Petzval_Imagon/PeachBlossomBokeh.html

[ross_wilson1]

I'm pretty sure Michael that the fact a rangefinder has no mirror is only relevent in the wide angle lenses from about 28mm and wider.

 

These wide lenses need to be very close to the film plane in order to be so wide. When the SLR came along they discovered there was a mirror in the the way, so they created what's called a retro focus design. This bounced the light back into the lens and then back towards the film plane again. I think it works like this: the distance the light travels in the retro focus design back into the lens and then out again to the film plane, is the distance it would normally travel to glass elements near the film plane that aren't there due to the mirror.

 

It's something like that I'm sure.

 

However, it has little to do with your aperture I believe, the depth of field will 'look' different even at the same aperture because lens manufacturing characteristics. If you're saying that you think there is a difference in the distance in focus at the same aperture, then, there shouldn't be, not that it would be at all easy to tell if not impossible.

 

Obviously feel free to correct me if I've gone wrong.

[blakley]

Michael,

 

Film-to-flange distance does vary, but nodal point to film distance does not - nodal point to film distance is the same for a 50mm lens on any camera in any format. Rangefinder lens designers have more freedom than SLR designers because they can put more rear elements in the nodal point to film distance (i.e. move the flange closer to the film) without having to worry about hitting a mirror.

 

DOF isn't affected EITHER by nodal point to film distance OR film-to-flange distance - it's affected only by the angle of view of the lens compared to the diagonal of the film format you're projecting the image onto.

[deardorff8x10]

Not sure I understand the angle of view aspect -- DOF of a 120 degree 110mm lens on 4x5

or 5x7 film has to be the same. I would like to understand this better.

 

I get the retrofocus, non-retrofocus, etc. It must be different optical engineering that leads

to different looks, e.g. as the post above notes, flatness of field is important and I assume

overall correction of point of focus of different colors probably are important in the look of

DOF. A meniscus sure looks different to an apo lens.

[vivek iyer]

Michael, Yes. There are other factors besides just the magnification. DOF is based on a subjective concept of COC.

 

Rangefinder lenses made for LTM/Leica M are generally better than the SLR counterparts in certain aspects. The 50/1.4 Nikkor (LTM) and the 50/1.4 Nikkor (F Mount) have different field of curvatures and actual (real life) resolving abilities.

[walterh]

Michael do not worry about the angle of view if you compare rangefinder with SLR using the same 24mmx36mm film format - the angle of view is the same for the same focal length. The magnification of the film image during printing to the same print size is also the same. Therefore the sharpness is also the same. This makes the comparison simple. Everything is the same - including depth of field.

 

All other factors like length of the lens, distance of rear element to film are as unimportant as weight of the lens. All have no effect.

 

If you want to be picky the only point that might make a very very small difference can be the difference between actual and nominal focal length. One "50mm" lens may be 47 mm, another type of "50mm" lens may be 53mm. Both would be sold as 50mm for convinience. Now we are talking about 6mm difference in focal length. That would make a difference in DOF but still hardly ever visible unless you shoot the same object under identical conditions and compare carefully.

 

The differences you see are most likely differing shooting conditions and sometimes different "characters" of lenses.

 

If we start to include different image formats we may as well start to include lens and back movements of view cameras we will get very far away from the original question.

[vivek iyer]

"The magnification of the film image during printing to the same print size is also the same. Therefore the sharpness is also the same. This makes the comparison simple. Everything is the same - including depth of field."

 

Interesting, Walter.

[madkins]

thanks so much for the help, folks. . . . i figured this would be something that someone with better knowledge of lens design/physics and all that would be able to answer (i'm pretty much in the dark on all but the most basic aspects of that).

 

from what all of you have stated, i figure it's just unique characteristics of the lenses that i used that are creating the differences. . . .

 

thanks again!

[stephen hazelton]

Minor variations in marked focal length versus actual or in marked aperture versus actual could affect the comparison as well. If you focused a bit farther in or out with one camera, it may not look like depth-of-field differences.

 

Depth of field calculations are usually made assuming a lens is a single piece of glass of zero thickness at the aperture. If fact, they're very 3-dimensional, and it's conceivable that variations in the arrangements of elements could affect depth of field somewhat.

[astral]

A note on retrofocus lens designs and dof, etc:

 

These are essentially reversed telephoto designs in which the effective focal length of the lens is less than its physical backfocus distance (ie the distance between the rear lens element and the image plane).

 

In principal, an additional wide angle 'supplementary' lens is placed in front of the main lens components, thus shifting the nodal point "forwards" (this is the origin of 'retro' - moves the nodal point "back" towards the front of the lens), thereby allowing the 'exit pupil' (rear elements) to be placed farther away from the film plane - primarily to allow space for a slr's mirror-box.

 

While all this has the effect of shortening the focal length, a retrofocus lens will invariably be physically longer than its focal length implies: some very short focal length retrofocus lenses are very wide at the front (the "supplementary" component) and pretty long too. A down-side of retrofocus designs is that field illumination (and sharpness) in the corners of the image can suffer, compared to non-retrofocus designs: modern formulations have 'largely' overcome this.

 

The design & construction of a lens has no practical bearing on the extent of depth of field - the diaphragm settings do, of course. But the placement of the diaphragm, as well as other aspects of the lens design, do impact significantly on the out of focus (and in focus) images, flatness of field, and control of chromatic and spherical abberations, etc. Lenses which have variable defocus are more complicated and require more explanation than any of us should really need to know! :-)

 

Fingers are crossed that I got all that right! It's amazing what gems are stored away in my memory .... the problem nowadays is getting them out when I need 'em!!

 

Alan

[astral]

Ooops - shudda checked it better!

 

In the above: "A down-side of retrofocus designs is that field illumination (and sharpness) in the corners of the image can suffer, compared to non-retrofocus designs: modern formulations have 'largely' overcome this."

 

Should read: "A down-side of non-retrofocus designs is that field illumination (and sharpness) in the corners of the image can suffer, compared to retrofocus designs: modern formulations have 'largely' overcome this."

 

Well, it is 2am here!

[Craig_Cooper11664875449]

Allan,

 

As a question given lens design points you raised in your post; By definition of what occurs at a lens nodal point (convergence/ intersection of the light rays) would this necessitate that by design an apeture diaphram be placed very close to the lens nodal point in order for everything to function when the apeture is shut down?

 

Craig

[dan flanders]

I've never been able to get a reall convincing answer but my terpsichorean angels tell me that the correct way to measure is the distance from the film plane to the object. Since the lens moves back and forth to focus it divides the overall distance into the image distance and object distance, all according to mathmatical formula. Ergo, if the aperture and object distance are constant the DOF would be also. Actually if there was any difference it would be so small as to be of interest only to the angels. I don't worry about it!

[blakley]

<a href="http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html">This link</a> has a great explanation; the <a href="http://en.wikipedia.org/wiki/Depth_of_field">Wikipedia entry</a> makes it clear that lens focal length is a factor, but not the only factor.

[astral]

Craig -

 

You may be right, but it also seems most likely that the practical/engineering problems in the lens design may be an overriding factor in diaphragm positioning. The actual position of the diapragm is an interesting one since it affects many aspects the final image. For example, Geoffrey Crawley suggested recently (Amateur Photographer (UK) article, sometime in the last few months I think) that it is the placement of the diaphragm which is primarily responsible for the curvature of field issues in lenses/images. I was a bit surprised by this and must re-read it as it seems rather counter-intuitive, like much in optics, physics and life generally.

 

Certainly the position of the diaphragm in modern, compared to older, Elmar 50mm lenses seems to be a significant factor in the many improvements seen in the current Elmar. However, I only have the old 50/28 Elmar and cannot make a direct comparison. I think Erwin Puts covers this issue on his website. If I was half as organised as I should be I'd be able to quickly find the Crawley & Puts articles and read them more carefully. I'd especially like to read all of Mr Crawley's technical articles that have appeared in AP, but taking photos is a bit more rewarding than keeping the 'grey matter' entertained.

 

The links Bob suggests seem to cover most of the issues nicely, and make up for the deficiencies in my ramblings! Anyway, where's Roger Hicks when you need him?

[peter]

Interesting question. One I have not thought about before so I will read the above inputs with interest when I get a moment. But first let me say this. With DIGITAL cameras it has been asserted by lens manufacturers (!!) that the angle at which the light hits each sensor is crucial to image quality because they interact differently to the way chemistry based film does (and I think there is a little lens over each CCD to help focus it, or so I have been told, I have never studied it.) This suggests that this could be an issue for digitals in some instances (eg with an ultrawide possibly (???)) But I should also acknowledge that others, including I think Irwin Puts, have said that this is a load of cock and bull designed to sell nice new plasticky lenses, custom made for digitals and does not reflect reality. Any views?

[vivek iyer]

Peter, Good point. Ultrawides and current day digitals don't get along all that well. The Zeiss Distagon 21mm is supposedly the only wide that would do well on digital. I don't know how true that is as I don't own any FF dSLRs with an EOS mount and there is little chance that I will pay a lot of money (current going prices) for an used C/Y mount 21 Distagon.

 

I am very doubtful that the 14mm Canon lens on 5D, etc is really worth the bother. Perhaps, some of the users can enlighten us.