Focal length - digital vs film

[radfordneal]

I'd agree that the whole "circle of confusion" thing is just

confusing. But that doesn't mean "depth of field" is an illusion,

or a matter of opinion. In particular, if you take a photo with

a 50mm lens at f/2 on a 35mm film camera, then make a 4x6 print,

and you also take the same shot (same field of view) using a 35mm

lens at f/2 on an APS-C size digital camera (crop factor 1.5), and

also make a 4x6 print, you will find that the circles in the

background from out-of-focus highlights are 50% bigger in the print

made with the film camera. To get the same effect as on film,

you'd need to use about a 35mm f/1.4 lens on the digital camera

(more precisely, f/1.3333).

[kelly_flanigan1]

DOF COC's go back to the 1930's; its nothing really new. PRE WW2 numbers are 1/2000 for 8mm cine; 1/1000 for 16mm cine; 1/500 for fine 35mm still like the Ektra; 1/200 for roll film folders; 2 minutes of arc for view cameras.<BR><BR>Smaller formats "tend" to be enlarged more; thus the tighter criteria. Folks in 1938 understood this, one wonders if the average chap today in 2007 is dumber in math or understanding things. <BR><BR>What really matters is that folks understand their tools. If one uses a Canon 5d then there is none of the equalvalent rubbish. <BR><BR>Folks 1/2 century ago used LTM and Exaktax lenses on 16mm cine with adapters; and didnt get confused; or use the equalvalent focal length terms. Maybe the slide rule era and less TV made folks better thinkers?

[kelly_flanigan1]

Teenage girls with cellphones are not confused about focal length, or perspective. They just move the phone away if they get the giant nose look. Since the actual focal length and sensor size is not known; they dont get bogged down in endless "equalvalent focal length confusion" like dslr users. They think more like a movie maker; distance to subject for perspective; and angular coverage that the phone shoots. Folks really should evolve and think in angle.

[peter k]

Hi all, just to add to the discussion on DOF. DOF is a function of lens to subject distance. A 50mm lens has the same depth of field as a 24 mm lens. You must change the camera position with the 24 mm lens so that the object being photographed is the same size in the view finder as it was with the 50mm lens. Now the DOF will be the same. Try it and you will see. Sorry to slightly disagree with some of the very knowlegable posters. Regards Peter

[kelly_flanigan1]

The DOF criteria are tighter on my 16mm Fastax movie camera lenses than a Leica M's or Nikon F's; due to the smaller COC. Thus a 50mm Fastax lens fast a different DOF table and tighter spaced DOF tick marks than a 50mm Nikkor for 35mm still camera. When the 50mm Nikkor is used on 16mm cine; one uses a more strict criteria for whats in focus; since 16mm is more strict than 35mm still. All this DOF stuff was known and published before ww2; its sad that folks dont understand the basics anymore.

[kelly_flanigan1]

A lens such as a 100mm can be used on a roll film folder with the typical 1/200 inch COC; or a 35mm still camera with a 1/30mm COC ; or a 16mm cine with a 1/1000 inch COC; or a 8mm cine with a 1/2000 inch COC.<BR><BR> Unless your know what the 100mm lens is used for; one cannot magically make a DOF table. <BR><BR>For a tight pixel pitch dslr today; the DOF table might be like a 16mm or 8mm cine camera of the 1930's; a tight spec.<BR><BR> If you go work for somebody; you really dont want to say a 100mm lens has the same DOF when used on any type of camera. <BR><BR>Experienced folks will sense the duffusity of your newbieness statements, and wonder what other gapping holes are in your photo experience.<BR><BR>A lens doesnt know what sensor or film its rays are hitting; or what the PURPOSE of the image is for; thus DOF tables and criteria are a starting point.

[radfordneal]

Responding to the following:

 

DOF is a function of lens to subject distance. A 50mm lens has the same depth of field as a 24 mm lens. You must change the camera position with the 24 mm lens so that the object being photographed is the same size in the view finder as it was with the 50mm lens. Now the DOF will be the same. Try it and you will see.

 

I say:

 

DOF is a function of lens to subject distance AND the size of the front element of the lens (strictly true only for a simple one-element lens, used without stopping down, but considering more complicated lenses and use when stopped down wouldn't add anything to this discussion). So what you say isn't true if the 24mm and 50mm lenses have the same f-ratio (eg, f/2). For it to be true, they'd need to have the same size front-element, which means they'd have different f-ratios. And yes, I've tried it and seen.

[tom_jenner1]

Wow, what a strange thread. Bob Atkins already explained all of this.

 

http://www.photo.net/learn/optics/dofdigital/

 

Some of you should really check this out; there seems to be quite a bit of confusion above.

[radfordneal]

Looking again at my reply above, I now realize that the post I was replying to was assuming the SAME camera in the two cases - not film versus digital. So my reply was confused. If you're using the same camera, a 50mm and 24mm lens will indeed give the same DOF at the same f-stop if you change position so that the subject is the same size in the viewfinder.

[johnw63]

Wow. Lots of well thought out posts, but still we have differing opinions.

 

I look at it this way...

 

The lens works the same way, optically, no matter WHAT you attach it to. The DoF does NOT change for any given lens. Sorry, optics and physics don't change because of your camera body. It is only when you put in on smaller capturing medium or larger medium that things get confusing for some. What you REALLY want is the LOOK to be the same. In most cases that does mean the field of view. In other words, what section of the real world is projected on the film, sensor, or little mythical charater who draws REALLY fast. Since, with Nikon anyway, the sensor is smaller than 35mm, you lose part of the shot you were going for. It fell off the edges of the medium. So, to make up for that, you get a wider lens, so the portion remaining on the sensor IS what you wanted before you took the shot. Now...if you back up, to get the shot your really wanted, rather than change lenses, you HAVE changed one of the things that contribute to the DoF; Lens to subject distance. But that is STILL not a change in the lens's properties. The Circle of Confusion didn't get smaller, you're just seeing LESS of it than before.

 

I figure in a number of years, it will be the FILM people needing to convert from one set of numbers instead of the other way around. I wonder if THEY will be confused ?

[photo5]

Weird, I never had trouble with this stuff because while I was shooting 35mm, I was also

shooting at the same time, 6x7cm and 4x5in format film. The 90mm was the standard lens

on the 6x7, and the 90mm was the wide lens on the 4x5. Each lens was the same focal

length, but the one for 4x5 had a larger image circle than the one for 6x7cm. I can

remember thinking the 90mm was about a 45mm, the 50mm was about a 24mm, and the

180mm was about a 90mm (thining in terms of 35mm format). So when I got into the Nikon

DX format, I wasn't too freaked out by the new numbers. It's all about coverage.

 

Dave

[blumesan]

Dave--

 

Naturally anyone with your experience with multiple formats would not be "freaked out" by these numbers. The same is true of anyone with experience using only 35mm with exchangable lenses. However I would venture to guess that a substantial fraction, perhaps even a majority of those buying and using dslrs today have not had that experience. Describing the lenses they will be buying by providing the "35mm effective focal length" has no frame of reference for them and is only confusing. If you doubt it, just look at the questions being asked in this and many other formats.

 

Cheers/Mike

[kelly_flanigan1]

Radford; in photography and filming one has to worry about exposure. Often one cannot open up the lens magically to make the shorter lens a faster fstop so the two cases have the same clear opening. Thus folks who work in filming and hopefully photography actually use fstops for DOF stuff; not clear aperture numbers. Thus with one is filming at dusk and one needs an F2 fstop; practical actual workers compare DOF tables for the 24 and 50mm lenses; and see that the 50mm one has a less useable zone that the actors can work in. This is basic stuff in movies; but maybe abit difficult for still photographers lost in the circle of confustion! :)<BR><BR>Here is a shot at F2.8 :<BR><BR><IMG SRC="http://i4.photobucket.com/albums/y148/ektar/hockey/tripods-371.jpg?t=1169448622"><BR><BR> Here is a shot with at F2.5<BR><BR><img src="http://i4.photobucket.com/albums/y148/ektar/scanback/SpeedGraphicF25AEsmall.jpg?t=1169448377"><BR><BR>Here is a shot at F2.8 :<BR><BR><img src="http://i4.photobucket.com/albums/y148/ektar/teletach/tripods-244.jpg?t=1169449293"><BR>OK <BR><BR><h3>*** POP TEST TIME! *** what do folks think the focal lengths are for the three shots?</h3> all are shot at about the same fstop of F2.8<BR><BR><BR>

[radfordneal]

POP TEST TIME! *** what do folks think the focal lengths are for the three shots?

 

There's no way we can possibly answer unless you tell us the size of the sensor/film in the camera you used (and how much you cropped, if any).

[kelly_flanigan1]

Little if any cropping.

[kelly_flanigan1]

The camera used or sensor used doesnt pin the DOF look; the focal length of the lens does At F2.8<BR><BR> Thus for each shot at F2.8 the focal length can be figured; since whats "in focus" in each image can be seen. <BR><BR>This is the type of thinking folks do in filming; understanding dof ; what zone is acceptable for the actors to appear in focus. <BR><BR>Thus a 8mm camera with a 12mm lens set at F2.8 will have a greater DOF than a 25mm lens on a 16mm cine at F2.8; greater than a 300mm F2.8 Nikkor on a Nikon F. <BR><BR>Seeing whats in focus at different distances at F2.8 in an image is enough to back out the focal length used in a scene.

[radfordneal]

I say we do need to know the size of the sensor/film to answer the quiz.

 

For the first photo, we can maybe get the answer, but only because we can figure out distances from the perspective. The hair on the

nearest person seems to be out of focus by about 2 mm, whereas the not-quite-so-near person in the centre seems to be in focus. The distances of these people seem to be about 60cm and 150cm. If I've

figured out the geometry right, we should multiply the 2mm by 150/90

to get the effective diameter of the lens, then multiply by 2.8

to get the focal length - 9.3mm. This could be off by quite a bit,

however, due to errors in judging the input quantities.

 

For the last photo, however, we can't get anywhere. It might have

been taken 2m from the subject, or it might have been taken 200m

from the subject (though admittedly you'd have had to be in a balloon

in that case). However, supposing it WAS taken from 200m away, we

can see that the grass about 3m behind the in-focus subject is about

3mm out of focus (and 3mm at 201m looks much the same as 3mm at

200m). So the geometry is that the cone of light reaching

the lens expands to a diameter of 3mm when we move 3m closer to the

lens, which means it would expand by 203/3 times as much when we

move all 203m to the lens, and hence the lens diameter must be

203mm. Multiplying by 2.8, we conclude that you used a 568mm lens.

Of course, the answer would be completely different if you were only

2m from the subject. If we knew the sensor/film size, we could

figure this out from the field of view, but as it is, you might

have been at any distance more than about a metre.

[caz_man]

If the standard focal length for 35 mm format is a 50mm lens then there must be a standard lens for digital format (the one used with 35mm style slr's) ??